Rename dhall_core to dhall_syntax

9 files changed, 3034 insertions, 0 deletions

diff --git a/dhall_syntax/src/context.rs b/dhall_syntax/src/context.rs
new file mode 100644
index 0000000..55bfff5
--- /dev/null
+++ b/dhall_syntax/src/context.rs
@@ -0,0 +1,80 @@
+use std::cmp::Eq;
+use std::collections::HashMap;
+use std::hash::Hash;
+
+/// A `(Context a)` associates `Text` labels with values of type `a`
+///
+/// The `Context` is used for type-checking when `(a = Expr X)`
+///
+/// * You create a `Context` using `empty` and `insert`
+/// * You transform a `Context` using `fmap`
+/// * You consume a `Context` using `lookup` and `toList`
+///
+/// The difference between a `Context` and a `Map` is that a `Context` lets you
+/// have multiple ordered occurrences of the same key and you can query for the
+/// `n`th occurrence of a given key.
+///
+#[derive(Debug, Clone)]
+pub struct Context<K: Eq + Hash, T>(HashMap<K, Vec<T>>);
+
+impl<K: Hash + Eq + Clone, T> Context<K, T> {
+    /// An empty context with no key-value pairs
+    pub fn new() -> Self {
+        Context(HashMap::new())
+    }
+
+    /// Look up a key by name and index
+    ///
+    /// ```c
+    /// lookup _ _         empty  = Nothing
+    /// lookup k 0 (insert k v c) = Just v
+    /// lookup k n (insert k v c) = lookup k (n - 1) c  -- 1 <= n
+    /// lookup k n (insert j v c) = lookup k  n      c  -- k /= j
+    /// ```
+    pub fn lookup<'a>(&'a self, k: &K, n: usize) -> Option<&'a T> {
+        self.0.get(k).and_then(|v| {
+            if n < v.len() {
+                v.get(v.len() - 1 - n)
+            } else {
+                None
+            }
+        })
+    }
+
+    pub fn map<U, F: Fn(&K, &T) -> U>(&self, f: F) -> Context<K, U> {
+        Context(
+            self.0
+                .iter()
+                .map(|(k, vs)| {
+                    ((*k).clone(), vs.iter().map(|v| f(k, v)).collect())
+                })
+                .collect(),
+        )
+    }
+
+    pub fn lookup_all<'a>(&'a self, k: &K) -> impl Iterator<Item = &T> {
+        self.0.get(k).into_iter().flat_map(|v| v.iter())
+    }
+
+    pub fn iter<'a>(&'a self) -> impl Iterator<Item = (&K, &T)> {
+        self.0
+            .iter()
+            .flat_map(|(k, vs)| vs.iter().map(move |v| (k, v)))
+    }
+
+    pub fn iter_keys<'a>(&'a self) -> impl Iterator<Item = (&K, &Vec<T>)> {
+        self.0.iter()
+    }
+}
+
+impl<K: Hash + Eq + Clone, T: Clone> Context<K, T> {
+    /// Add a key-value pair to the `Context`
+    pub fn insert(&self, k: K, v: T) -> Self {
+        let mut ctx = (*self).clone();
+        {
+            let m = ctx.0.entry(k).or_insert_with(Vec::new);
+            m.push(v);
+        }
+        ctx
+    }
+}
diff --git a/dhall_syntax/src/core.rs b/dhall_syntax/src/core.rs
new file mode 100644
index 0000000..3db07dd
--- /dev/null
+++ b/dhall_syntax/src/core.rs
@@ -0,0 +1,563 @@
+#![allow(non_snake_case)]
+use std::collections::BTreeMap;
+use std::collections::HashMap;
+use std::rc::Rc;
+
+use crate::context::Context;
+use crate::visitor;
+use crate::*;
+
+pub type Integer = isize;
+pub type Natural = usize;
+pub type Double = NaiveDouble;
+
+/// An empty type
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+pub enum X {}
+
+pub fn trivial_result<T>(x: Result<T, X>) -> T {
+    match x {
+        Ok(x) => x,
+        Err(e) => match e {},
+    }
+}
+
+/// Double with bitwise equality
+#[derive(Debug, Copy, Clone)]
+pub struct NaiveDouble(f64);
+
+impl PartialEq for NaiveDouble {
+    fn eq(&self, other: &Self) -> bool {
+        self.0.to_bits() == other.0.to_bits()
+    }
+}
+
+impl Eq for NaiveDouble {}
+
+impl From<f64> for NaiveDouble {
+    fn from(x: f64) -> Self {
+        NaiveDouble(x)
+    }
+}
+
+impl From<NaiveDouble> for f64 {
+    fn from(x: NaiveDouble) -> f64 {
+        x.0
+    }
+}
+
+/// Constants for a pure type system
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+pub enum Const {
+    Type,
+    Kind,
+    Sort,
+}
+
+/// Bound variable
+///
+/// The `Label` field is the variable's name (i.e. \"`x`\").
+/// The `Int` field is a DeBruijn index.
+/// See dhall-lang/standard/semantics.md for details
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct V<Label>(pub Label, pub usize);
+
+// This is only for the specific `Label` type, not generic
+impl From<Label> for V<Label> {
+    fn from(x: Label) -> V<Label> {
+        V(x, 0)
+    }
+}
+impl<'a> From<&'a Label> for V<Label> {
+    fn from(x: &'a Label) -> V<Label> {
+        V(x.clone(), 0)
+    }
+}
+
+// Definition order must match precedence order for
+// pretty-printing to work correctly
+#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
+pub enum BinOp {
+    /// x ? y
+    ImportAlt,
+    /// x || y`
+    BoolOr,
+    /// x + y`
+    NaturalPlus,
+    /// x ++ y`
+    TextAppend,
+    /// x # y
+    ListAppend,
+    /// x && y`
+    BoolAnd,
+    /// x ∧ y`
+    Combine,
+    /// x // y
+    Prefer,
+    /// x //\\ y
+    CombineTypes,
+    /// x * y`
+    NaturalTimes,
+    /// x == y`
+    BoolEQ,
+    /// x != y`
+    BoolNE,
+}
+
+/// Built-ins
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+pub enum Builtin {
+    Bool,
+    Natural,
+    Integer,
+    Double,
+    Text,
+    List,
+    Optional,
+    OptionalNone,
+    NaturalBuild,
+    NaturalFold,
+    NaturalIsZero,
+    NaturalEven,
+    NaturalOdd,
+    NaturalToInteger,
+    NaturalShow,
+    IntegerToDouble,
+    IntegerShow,
+    DoubleShow,
+    ListBuild,
+    ListFold,
+    ListLength,
+    ListHead,
+    ListLast,
+    ListIndexed,
+    ListReverse,
+    OptionalFold,
+    OptionalBuild,
+    TextShow,
+}
+
+pub type ParsedExpr = SubExpr<X, Import>;
+pub type ResolvedExpr = SubExpr<X, X>;
+pub type DhallExpr = ResolvedExpr;
+
+#[derive(Debug, PartialEq, Eq)]
+pub struct SubExpr<Note, Embed>(pub Rc<Expr<Note, Embed>>);
+
+pub type Expr<Note, Embed> = ExprF<SubExpr<Note, Embed>, Label, Note, Embed>;
+
+/// Syntax tree for expressions
+// Having the recursion out of the enum definition enables writing
+// much more generic code and improves pattern-matching behind
+// smart pointers.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum ExprF<SubExpr, Label, Note, Embed> {
+    Const(Const),
+    ///  `x`
+    ///  `x@n`
+    Var(V<Label>),
+    ///  `λ(x : A) -> b`
+    Lam(Label, SubExpr, SubExpr),
+    ///  `A -> B`
+    ///  `∀(x : A) -> B`
+    Pi(Label, SubExpr, SubExpr),
+    ///  `f a`
+    App(SubExpr, SubExpr),
+    ///  `let x     = r in e`
+    ///  `let x : t = r in e`
+    Let(Label, Option<SubExpr>, SubExpr, SubExpr),
+    ///  `x : t`
+    Annot(SubExpr, SubExpr),
+    /// Built-in values
+    Builtin(Builtin),
+    // Binary operations
+    BinOp(BinOp, SubExpr, SubExpr),
+    ///  `True`
+    BoolLit(bool),
+    ///  `if x then y else z`
+    BoolIf(SubExpr, SubExpr, SubExpr),
+    ///  `1`
+    NaturalLit(Natural),
+    ///  `+2`
+    IntegerLit(Integer),
+    ///  `3.24`
+    DoubleLit(Double),
+    ///  `"Some ${interpolated} text"`
+    TextLit(InterpolatedText<SubExpr>),
+    ///  `[] : List t`
+    EmptyListLit(SubExpr),
+    ///  `[x, y, z]`
+    NEListLit(Vec<SubExpr>),
+    /// Deprecated Optional literal form
+    ///  `[] : Optional a`
+    ///  `[x] : Optional a`
+    OldOptionalLit(Option<SubExpr>, SubExpr),
+    ///  `Some e`
+    SomeLit(SubExpr),
+    ///  `{ k1 : t1, k2 : t1 }`
+    RecordType(BTreeMap<Label, SubExpr>),
+    ///  `{ k1 = v1, k2 = v2 }`
+    RecordLit(BTreeMap<Label, SubExpr>),
+    ///  `< k1 : t1, k2 >`
+    UnionType(BTreeMap<Label, Option<SubExpr>>),
+    ///  `< k1 = t1, k2 : t2, k3 >`
+    UnionLit(Label, SubExpr, BTreeMap<Label, Option<SubExpr>>),
+    ///  `merge x y : t`
+    Merge(SubExpr, SubExpr, Option<SubExpr>),
+    ///  `e.x`
+    Field(SubExpr, Label),
+    ///  `e.{ x, y, z }`
+    Projection(SubExpr, Vec<Label>),
+    /// Annotation on the AST. Unused for now but could hold e.g. file location information
+    Note(Note, SubExpr),
+    /// Embeds an import or the result of resolving the import
+    Embed(Embed),
+}
+
+impl<SE, L, N, E> ExprF<SE, L, N, E> {
+    pub(crate) fn visit<'a, V, Return>(&'a self, v: V) -> Return
+    where
+        V: visitor::GenericVisitor<&'a ExprF<SE, L, N, E>, Return>,
+    {
+        v.visit(self)
+    }
+
+    fn traverse_ref_with_special_handling_of_binders<'a, SE2, L2, N2, E2, Err>(
+        &'a self,
+        visit_subexpr: impl FnMut(&'a SE) -> Result<SE2, Err>,
+        visit_under_binder: impl FnOnce(&'a L, &'a SE) -> Result<SE2, Err>,
+        visit_note: impl FnOnce(&'a N) -> Result<N2, Err>,
+        visit_embed: impl FnOnce(&'a E) -> Result<E2, Err>,
+        visit_label: impl FnMut(&'a L) -> Result<L2, Err>,
+    ) -> Result<ExprF<SE2, L2, N2, E2>, Err>
+    where
+        L: Ord,
+        L2: Ord,
+    {
+        self.visit(visitor::TraverseRefWithBindersVisitor {
+            visit_subexpr,
+            visit_under_binder,
+            visit_note,
+            visit_embed,
+            visit_label,
+        })
+    }
+
+    fn traverse_ref<'a, SE2, L2, N2, E2, Err>(
+        &'a self,
+        visit_subexpr: impl FnMut(&'a SE) -> Result<SE2, Err>,
+        visit_note: impl FnOnce(&'a N) -> Result<N2, Err>,
+        visit_embed: impl FnOnce(&'a E) -> Result<E2, Err>,
+        visit_label: impl FnMut(&'a L) -> Result<L2, Err>,
+    ) -> Result<ExprF<SE2, L2, N2, E2>, Err>
+    where
+        L: Ord,
+        L2: Ord,
+    {
+        self.visit(visitor::TraverseRefVisitor {
+            visit_subexpr,
+            visit_note,
+            visit_embed,
+            visit_label,
+        })
+    }
+
+    pub fn map_ref_with_special_handling_of_binders<'a, SE2, L2, N2, E2>(
+        &'a self,
+        mut map_subexpr: impl FnMut(&'a SE) -> SE2,
+        mut map_under_binder: impl FnMut(&'a L, &'a SE) -> SE2,
+        map_note: impl FnOnce(&'a N) -> N2,
+        map_embed: impl FnOnce(&'a E) -> E2,
+        mut map_label: impl FnMut(&'a L) -> L2,
+    ) -> ExprF<SE2, L2, N2, E2>
+    where
+        L: Ord,
+        L2: Ord,
+    {
+        trivial_result(self.traverse_ref_with_special_handling_of_binders(
+            |x| Ok(map_subexpr(x)),
+            |l, x| Ok(map_under_binder(l, x)),
+            |x| Ok(map_note(x)),
+            |x| Ok(map_embed(x)),
+            |x| Ok(map_label(x)),
+        ))
+    }
+
+    pub fn map_ref<'a, SE2, L2, N2, E2>(
+        &'a self,
+        mut map_subexpr: impl FnMut(&'a SE) -> SE2,
+        map_note: impl FnOnce(&'a N) -> N2,
+        map_embed: impl FnOnce(&'a E) -> E2,
+        mut map_label: impl FnMut(&'a L) -> L2,
+    ) -> ExprF<SE2, L2, N2, E2>
+    where
+        L: Ord,
+        L2: Ord,
+    {
+        trivial_result(self.traverse_ref(
+            |x| Ok(map_subexpr(x)),
+            |x| Ok(map_note(x)),
+            |x| Ok(map_embed(x)),
+            |x| Ok(map_label(x)),
+        ))
+    }
+
+    pub fn traverse_ref_simple<'a, SE2, Err>(
+        &'a self,
+        visit_subexpr: impl FnMut(&'a SE) -> Result<SE2, Err>,
+    ) -> Result<ExprF<SE2, L, N, E>, Err>
+    where
+        L: Ord + Clone,
+        N: Clone,
+        E: Clone,
+    {
+        self.traverse_ref(
+            visit_subexpr,
+            |x| Ok(N::clone(x)),
+            |x| Ok(E::clone(x)),
+            |x| Ok(L::clone(x)),
+        )
+    }
+
+    pub fn map_ref_simple<'a, SE2>(
+        &'a self,
+        map_subexpr: impl Fn(&'a SE) -> SE2,
+    ) -> ExprF<SE2, L, N, E>
+    where
+        L: Ord + Clone,
+        N: Clone,
+        E: Clone,
+    {
+        self.map_ref(map_subexpr, N::clone, E::clone, L::clone)
+    }
+}
+
+impl<N, E> Expr<N, E> {
+    fn traverse_embed<E2, Err>(
+        &self,
+        visit_embed: impl FnMut(&E) -> Result<E2, Err>,
+    ) -> Result<Expr<N, E2>, Err>
+    where
+        N: Clone,
+    {
+        self.visit(&mut visitor::TraverseEmbedVisitor(visit_embed))
+    }
+
+    fn map_embed<E2>(&self, mut map_embed: impl FnMut(&E) -> E2) -> Expr<N, E2>
+    where
+        N: Clone,
+    {
+        trivial_result(self.traverse_embed(|x| Ok(map_embed(x))))
+    }
+
+    pub fn roll(&self) -> SubExpr<N, E>
+    where
+        N: Clone,
+        E: Clone,
+    {
+        rc(ExprF::clone(self))
+    }
+
+    pub fn squash_embed<E2>(
+        &self,
+        f: impl FnMut(&E) -> SubExpr<N, E2>,
+    ) -> SubExpr<N, E2>
+    where
+        N: Clone,
+    {
+        trivial_result(self.visit(&mut visitor::SquashEmbedVisitor(f)))
+    }
+}
+
+impl<E: Clone> Expr<X, E> {
+    pub fn note_absurd<N>(&self) -> Expr<N, E> {
+        self.visit(&mut visitor::NoteAbsurdVisitor)
+    }
+}
+
+impl<N: Clone> Expr<N, X> {
+    pub fn embed_absurd<E>(&self) -> Expr<N, E> {
+        self.visit(&mut visitor::EmbedAbsurdVisitor)
+    }
+}
+
+impl<N, E> SubExpr<N, E> {
+    pub fn as_ref(&self) -> &Expr<N, E> {
+        self.0.as_ref()
+    }
+
+    pub fn traverse_embed<E2, Err>(
+        &self,
+        visit_embed: impl FnMut(&E) -> Result<E2, Err>,
+    ) -> Result<SubExpr<N, E2>, Err>
+    where
+        N: Clone,
+    {
+        Ok(rc(self.as_ref().traverse_embed(visit_embed)?))
+    }
+
+    pub fn map_embed<E2>(
+        &self,
+        map_embed: impl FnMut(&E) -> E2,
+    ) -> SubExpr<N, E2>
+    where
+        N: Clone,
+    {
+        rc(self.as_ref().map_embed(map_embed))
+    }
+
+    pub fn map_subexprs_with_special_handling_of_binders<'a>(
+        &'a self,
+        map_expr: impl FnMut(&'a Self) -> Self,
+        map_under_binder: impl FnMut(&'a Label, &'a Self) -> Self,
+    ) -> Self {
+        match self.as_ref() {
+            ExprF::Embed(_) => SubExpr::clone(self),
+            // Recursive call
+            // TODO: don't discard the note !
+            ExprF::Note(_, e) => e
+                .map_subexprs_with_special_handling_of_binders(
+                    map_expr,
+                    map_under_binder,
+                ),
+            // Call ExprF::map_ref
+            e => rc(e.map_ref_with_special_handling_of_binders(
+                map_expr,
+                map_under_binder,
+                |_| unreachable!(),
+                |_| unreachable!(),
+                Label::clone,
+            )),
+        }
+    }
+
+    pub fn unroll(&self) -> Expr<N, E>
+    where
+        N: Clone,
+        E: Clone,
+    {
+        ExprF::clone(self.as_ref())
+    }
+
+    pub fn unnote(&self) -> SubExpr<X, E>
+    where
+        E: Clone,
+    {
+        rc(self.as_ref().visit(&mut visitor::UnNoteVisitor))
+    }
+
+    /// `shift` is used by both normalization and type-checking to avoid variable
+    /// capture by shifting variable indices
+    /// See https://github.com/dhall-lang/dhall-lang/blob/master/standard/semantics.md#shift
+    /// for details
+    pub fn shift(&self, delta: isize, var: &V<Label>) -> Self {
+        match self.as_ref() {
+            ExprF::Var(v) => rc(ExprF::Var(v.shift(delta, var))),
+            _ => self.map_subexprs_with_special_handling_of_binders(
+                |e| e.shift(delta, var),
+                |x: &Label, e| e.shift(delta, &var.shift(1, &x.into())),
+            ),
+        }
+    }
+
+    pub fn subst_shift(&self, var: &V<Label>, val: &Self) -> Self {
+        match self.as_ref() {
+            ExprF::Var(v) if v == var => val.clone(),
+            ExprF::Var(v) => rc(ExprF::Var(v.shift(-1, var))),
+            _ => self.map_subexprs_with_special_handling_of_binders(
+                |e| e.subst_shift(var, val),
+                |x: &Label, e| {
+                    e.subst_shift(
+                        &var.shift(1, &x.into()),
+                        &val.shift(1, &x.into()),
+                    )
+                },
+            ),
+        }
+    }
+}
+
+impl<N: Clone> SubExpr<N, X> {
+    pub fn embed_absurd<T>(&self) -> SubExpr<N, T> {
+        rc(self.as_ref().embed_absurd())
+    }
+}
+
+impl<E: Clone> SubExpr<X, E> {
+    pub fn note_absurd<N>(&self) -> SubExpr<N, E> {
+        rc(self.as_ref().note_absurd())
+    }
+}
+
+impl<N, E> Clone for SubExpr<N, E> {
+    fn clone(&self) -> Self {
+        SubExpr(Rc::clone(&self.0))
+    }
+}
+
+// Should probably rename this
+pub fn rc<N, E>(x: Expr<N, E>) -> SubExpr<N, E> {
+    SubExpr(Rc::new(x))
+}
+
+/// Add an isize to an usize
+/// Panics on over/underflow
+fn add_ui(u: usize, i: isize) -> usize {
+    if i < 0 {
+        u.checked_sub(i.checked_neg().unwrap() as usize).unwrap()
+    } else {
+        u.checked_add(i as usize).unwrap()
+    }
+}
+
+impl<Label: PartialEq + Clone> V<Label> {
+    pub fn shift(&self, delta: isize, var: &V<Label>) -> Self {
+        let V(x, n) = var;
+        let V(y, m) = self;
+        if x == y && n <= m {
+            V(y.clone(), add_ui(*m, delta))
+        } else {
+            V(y.clone(), *m)
+        }
+    }
+}
+
+/// `shift` is used by both normalization and type-checking to avoid variable
+/// capture by shifting variable indices
+/// See https://github.com/dhall-lang/dhall-lang/blob/master/standard/semantics.md#shift
+/// for details
+///
+pub fn shift<N, E>(
+    delta: isize,
+    var: &V<Label>,
+    in_expr: &SubExpr<N, E>,
+) -> SubExpr<N, E> {
+    in_expr.shift(delta, var)
+}
+
+pub fn shift0_multiple<N, E>(
+    deltas: &HashMap<Label, isize>,
+    in_expr: &SubExpr<N, E>,
+) -> SubExpr<N, E> {
+    shift0_multiple_inner(&Context::new(), deltas, in_expr)
+}
+
+fn shift0_multiple_inner<N, E>(
+    ctx: &Context<Label, ()>,
+    deltas: &HashMap<Label, isize>,
+    in_expr: &SubExpr<N, E>,
+) -> SubExpr<N, E> {
+    use crate::ExprF::*;
+    match in_expr.as_ref() {
+        Var(V(y, m)) if ctx.lookup(y, *m).is_none() => {
+            let delta = deltas.get(y).unwrap_or(&0);
+            rc(Var(V(y.clone(), add_ui(*m, *delta))))
+        }
+        _ => in_expr.map_subexprs_with_special_handling_of_binders(
+            |e| shift0_multiple_inner(ctx, deltas, e),
+            |x: &Label, e| {
+                shift0_multiple_inner(&ctx.insert(x.clone(), ()), deltas, e)
+            },
+        ),
+    }
+}
diff --git a/dhall_syntax/src/import.rs b/dhall_syntax/src/import.rs
new file mode 100644
index 0000000..00f293c
--- /dev/null
+++ b/dhall_syntax/src/import.rs
@@ -0,0 +1,64 @@
+use std::path::PathBuf;
+
+/// The beginning of a file path which anchors subsequent path components
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
+pub enum FilePrefix {
+    /// Absolute path
+    Absolute,
+    /// Path relative to .
+    Here,
+    /// Path relative to ..
+    Parent,
+    /// Path relative to ~
+    Home,
+}
+
+/// The location of import (i.e. local vs. remote vs. environment)
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub enum ImportLocation {
+    Local(FilePrefix, PathBuf),
+    Remote(URL),
+    Env(String),
+    Missing,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub struct URL {
+    pub scheme: Scheme,
+    pub authority: String,
+    pub path: PathBuf,
+    pub query: Option<String>,
+    pub headers: Option<Box<ImportHashed>>,
+}
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
+pub enum Scheme {
+    HTTP,
+    HTTPS,
+}
+
+/// How to interpret the import's contents (i.e. as Dhall code or raw text)
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
+pub enum ImportMode {
+    Code,
+    RawText,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub struct Hash {
+    pub protocol: String,
+    pub hash: String,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub struct ImportHashed {
+    pub location: ImportLocation,
+    pub hash: Option<Hash>,
+}
+
+/// Reference to an external resource
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub struct Import {
+    pub mode: ImportMode,
+    pub location_hashed: ImportHashed,
+}
diff --git a/dhall_syntax/src/label.rs b/dhall_syntax/src/label.rs
new file mode 100644
index 0000000..43c3f53
--- /dev/null
+++ b/dhall_syntax/src/label.rs
@@ -0,0 +1,34 @@
+use std::rc::Rc;
+
+// The type for labels throughout the AST
+// It owns the data because otherwise lifetimes would make recursive imports impossible
+#[derive(Debug, Clone, PartialEq, Eq, Hash, PartialOrd, Ord)]
+pub struct Label(Rc<str>);
+
+impl From<String> for Label {
+    fn from(s: String) -> Self {
+        let s: &str = &s;
+        Label(s.into())
+    }
+}
+
+impl<'a> From<&'a str> for Label {
+    fn from(s: &'a str) -> Self {
+        Label(Rc::from(s))
+    }
+}
+
+impl From<&Label> for String {
+    fn from(x: &Label) -> String {
+        x.0.as_ref().to_owned()
+    }
+}
+
+impl Label {
+    pub fn from_str(s: &str) -> Label {
+        Label(s.into())
+    }
+    pub fn as_ref(&self) -> &str {
+        self.0.as_ref()
+    }
+}
diff --git a/dhall_syntax/src/lib.rs b/dhall_syntax/src/lib.rs
new file mode 100644
index 0000000..3db8222
--- /dev/null
+++ b/dhall_syntax/src/lib.rs
@@ -0,0 +1,28 @@
+#![feature(trace_macros)]
+#![feature(slice_patterns)]
+#![allow(
+    clippy::many_single_char_names,
+    clippy::should_implement_trait,
+    clippy::new_without_default,
+    clippy::type_complexity
+)]
+
+//! This crate contains the core AST-handling primitives for the [dhall-rust][dhall-rust] crate.
+//! This is highly unstable and breaks regularly; use at your own risk.
+//!
+//! [dhall-rust]: https://github.com/Nadrieril/dhall-rust
+
+mod core;
+pub use crate::core::*;
+mod import;
+pub use crate::import::*;
+mod label;
+pub use crate::label::*;
+mod text;
+pub use crate::text::*;
+mod printer;
+pub use crate::printer::*;
+mod parser;
+pub use crate::parser::*;
+pub mod context;
+pub mod visitor;
diff --git a/dhall_syntax/src/parser.rs b/dhall_syntax/src/parser.rs
new file mode 100644
index 0000000..12383d4
--- /dev/null
+++ b/dhall_syntax/src/parser.rs
@@ -0,0 +1,984 @@
+use itertools::Itertools;
+use pest::iterators::Pair;
+use pest::Parser;
+pub use pest::Span;
+use std::borrow::Cow;
+use std::collections::BTreeMap;
+use std::path::PathBuf;
+
+use dhall_generated_parser::{DhallParser, Rule};
+
+use crate::*;
+
+// This file consumes the parse tree generated by pest and turns it into
+// our own AST. All those custom macros should eventually moved into
+// their own crate because they are quite general and useful. For now they
+// are here and hopefully you can figure out how they work.
+
+use crate::ExprF::*;
+
+type ParsedExpr<'a> = Expr<Span<'a>, Import>;
+type ParsedSubExpr<'a> = SubExpr<Span<'a>, Import>;
+type ParsedText<'a> = InterpolatedText<SubExpr<Span<'a>, Import>>;
+type ParsedTextContents<'a> =
+    InterpolatedTextContents<SubExpr<Span<'a>, Import>>;
+
+pub type ParseError = pest::error::Error<Rule>;
+
+pub type ParseResult<T> = Result<T, ParseError>;
+
+fn rc(x: ParsedExpr<'_>) -> ParsedSubExpr<'_> {
+    crate::rc(x)
+}
+
+fn spanned<'a>(_span: Span<'a>, x: ParsedExpr<'a>) -> ParsedExpr<'a> {
+    x
+    // This breaks equality testing; I need to fix that first
+    // Note(span, rc(x))
+}
+
+#[derive(Debug)]
+enum Either<A, B> {
+    Left(A),
+    Right(B),
+}
+
+impl crate::Builtin {
+    pub fn parse(s: &str) -> Option<Self> {
+        use crate::Builtin::*;
+        match s {
+            "Bool" => Some(Bool),
+            "Natural" => Some(Natural),
+            "Integer" => Some(Integer),
+            "Double" => Some(Double),
+            "Text" => Some(Text),
+            "List" => Some(List),
+            "Optional" => Some(Optional),
+            "None" => Some(OptionalNone),
+            "Natural/build" => Some(NaturalBuild),
+            "Natural/fold" => Some(NaturalFold),
+            "Natural/isZero" => Some(NaturalIsZero),
+            "Natural/even" => Some(NaturalEven),
+            "Natural/odd" => Some(NaturalOdd),
+            "Natural/toInteger" => Some(NaturalToInteger),
+            "Natural/show" => Some(NaturalShow),
+            "Integer/toDouble" => Some(IntegerToDouble),
+            "Integer/show" => Some(IntegerShow),
+            "Double/show" => Some(DoubleShow),
+            "List/build" => Some(ListBuild),
+            "List/fold" => Some(ListFold),
+            "List/length" => Some(ListLength),
+            "List/head" => Some(ListHead),
+            "List/last" => Some(ListLast),
+            "List/indexed" => Some(ListIndexed),
+            "List/reverse" => Some(ListReverse),
+            "Optional/fold" => Some(OptionalFold),
+            "Optional/build" => Some(OptionalBuild),
+            "Text/show" => Some(TextShow),
+            _ => None,
+        }
+    }
+}
+
+pub fn custom_parse_error(pair: &Pair<Rule>, msg: String) -> ParseError {
+    let msg =
+        format!("{} while matching on:\n{}", msg, debug_pair(pair.clone()));
+    let e = pest::error::ErrorVariant::CustomError { message: msg };
+    pest::error::Error::new_from_span(e, pair.as_span())
+}
+
+fn debug_pair(pair: Pair<Rule>) -> String {
+    use std::fmt::Write;
+    let mut s = String::new();
+    fn aux(s: &mut String, indent: usize, prefix: String, pair: Pair<Rule>) {
+        let indent_str = "| ".repeat(indent);
+        let rule = pair.as_rule();
+        let contents = pair.as_str();
+        let mut inner = pair.into_inner();
+        let mut first = true;
+        while let Some(p) = inner.next() {
+            if first {
+                first = false;
+                let last = inner.peek().is_none();
+                if last && p.as_str() == contents {
+                    let prefix = format!("{}{:?} > ", prefix, rule);
+                    aux(s, indent, prefix, p);
+                    continue;
+                } else {
+                    writeln!(
+                        s,
+                        r#"{}{}{:?}: "{}""#,
+                        indent_str, prefix, rule, contents
+                    )
+                    .unwrap();
+                }
+            }
+            aux(s, indent + 1, "".into(), p);
+        }
+        if first {
+            writeln!(
+                s,
+                r#"{}{}{:?}: "{}""#,
+                indent_str, prefix, rule, contents
+            )
+            .unwrap();
+        }
+    }
+    aux(&mut s, 0, "".into(), pair);
+    s
+}
+
+macro_rules! make_parser {
+    (@pattern, rule, $name:ident) => (Rule::$name);
+    (@pattern, token_rule, $name:ident) => (Rule::$name);
+    (@pattern, rule_group, $name:ident) => (_);
+    (@filter, rule) => (true);
+    (@filter, token_rule) => (true);
+    (@filter, rule_group) => (false);
+
+    (@body,
+        $pair:expr,
+        $children:expr,
+        rule!( $name:ident<$o:ty>; $($args:tt)* )
+    ) => (
+        make_parser!(@body,
+            $pair,
+            $children,
+            rule!( $name<$o> as $name; $($args)* )
+        )
+    );
+    (@body,
+        $pair:expr,
+        $children:expr,
+        rule!(
+            $name:ident<$o:ty>
+            as $group:ident;
+            captured_str!($x:pat) => $body:expr
+        )
+    ) => ({
+        let $x = $pair.as_str();
+        let res: $o = $body;
+        Ok(ParsedValue::$group(res))
+    });
+    (@body,
+        $pair:expr,
+        $children:expr,
+        rule!(
+            $name:ident<$o:ty>
+            as $group:ident;
+            children!( $( [$($args:tt)*] => $body:expr ),* $(,)* )
+        )
+    ) => ({
+        #[allow(unused_imports)]
+        use ParsedValue::*;
+        #[allow(unreachable_code)]
+        let res: $o = improved_slice_patterns::match_vec!($children;
+            $( [$($args)*] => $body, )*
+            [x..] => Err(
+                format!("Unexpected children: {:?}", x.collect::<Vec<_>>())
+            )?,
+        ).map_err(|_| -> String { unreachable!() })?;
+        Ok(ParsedValue::$group(res))
+    });
+    (@body,
+        $pair:expr,
+        $children:expr,
+        rule!(
+            $name:ident<$o:ty>
+            as $group:ident;
+            $span:ident;
+            $($args:tt)*
+        )
+    ) => ({
+        let $span = $pair.as_span();
+        make_parser!(@body,
+            $pair,
+            $children,
+            rule!(
+                $name<$o>
+                as $group;
+                $($args)*
+            )
+        )
+    });
+    (@body,
+        $pair:expr,
+        $children:expr,
+        token_rule!($name:ident<$o:ty>)
+    ) => ({
+        Ok(ParsedValue::$name(()))
+    });
+    (@body, $pair:expr, $children:expr, rule_group!( $name:ident<$o:ty> )) => (
+        unreachable!()
+    );
+
+    ($( $submac:ident!( $name:ident<$o:ty> $($args:tt)* ); )*) => (
+        #[allow(non_camel_case_types, dead_code, clippy::large_enum_variant)]
+        #[derive(Debug)]
+        enum ParsedValue<'a> {
+            $( $name($o), )*
+        }
+
+        fn parse_any<'a>(pair: Pair<'a, Rule>, children: Vec<ParsedValue<'a>>)
+                -> Result<ParsedValue<'a>, String> {
+            match pair.as_rule() {
+                $(
+                    make_parser!(@pattern, $submac, $name)
+                    if make_parser!(@filter, $submac)
+                    => make_parser!(@body, pair, children,
+                                           $submac!( $name<$o> $($args)* ))
+                    ,
+                )*
+                r => Err(format!("Unexpected {:?}", r)),
+            }
+        }
+    );
+}
+
+// Non-recursive implementation to avoid stack overflows
+fn do_parse<'a>(initial_pair: Pair<'a, Rule>) -> ParseResult<ParsedValue<'a>> {
+    enum StackFrame<'a> {
+        Unprocessed(Pair<'a, Rule>),
+        Processed(Pair<'a, Rule>, usize),
+    }
+    use StackFrame::*;
+    let mut pairs_stack: Vec<StackFrame> =
+        vec![Unprocessed(initial_pair.clone())];
+    let mut values_stack: Vec<ParsedValue> = vec![];
+    while let Some(p) = pairs_stack.pop() {
+        match p {
+            Unprocessed(mut pair) => loop {
+                let mut pairs: Vec<_> = pair.clone().into_inner().collect();
+                let n_children = pairs.len();
+                if n_children == 1 && can_be_shortcutted(pair.as_rule()) {
+                    pair = pairs.pop().unwrap();
+                    continue;
+                } else {
+                    pairs_stack.push(Processed(pair, n_children));
+                    pairs_stack
+                        .extend(pairs.into_iter().map(StackFrame::Unprocessed));
+                    break;
+                }
+            },
+            Processed(pair, n) => {
+                let mut children: Vec<_> =
+                    values_stack.split_off(values_stack.len() - n);
+                children.reverse();
+                let val = match parse_any(pair.clone(), children) {
+                    Ok(v) => v,
+                    Err(msg) => Err(custom_parse_error(&pair, msg))?,
+                };
+                values_stack.push(val);
+            }
+        }
+    }
+    Ok(values_stack.pop().unwrap())
+}
+
+// List of rules that can be shortcutted if they have a single child
+fn can_be_shortcutted(rule: Rule) -> bool {
+    use Rule::*;
+    match rule {
+        expression
+        | import_alt_expression
+        | or_expression
+        | plus_expression
+        | text_append_expression
+        | list_append_expression
+        | and_expression
+        | combine_expression
+        | prefer_expression
+        | combine_types_expression
+        | times_expression
+        | equal_expression
+        | not_equal_expression
+        | application_expression
+        | first_application_expression
+        | selector_expression
+        | annotated_expression => true,
+        _ => false,
+    }
+}
+
+make_parser! {
+    token_rule!(EOI<()>);
+
+    rule!(simple_label<Label>;
+        captured_str!(s) => Label::from(s.trim().to_owned())
+    );
+    rule!(quoted_label<Label>;
+        captured_str!(s) => Label::from(s.trim().to_owned())
+    );
+    rule!(label<Label>; children!(
+        [simple_label(l)] => l,
+        [quoted_label(l)] => l,
+    ));
+
+    rule!(double_quote_literal<ParsedText<'a>>; children!(
+        [double_quote_chunk(chunks)..] => {
+            chunks.collect()
+        }
+    ));
+
+    rule!(double_quote_chunk<ParsedTextContents<'a>>; children!(
+        [interpolation(e)] => {
+            InterpolatedTextContents::Expr(rc(e))
+        },
+        [double_quote_escaped(s)] => {
+            InterpolatedTextContents::Text(s)
+        },
+        [double_quote_char(s)] => {
+            InterpolatedTextContents::Text(s.to_owned())
+        },
+    ));
+    rule!(double_quote_escaped<String>;
+        captured_str!(s) => {
+            match s {
+                "\"" => "\"".to_owned(),
+                "$" => "$".to_owned(),
+                "\\" => "\\".to_owned(),
+                "/" => "/".to_owned(),
+                "b" => "\u{0008}".to_owned(),
+                "f" => "\u{000C}".to_owned(),
+                "n" => "\n".to_owned(),
+                "r" => "\r".to_owned(),
+                "t" => "\t".to_owned(),
+                _ => {
+                    // "uXXXX"
+                    use std::convert::TryFrom;
+                    let c = u16::from_str_radix(&s[1..5], 16).unwrap();
+                    let c = char::try_from(u32::from(c)).unwrap();
+                    std::iter::once(c).collect()
+                }
+            }
+        }
+    );
+    rule!(double_quote_char<&'a str>;
+        captured_str!(s) => s
+    );
+
+    rule!(single_quote_literal<ParsedText<'a>>; children!(
+        [single_quote_continue(lines)] => {
+            let space = InterpolatedTextContents::Text(" ".to_owned());
+            let newline = InterpolatedTextContents::Text("\n".to_owned());
+            let min_indent = lines
+                .iter()
+                .map(|l| {
+                    l.iter().rev().take_while(|c| **c == space).count()
+                })
+                .min()
+                .unwrap();
+
+            lines
+                .into_iter()
+                .rev()
+                .map(|mut l| { l.split_off(l.len() - min_indent); l })
+                .intersperse(vec![newline])
+                .flat_map(|x| x.into_iter().rev())
+                .collect::<ParsedText>()
+        }
+    ));
+    rule!(single_quote_char<&'a str>;
+        captured_str!(s) => s
+    );
+    rule!(escaped_quote_pair<&'a str>;
+        captured_str!(_) => "''"
+    );
+    rule!(escaped_interpolation<&'a str>;
+        captured_str!(_) => "${"
+    );
+    rule!(interpolation<ParsedExpr<'a>>; children!(
+        [expression(e)] => e
+    ));
+
+    rule!(single_quote_continue<Vec<Vec<ParsedTextContents<'a>>>>; children!(
+        [interpolation(c), single_quote_continue(lines)] => {
+            let c = InterpolatedTextContents::Expr(rc(c));
+            let mut lines = lines;
+            lines.last_mut().unwrap().push(c);
+            lines
+        },
+        [escaped_quote_pair(c), single_quote_continue(lines)] => {
+            let c = InterpolatedTextContents::Text(c.to_owned());
+            let mut lines = lines;
+            lines.last_mut().unwrap().push(c);
+            lines
+        },
+        [escaped_interpolation(c), single_quote_continue(lines)] => {
+            let c = InterpolatedTextContents::Text(c.to_owned());
+            let mut lines = lines;
+            lines.last_mut().unwrap().push(c);
+            lines
+        },
+        [single_quote_char("\n"), single_quote_continue(lines)] => {
+            let mut lines = lines;
+            lines.push(vec![]);
+            lines
+        },
+        [single_quote_char(c), single_quote_continue(lines)] => {
+            let c = InterpolatedTextContents::Text(c.to_owned());
+            let mut lines = lines;
+            lines.last_mut().unwrap().push(c);
+            lines
+        },
+        [] => {
+            vec![vec![]]
+        },
+    ));
+
+    rule!(builtin<ParsedExpr<'a>>; span;
+        captured_str!(s) => {
+            spanned(span, match crate::Builtin::parse(s) {
+                Some(b) => Builtin(b),
+                None => match s {
+                    "True" => BoolLit(true),
+                    "False" => BoolLit(false),
+                    "Type" => Const(crate::Const::Type),
+                    "Kind" => Const(crate::Const::Kind),
+                    "Sort" => Const(crate::Const::Sort),
+                    _ => Err(
+                        format!("Unrecognized builtin: '{}'", s)
+                    )?,
+                }
+            })
+        }
+    );
+
+    token_rule!(NaN<()>);
+    token_rule!(minus_infinity_literal<()>);
+    token_rule!(plus_infinity_literal<()>);
+
+    rule!(numeric_double_literal<core::Double>;
+        captured_str!(s) => {
+            let s = s.trim();
+            match s.parse::<f64>() {
+                Ok(x) if x.is_infinite() =>
+                    Err(format!("Overflow while parsing double literal '{}'", s))?,
+                Ok(x) => NaiveDouble::from(x),
+                Err(e) => Err(format!("{}", e))?,
+            }
+        }
+    );
+
+    rule!(double_literal<core::Double>; children!(
+        [numeric_double_literal(n)] => n,
+        [minus_infinity_literal(n)] => std::f64::NEG_INFINITY.into(),
+        [plus_infinity_literal(n)] => std::f64::INFINITY.into(),
+        [NaN(n)] => std::f64::NAN.into(),
+    ));
+
+    rule!(natural_literal<core::Natural>;
+        captured_str!(s) => {
+            s.trim()
+                .parse()
+                .map_err(|e| format!("{}", e))?
+        }
+    );
+
+    rule!(integer_literal<core::Integer>;
+        captured_str!(s) => {
+            s.trim()
+                .parse()
+                .map_err(|e| format!("{}", e))?
+        }
+    );
+
+    rule!(identifier<ParsedExpr<'a>> as expression; span; children!(
+        [variable(v)] => {
+            spanned(span, Var(v))
+        },
+        [builtin(e)] => e,
+    ));
+
+    rule!(variable<V<Label>>; children!(
+        [label(l), natural_literal(idx)] => {
+            V(l, idx)
+        },
+        [label(l)] => {
+            V(l, 0)
+        },
+    ));
+
+    rule!(unquoted_path_component<&'a str>; captured_str!(s) => s);
+    rule!(quoted_path_component<&'a str>; captured_str!(s) => s);
+    rule!(path_component<String>; children!(
+        [unquoted_path_component(s)] => {
+            percent_encoding::percent_decode(s.as_bytes())
+                .decode_utf8_lossy()
+                .into_owned()
+        },
+        [quoted_path_component(s)] => s.to_string(),
+    ));
+    rule!(path<PathBuf>; children!(
+        [path_component(components)..] => {
+            components.collect()
+        }
+    ));
+
+    rule_group!(local<(FilePrefix, PathBuf)>);
+
+    rule!(parent_path<(FilePrefix, PathBuf)> as local; children!(
+        [path(p)] => (FilePrefix::Parent, p)
+    ));
+    rule!(here_path<(FilePrefix, PathBuf)> as local; children!(
+        [path(p)] => (FilePrefix::Here, p)
+    ));
+    rule!(home_path<(FilePrefix, PathBuf)> as local; children!(
+        [path(p)] => (FilePrefix::Home, p)
+    ));
+    rule!(absolute_path<(FilePrefix, PathBuf)> as local; children!(
+        [path(p)] => (FilePrefix::Absolute, p)
+    ));
+
+    rule!(scheme<Scheme>; captured_str!(s) => match s {
+        "http" => Scheme::HTTP,
+        "https" => Scheme::HTTPS,
+        _ => unreachable!(),
+    });
+
+    rule!(http_raw<URL>; children!(
+        [scheme(sch), authority(auth), path(p)] => URL {
+            scheme: sch,
+            authority: auth,
+            path: p,
+            query: None,
+            headers: None,
+        },
+        [scheme(sch), authority(auth), path(p), query(q)] => URL {
+            scheme: sch,
+            authority: auth,
+            path: p,
+            query: Some(q),
+            headers: None,
+        },
+    ));
+
+    rule!(authority<String>; captured_str!(s) => s.to_owned());
+
+    rule!(query<String>; captured_str!(s) => s.to_owned());
+
+    rule!(http<URL>; children!(
+        [http_raw(url)] => url,
+        [http_raw(url), import_hashed(ih)] =>
+            URL { headers: Some(Box::new(ih)), ..url },
+    ));
+
+    rule!(env<String>; children!(
+        [bash_environment_variable(s)] => s,
+        [posix_environment_variable(s)] => s,
+    ));
+    rule!(bash_environment_variable<String>; captured_str!(s) => s.to_owned());
+    rule!(posix_environment_variable<String>; children!(
+        [posix_environment_variable_character(chars)..] => {
+            chars.collect()
+        },
+    ));
+    rule!(posix_environment_variable_character<Cow<'a, str>>;
+        captured_str!(s) => {
+            match s {
+                "\\\"" => Cow::Owned("\"".to_owned()),
+                "\\\\" => Cow::Owned("\\".to_owned()),
+                "\\a" =>  Cow::Owned("\u{0007}".to_owned()),
+                "\\b" =>  Cow::Owned("\u{0008}".to_owned()),
+                "\\f" =>  Cow::Owned("\u{000C}".to_owned()),
+                "\\n" =>  Cow::Owned("\n".to_owned()),
+                "\\r" =>  Cow::Owned("\r".to_owned()),
+                "\\t" =>  Cow::Owned("\t".to_owned()),
+                "\\v" =>  Cow::Owned("\u{000B}".to_owned()),
+                _ => Cow::Borrowed(s)
+            }
+        }
+    );
+
+    token_rule!(missing<()>);
+
+    rule!(import_type<ImportLocation>; children!(
+        [missing(_)] => {
+            ImportLocation::Missing
+        },
+        [env(e)] => {
+            ImportLocation::Env(e)
+        },
+        [http(url)] => {
+            ImportLocation::Remote(url)
+        },
+        [local((prefix, p))] => {
+            ImportLocation::Local(prefix, p)
+        },
+    ));
+
+    rule!(hash<Hash>; captured_str!(s) =>
+        Hash {
+            protocol: s.trim()[..6].to_owned(),
+            hash: s.trim()[7..].to_owned(),
+        }
+    );
+
+    rule!(import_hashed<ImportHashed>; children!(
+        [import_type(location)] =>
+            ImportHashed { location, hash: None },
+        [import_type(location), hash(h)] =>
+            ImportHashed { location, hash: Some(h) },
+    ));
+
+    token_rule!(Text<()>);
+
+    rule!(import<ParsedExpr<'a>> as expression; span; children!(
+        [import_hashed(location_hashed)] => {
+            spanned(span, Embed(Import {
+                mode: ImportMode::Code,
+                location_hashed
+            }))
+        },
+        [import_hashed(location_hashed), Text(_)] => {
+            spanned(span, Embed(Import {
+                mode: ImportMode::RawText,
+                location_hashed
+            }))
+        },
+    ));
+
+    token_rule!(lambda<()>);
+    token_rule!(forall<()>);
+    token_rule!(arrow<()>);
+    token_rule!(merge<()>);
+    token_rule!(if_<()>);
+    token_rule!(in_<()>);
+
+    rule!(expression<ParsedExpr<'a>> as expression; span; children!(
+        [lambda(()), label(l), expression(typ),
+                arrow(()), expression(body)] => {
+            spanned(span, Lam(l, rc(typ), rc(body)))
+        },
+        [if_(()), expression(cond), expression(left), expression(right)] => {
+            spanned(span, BoolIf(rc(cond), rc(left), rc(right)))
+        },
+        [let_binding(bindings).., in_(()), expression(final_expr)] => {
+            bindings.rev().fold(
+                final_expr,
+                |acc, x| Let(x.0, x.1, x.2, rc(acc))
+            )
+        },
+        [forall(()), label(l), expression(typ),
+                arrow(()), expression(body)] => {
+            spanned(span, Pi(l, rc(typ), rc(body)))
+        },
+        [expression(typ), arrow(()), expression(body)] => {
+            spanned(span, Pi("_".into(), rc(typ), rc(body)))
+        },
+        [merge(()), expression(x), expression(y), expression(z)] => {
+            spanned(span, Merge(rc(x), rc(y), Some(rc(z))))
+        },
+        [expression(e)] => e,
+    ));
+
+    rule!(let_binding<(Label, Option<ParsedSubExpr<'a>>, ParsedSubExpr<'a>)>;
+            children!(
+        [label(name), expression(annot), expression(expr)] =>
+            (name, Some(rc(annot)), rc(expr)),
+        [label(name), expression(expr)] =>
+            (name, None, rc(expr)),
+    ));
+
+    token_rule!(List<()>);
+    token_rule!(Optional<()>);
+
+    rule!(empty_collection<ParsedExpr<'a>> as expression; span; children!(
+        [List(_), expression(t)] => {
+            spanned(span, EmptyListLit(rc(t)))
+        },
+        [Optional(_), expression(t)] => {
+            spanned(span, OldOptionalLit(None, rc(t)))
+        },
+    ));
+
+    rule!(non_empty_optional<ParsedExpr<'a>> as expression; span; children!(
+        [expression(x), Optional(_), expression(t)] => {
+            spanned(span, OldOptionalLit(Some(rc(x)), rc(t)))
+        }
+    ));
+
+    rule!(import_alt_expression<ParsedExpr<'a>> as expression; children!(
+        [expression(e)] => e,
+        [expression(first), expression(rest)..] => {
+            let o = crate::BinOp::ImportAlt;
+            rest.fold(first, |acc, e| BinOp(o, rc(acc), rc(e)))
+        },
+    ));
+    rule!(or_expression<ParsedExpr<'a>> as expression; children!(
+        [expression(e)] => e,
+        [expression(first), expression(rest)..] => {
+            let o = crate::BinOp::BoolOr;
+            rest.fold(first, |acc, e| BinOp(o, rc(acc), rc(e)))
+        },
+    ));
+    rule!(plus_expression<ParsedExpr<'a>> as expression; children!(
+        [expression(e)] => e,
+        [expression(first), expression(rest)..] => {
+            let o = crate::BinOp::NaturalPlus;
+            rest.fold(first, |acc, e| BinOp(o, rc(acc), rc(e)))
+        },
+    ));
+    rule!(text_append_expression<ParsedExpr<'a>> as expression; children!(
+        [expression(e)] => e,
+        [expression(first), expression(rest)..] => {
+            let o = crate::BinOp::TextAppend;
+            rest.fold(first, |acc, e| BinOp(o, rc(acc), rc(e)))
+        },
+    ));
+    rule!(list_append_expression<ParsedExpr<'a>> as expression; children!(
+        [expression(e)] => e,
+        [expression(first), expression(rest)..] => {
+            let o = crate::BinOp::ListAppend;
+            rest.fold(first, |acc, e| BinOp(o, rc(acc), rc(e)))
+        },
+    ));
+    rule!(and_expression<ParsedExpr<'a>> as expression; children!(
+        [expression(e)] => e,
+        [expression(first), expression(rest)..] => {
+            let o = crate::BinOp::BoolAnd;
+            rest.fold(first, |acc, e| BinOp(o, rc(acc), rc(e)))
+        },
+    ));
+    rule!(combine_expression<ParsedExpr<'a>> as expression; children!(
+        [expression(e)] => e,
+        [expression(first), expression(rest)..] => {
+            let o = crate::BinOp::Combine;
+            rest.fold(first, |acc, e| BinOp(o, rc(acc), rc(e)))
+        },
+    ));
+    rule!(prefer_expression<ParsedExpr<'a>> as expression; children!(
+        [expression(e)] => e,
+        [expression(first), expression(rest)..] => {
+            let o = crate::BinOp::Prefer;
+            rest.fold(first, |acc, e| BinOp(o, rc(acc), rc(e)))
+        },
+    ));
+    rule!(combine_types_expression<ParsedExpr<'a>> as expression; children!(
+        [expression(e)] => e,
+        [expression(first), expression(rest)..] => {
+            let o = crate::BinOp::CombineTypes;
+            rest.fold(first, |acc, e| BinOp(o, rc(acc), rc(e)))
+        },
+    ));
+    rule!(times_expression<ParsedExpr<'a>> as expression; children!(
+        [expression(e)] => e,
+        [expression(first), expression(rest)..] => {
+            let o = crate::BinOp::NaturalTimes;
+            rest.fold(first, |acc, e| BinOp(o, rc(acc), rc(e)))
+        },
+    ));
+    rule!(equal_expression<ParsedExpr<'a>> as expression; children!(
+        [expression(e)] => e,
+        [expression(first), expression(rest)..] => {
+            let o = crate::BinOp::BoolEQ;
+            rest.fold(first, |acc, e| BinOp(o, rc(acc), rc(e)))
+        },
+    ));
+    rule!(not_equal_expression<ParsedExpr<'a>> as expression; children!(
+        [expression(e)] => e,
+        [expression(first), expression(rest)..] => {
+            let o = crate::BinOp::BoolNE;
+            rest.fold(first, |acc, e| BinOp(o, rc(acc), rc(e)))
+        },
+    ));
+
+    rule!(annotated_expression<ParsedExpr<'a>> as expression; span; children!(
+        [expression(e)] => e,
+        [expression(e), expression(annot)] => {
+            spanned(span, Annot(rc(e), rc(annot)))
+        },
+    ));
+
+    token_rule!(Some_<()>);
+
+    rule!(application_expression<ParsedExpr<'a>> as expression; children!(
+        [expression(e)] => e,
+        [expression(first), expression(rest)..] => {
+            rest.fold(first, |acc, e| App(rc(acc), rc(e)))
+        },
+    ));
+
+    rule!(first_application_expression<ParsedExpr<'a>> as expression; span;
+            children!(
+        [expression(e)] => e,
+        [Some_(()), expression(e)] => {
+            spanned(span, SomeLit(rc(e)))
+        },
+        [merge(()), expression(x), expression(y)] => {
+            spanned(span, Merge(rc(x), rc(y), None))
+        },
+    ));
+
+    rule!(selector_expression<ParsedExpr<'a>> as expression; children!(
+        [expression(e)] => e,
+        [expression(first), selector(rest)..] => {
+            rest.fold(first, |acc, e| match e {
+                Either::Left(l) => Field(rc(acc), l),
+                Either::Right(ls) => Projection(rc(acc), ls),
+            })
+        }
+    ));
+
+    rule!(selector<Either<Label, Vec<Label>>>; children!(
+        [label(l)] => Either::Left(l),
+        [labels(ls)] => Either::Right(ls),
+    ));
+
+    rule!(labels<Vec<Label>>; children!(
+        [label(ls)..] => ls.collect(),
+    ));
+
+    rule!(primitive_expression<ParsedExpr<'a>> as expression; span; children!(
+        [double_literal(n)] => spanned(span, DoubleLit(n)),
+        [natural_literal(n)] => spanned(span, NaturalLit(n)),
+        [integer_literal(n)] => spanned(span, IntegerLit(n)),
+        [double_quote_literal(s)] => spanned(span, TextLit(s)),
+        [single_quote_literal(s)] => spanned(span, TextLit(s)),
+        [expression(e)] => e,
+    ));
+
+    rule!(empty_record_literal<ParsedExpr<'a>> as expression; span;
+        captured_str!(_) => spanned(span, RecordLit(BTreeMap::new()))
+    );
+
+    rule!(empty_record_type<ParsedExpr<'a>> as expression; span;
+        captured_str!(_) => spanned(span, RecordType(BTreeMap::new()))
+    );
+
+    rule!(non_empty_record_type_or_literal<ParsedExpr<'a>> as expression; span;
+          children!(
+        [label(first_label), non_empty_record_type(rest)] => {
+            let (first_expr, mut map) = rest;
+            map.insert(first_label, rc(first_expr));
+            spanned(span, RecordType(map))
+        },
+        [label(first_label), non_empty_record_literal(rest)] => {
+            let (first_expr, mut map) = rest;
+            map.insert(first_label, rc(first_expr));
+            spanned(span, RecordLit(map))
+        },
+    ));
+
+    rule!(non_empty_record_type
+          <(ParsedExpr<'a>, BTreeMap<Label, ParsedSubExpr<'a>>)>; children!(
+        [expression(expr), record_type_entry(entries)..] => {
+            (expr, entries.collect())
+        }
+    ));
+
+    rule!(record_type_entry<(Label, ParsedSubExpr<'a>)>; children!(
+        [label(name), expression(expr)] => (name, rc(expr))
+    ));
+
+    rule!(non_empty_record_literal
+          <(ParsedExpr<'a>, BTreeMap<Label, ParsedSubExpr<'a>>)>; children!(
+        [expression(expr), record_literal_entry(entries)..] => {
+            (expr, entries.collect())
+        }
+    ));
+
+    rule!(record_literal_entry<(Label, ParsedSubExpr<'a>)>; children!(
+        [label(name), expression(expr)] => (name, rc(expr))
+    ));
+
+    rule!(union_type_or_literal<ParsedExpr<'a>> as expression; span; children!(
+        [empty_union_type(_)] => {
+            spanned(span, UnionType(BTreeMap::new()))
+        },
+        [non_empty_union_type_or_literal((Some((l, e)), entries))] => {
+            spanned(span, UnionLit(l, e, entries))
+        },
+        [non_empty_union_type_or_literal((None, entries))] => {
+            spanned(span, UnionType(entries))
+        },
+    ));
+
+    token_rule!(empty_union_type<()>);
+
+    rule!(non_empty_union_type_or_literal
+          <(Option<(Label, ParsedSubExpr<'a>)>,
+            BTreeMap<Label, Option<ParsedSubExpr<'a>>>)>;
+            children!(
+        [label(l), union_literal_variant_value((e, entries))] => {
+            (Some((l, e)), entries)
+        },
+        [label(l), union_type_or_literal_variant_type((e, rest))] => {
+            let (x, mut entries) = rest;
+            entries.insert(l, e);
+            (x, entries)
+        },
+    ));
+
+    rule!(union_literal_variant_value
+          <(ParsedSubExpr<'a>, BTreeMap<Label, Option<ParsedSubExpr<'a>>>)>;
+            children!(
+        [expression(e), union_type_entry(entries)..] => {
+            (rc(e), entries.collect())
+        },
+    ));
+
+    rule!(union_type_entry<(Label, Option<ParsedSubExpr<'a>>)>; children!(
+        [label(name), expression(expr)] => (name, Some(rc(expr))),
+        [label(name)] => (name, None),
+    ));
+
+    // TODO: unary union variants
+    rule!(union_type_or_literal_variant_type
+          <(Option<ParsedSubExpr<'a>>,
+            (Option<(Label, ParsedSubExpr<'a>)>,
+             BTreeMap<Label, Option<ParsedSubExpr<'a>>>))>;
+                children!(
+        [expression(e), non_empty_union_type_or_literal(rest)] => {
+            (Some(rc(e)), rest)
+        },
+        [expression(e)] => {
+            (Some(rc(e)), (None, BTreeMap::new()))
+        },
+        [non_empty_union_type_or_literal(rest)] => {
+            (None, rest)
+        },
+        [] => {
+            (None, (None, BTreeMap::new()))
+        },
+    ));
+
+    rule!(non_empty_list_literal<ParsedExpr<'a>> as expression; span;
+          children!(
+        [expression(items)..] => spanned(
+            span,
+            NEListLit(items.map(rc).collect())
+        )
+    ));
+
+    rule!(final_expression<ParsedExpr<'a>> as expression; children!(
+        [expression(e), EOI(_eoi)] => e
+    ));
+}
+
+pub fn parse_expr<'a>(s: &'a str) -> ParseResult<ParsedSubExpr<'a>> {
+    let mut pairs = DhallParser::parse(Rule::final_expression, s)?;
+    let expr = do_parse(pairs.next().unwrap())?;
+    assert_eq!(pairs.next(), None);
+    match expr {
+        ParsedValue::expression(e) => Ok(rc(e)),
+        _ => unreachable!(),
+    }
+    // Ok(rc(BoolLit(false)))
+}
+
+#[test]
+fn test_parse() {
+    // let expr = r#"{ x = "foo", y = 4 }.x"#;
+    // let expr = r#"(1 + 2) * 3"#;
+    let expr = r#"(1) + 3 * 5"#;
+    println!("{:?}", parse_expr(expr));
+    match parse_expr(expr) {
+        Err(e) => {
+            println!("{:?}", e);
+            println!("{}", e);
+        }
+        ok => println!("{:?}", ok),
+    };
+    // assert!(false);
+}
diff --git a/dhall_syntax/src/printer.rs b/dhall_syntax/src/printer.rs
new file mode 100644
index 0000000..704000a
--- /dev/null
+++ b/dhall_syntax/src/printer.rs
@@ -0,0 +1,498 @@
+use crate::*;
+use itertools::Itertools;
+use std::fmt::{self, Display};
+
+/// Generic instance that delegates to subexpressions
+impl<SE: Display + Clone, N, E: Display> Display for ExprF<SE, Label, N, E> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        use crate::ExprF::*;
+        match self {
+            Lam(a, b, c) => {
+                write!(f, "λ({} : {}) → {}", a, b, c)?;
+            }
+            BoolIf(a, b, c) => {
+                write!(f, "if {} then {} else {}", a, b, c)?;
+            }
+            Pi(a, b, c) if &String::from(a) == "_" => {
+                write!(f, "{} → {}", b, c)?;
+            }
+            Pi(a, b, c) => {
+                write!(f, "∀({} : {}) → {}", a, b, c)?;
+            }
+            Let(a, b, c, d) => {
+                write!(f, "let {}", a)?;
+                if let Some(b) = b {
+                    write!(f, " : {}", b)?;
+                }
+                write!(f, " = {} in {}", c, d)?;
+            }
+            EmptyListLit(t) => {
+                write!(f, "[] : List {}", t)?;
+            }
+            NEListLit(es) => {
+                fmt_list("[", ", ", "]", es, f, Display::fmt)?;
+            }
+            OldOptionalLit(None, t) => {
+                write!(f, "[] : Optional {}", t)?;
+            }
+            OldOptionalLit(Some(x), t) => {
+                write!(f, "[{}] : Optional {}", x, t)?;
+            }
+            SomeLit(e) => {
+                write!(f, "Some {}", e)?;
+            }
+            Merge(a, b, c) => {
+                write!(f, "merge {} {}", a, b)?;
+                if let Some(c) = c {
+                    write!(f, " : {}", c)?;
+                }
+            }
+            Annot(a, b) => {
+                write!(f, "{} : {}", a, b)?;
+            }
+            ExprF::BinOp(op, a, b) => {
+                write!(f, "{} {} {}", a, op, b)?;
+            }
+            ExprF::App(a, b) => {
+                write!(f, "{} {}", a, b)?;
+            }
+            Field(a, b) => {
+                write!(f, "{}.{}", a, b)?;
+            }
+            Projection(e, ls) => {
+                write!(f, "{}.", e)?;
+                fmt_list("{ ", ", ", " }", ls, f, Display::fmt)?;
+            }
+            Var(a) => a.fmt(f)?,
+            Const(k) => k.fmt(f)?,
+            Builtin(v) => v.fmt(f)?,
+            BoolLit(true) => f.write_str("True")?,
+            BoolLit(false) => f.write_str("False")?,
+            NaturalLit(a) => a.fmt(f)?,
+            IntegerLit(a) if *a >= 0 => {
+                f.write_str("+")?;
+                a.fmt(f)?;
+            }
+            IntegerLit(a) => a.fmt(f)?,
+            DoubleLit(a) => a.fmt(f)?,
+            TextLit(a) => a.fmt(f)?,
+            RecordType(a) if a.is_empty() => f.write_str("{}")?,
+            RecordType(a) => fmt_list("{ ", ", ", " }", a, f, |(k, t), f| {
+                write!(f, "{} : {}", k, t)
+            })?,
+            RecordLit(a) if a.is_empty() => f.write_str("{=}")?,
+            RecordLit(a) => fmt_list("{ ", ", ", " }", a, f, |(k, v), f| {
+                write!(f, "{} = {}", k, v)
+            })?,
+            UnionType(a) => fmt_list("< ", " | ", " >", a, f, |(k, v), f| {
+                write!(f, "{}", k)?;
+                if let Some(v) = v {
+                    write!(f, ": {}", v)?;
+                }
+                Ok(())
+            })?,
+            UnionLit(a, b, c) => {
+                write!(f, "< {} = {}", a, b)?;
+                for (k, v) in c {
+                    write!(f, " | {}", k)?;
+                    if let Some(v) = v {
+                        write!(f, ": {}", v)?;
+                    }
+                }
+                f.write_str(" >")?
+            }
+            Embed(a) => a.fmt(f)?,
+            Note(_, b) => b.fmt(f)?,
+        }
+        Ok(())
+    }
+}
+
+// There is a one-to-one correspondence between the formatter and the grammar. Each phase is
+// named after a corresponding grammar group, and the structure of the formatter reflects
+// the relationship between the corresponding grammar rules. This leads to the nice property
+// of automatically getting all the parentheses and precedences right.
+#[derive(Debug, Copy, Clone, Ord, PartialOrd, Eq, PartialEq)]
+enum PrintPhase {
+    Base,
+    Operator,
+    BinOp(core::BinOp),
+    App,
+    Import,
+    Primitive,
+}
+
+// Wraps an Expr with a phase, so that phase selsction can be done
+// separate from the actual printing
+#[derive(Clone)]
+struct PhasedExpr<'a, S, A>(&'a SubExpr<S, A>, PrintPhase);
+
+impl<'a, S: Clone, A: Display + Clone> Display for PhasedExpr<'a, S, A> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        self.0.as_ref().fmt_phase(f, self.1)
+    }
+}
+
+impl<'a, S: Clone, A: Display + Clone> PhasedExpr<'a, S, A> {
+    fn phase(self, phase: PrintPhase) -> PhasedExpr<'a, S, A> {
+        PhasedExpr(self.0, phase)
+    }
+}
+
+impl<S: Clone, A: Display + Clone> Expr<S, A> {
+    fn fmt_phase(
+        &self,
+        f: &mut fmt::Formatter,
+        mut phase: PrintPhase,
+    ) -> Result<(), fmt::Error> {
+        use crate::ExprF::*;
+        use PrintPhase::*;
+
+        let needs_paren = match self {
+            Lam(_, _, _)
+            | BoolIf(_, _, _)
+            | Pi(_, _, _)
+            | Let(_, _, _, _)
+            | EmptyListLit(_)
+            | NEListLit(_)
+            | OldOptionalLit(_, _)
+            | SomeLit(_)
+            | Merge(_, _, _)
+            | Annot(_, _)
+                if phase > Base =>
+            {
+                true
+            }
+            // Precedence is magically handled by the ordering of BinOps.
+            ExprF::BinOp(op, _, _) if phase > PrintPhase::BinOp(*op) => true,
+            ExprF::App(_, _) if phase > PrintPhase::App => true,
+            Field(_, _) | Projection(_, _) if phase > Import => true,
+            _ => false,
+        };
+
+        if needs_paren {
+            phase = Base;
+        }
+
+        // Annotate subexpressions with the appropriate phase, defaulting to Base
+        let phased_self = match self.map_ref_simple(|e| PhasedExpr(e, Base)) {
+            Pi(a, b, c) => {
+                if &String::from(&a) == "_" {
+                    Pi(a, b.phase(Operator), c)
+                } else {
+                    Pi(a, b, c)
+                }
+            }
+            Merge(a, b, c) => Merge(
+                a.phase(Import),
+                b.phase(Import),
+                c.map(|x| x.phase(PrintPhase::App)),
+            ),
+            Annot(a, b) => Annot(a.phase(Operator), b),
+            ExprF::BinOp(op, a, b) => ExprF::BinOp(
+                op,
+                a.phase(PrintPhase::BinOp(op)),
+                b.phase(PrintPhase::BinOp(op)),
+            ),
+            EmptyListLit(t) => EmptyListLit(t.phase(Import)),
+            OldOptionalLit(x, t) => OldOptionalLit(x, t.phase(Import)),
+            SomeLit(e) => SomeLit(e.phase(Import)),
+            ExprF::App(f, a) => ExprF::App(f.phase(Import), a.phase(Import)),
+            Field(a, b) => Field(a.phase(Primitive), b),
+            Projection(e, ls) => Projection(e.phase(Primitive), ls),
+            Note(n, b) => Note(n, b.phase(phase)),
+            e => e,
+        };
+
+        if needs_paren {
+            f.write_str("(")?;
+        }
+
+        // Uses the ExprF<PhasedExpr<_>, _, _, _> instance
+        phased_self.fmt(f)?;
+
+        if needs_paren {
+            f.write_str(")")?;
+        }
+        Ok(())
+    }
+}
+
+impl<S: Clone, A: Display + Clone> Display for SubExpr<S, A> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        self.as_ref().fmt_phase(f, PrintPhase::Base)
+    }
+}
+
+fn fmt_list<T, I, F>(
+    open: &str,
+    sep: &str,
+    close: &str,
+    it: I,
+    f: &mut fmt::Formatter,
+    func: F,
+) -> Result<(), fmt::Error>
+where
+    I: IntoIterator<Item = T>,
+    F: Fn(T, &mut fmt::Formatter) -> Result<(), fmt::Error>,
+{
+    f.write_str(open)?;
+    for (i, x) in it.into_iter().enumerate() {
+        if i > 0 {
+            f.write_str(sep)?;
+        }
+        func(x, f)?;
+    }
+    f.write_str(close)
+}
+
+impl<SubExpr: Display + Clone> Display for InterpolatedText<SubExpr> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        f.write_str("\"")?;
+        for x in self.iter() {
+            match x {
+                InterpolatedTextContents::Text(a) => {
+                    for c in a.chars() {
+                        match c {
+                            '\\' => f.write_str("\\\\"),
+                            '"' => f.write_str("\\\""),
+                            '$' => f.write_str("\\$"),
+                            '\u{0008}' => f.write_str("\\b"),
+                            '\u{000C}' => f.write_str("\\f"),
+                            '\n' => f.write_str("\\n"),
+                            '\r' => f.write_str("\\r"),
+                            '\t' => f.write_str("\\t"),
+                            c => write!(f, "{}", c),
+                        }?;
+                    }
+                }
+                InterpolatedTextContents::Expr(e) => {
+                    f.write_str("${ ")?;
+                    e.fmt(f)?;
+                    f.write_str(" }")?;
+                }
+            }
+        }
+        f.write_str("\"")?;
+        Ok(())
+    }
+}
+
+impl Display for Const {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        <Self as fmt::Debug>::fmt(self, f)
+    }
+}
+
+impl Display for BinOp {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        use crate::BinOp::*;
+        f.write_str(match self {
+            BoolOr => "||",
+            TextAppend => "++",
+            NaturalPlus => "+",
+            BoolAnd => "&&",
+            Combine => "/\\",
+            NaturalTimes => "*",
+            BoolEQ => "==",
+            BoolNE => "!=",
+            CombineTypes => "//\\\\",
+            ImportAlt => "?",
+            Prefer => "//",
+            ListAppend => "#",
+        })
+    }
+}
+
+impl Display for NaiveDouble {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        let v = f64::from(*self);
+        if v == std::f64::INFINITY {
+            f.write_str("Infinity")
+        } else if v == std::f64::NEG_INFINITY {
+            f.write_str("-Infinity")
+        } else if v.is_nan() {
+            f.write_str("NaN")
+        } else {
+            let s = format!("{}", v);
+            if s.contains('e') || s.contains('.') {
+                f.write_str(&s)
+            } else {
+                write!(f, "{}.0", s)
+            }
+        }
+    }
+}
+
+impl Display for Label {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        // TODO: distinguish between reserved and nonreserved locations for quoting builtins
+        let s = String::from(self);
+        let is_reserved = match s.as_str() {
+            "let" | "in" | "if" | "then" | "else" | "Type" | "Kind"
+            | "Sort" | "True" | "False" => true,
+            _ => crate::Builtin::parse(&s).is_some(),
+        };
+        if !is_reserved && s.chars().all(|c| c.is_ascii_alphanumeric()) {
+            write!(f, "{}", s)
+        } else {
+            write!(f, "`{}`", s)
+        }
+    }
+}
+
+impl Display for Hash {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        write!(f, "{}:{}", self.protocol, self.hash)
+    }
+}
+impl Display for ImportHashed {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        use std::path::PathBuf;
+        use FilePrefix::*;
+        use ImportLocation::*;
+        let quoted_path_component = |s: &str| -> String {
+            if s.chars().all(|c| c.is_ascii_alphanumeric()) {
+                s.to_owned()
+            } else {
+                format!("\"{}\"", s)
+            }
+        };
+        let fmt_path = |f: &mut fmt::Formatter, p: &PathBuf| {
+            let res: String = p
+                .iter()
+                .map(|c| quoted_path_component(c.to_string_lossy().as_ref()))
+                .join("/");
+            f.write_str(&res)
+        };
+
+        match &self.location {
+            Local(prefix, path) => {
+                let prefix = match prefix {
+                    Here => ".",
+                    Parent => "..",
+                    Home => "~",
+                    Absolute => "",
+                };
+                write!(f, "{}/", prefix)?;
+                fmt_path(f, path)?;
+            }
+            Remote(url) => {
+                write!(f, "{}://{}/", url.scheme, url.authority,)?;
+                fmt_path(f, &url.path)?;
+                if let Some(q) = &url.query {
+                    write!(f, "?{}", q)?
+                }
+                if let Some(h) = &url.headers {
+                    write!(f, " using ({})", h)?
+                }
+            }
+            Env(s) => {
+                write!(f, "env:")?;
+                if s.chars().all(|c| c.is_ascii_alphanumeric()) {
+                    write!(f, "{}", s)?;
+                } else {
+                    write!(f, "\"")?;
+                    for c in s.chars() {
+                        match c {
+                            '"' => f.write_str("\\\"")?,
+                            '\\' => f.write_str("\\\\")?,
+                            '\u{0007}' => f.write_str("\\a")?,
+                            '\u{0008}' => f.write_str("\\b")?,
+                            '\u{000C}' => f.write_str("\\f")?,
+                            '\n' => f.write_str("\\n")?,
+                            '\r' => f.write_str("\\r")?,
+                            '\t' => f.write_str("\\t")?,
+                            '\u{000B}' => f.write_str("\\v")?,
+                            _ => write!(f, "{}", c)?,
+                        }
+                    }
+                    write!(f, "\"")?;
+                }
+            }
+            Missing => {
+                write!(f, "missing")?;
+            }
+        }
+        if let Some(hash) = &self.hash {
+            write!(f, " ")?;
+            hash.fmt(f)?;
+        }
+        Ok(())
+    }
+}
+
+impl Display for Import {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        self.location_hashed.fmt(f)?;
+        use ImportMode::*;
+        match self.mode {
+            Code => {}
+            RawText => write!(f, " as Text")?,
+        }
+        Ok(())
+    }
+}
+
+impl Display for Builtin {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        use crate::Builtin::*;
+        f.write_str(match *self {
+            Bool => "Bool",
+            Natural => "Natural",
+            Integer => "Integer",
+            Double => "Double",
+            Text => "Text",
+            List => "List",
+            Optional => "Optional",
+            OptionalNone => "None",
+            NaturalBuild => "Natural/build",
+            NaturalFold => "Natural/fold",
+            NaturalIsZero => "Natural/isZero",
+            NaturalEven => "Natural/even",
+            NaturalOdd => "Natural/odd",
+            NaturalToInteger => "Natural/toInteger",
+            NaturalShow => "Natural/show",
+            IntegerToDouble => "Integer/toDouble",
+            IntegerShow => "Integer/show",
+            DoubleShow => "Double/show",
+            ListBuild => "List/build",
+            ListFold => "List/fold",
+            ListLength => "List/length",
+            ListHead => "List/head",
+            ListLast => "List/last",
+            ListIndexed => "List/indexed",
+            ListReverse => "List/reverse",
+            OptionalFold => "Optional/fold",
+            OptionalBuild => "Optional/build",
+            TextShow => "Text/show",
+        })
+    }
+}
+
+impl Display for Scheme {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        use crate::Scheme::*;
+        f.write_str(match *self {
+            HTTP => "http",
+            HTTPS => "https",
+        })
+    }
+}
+
+impl<Label: Display> Display for V<Label> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        let V(x, n) = self;
+        x.fmt(f)?;
+        if *n != 0 {
+            write!(f, "@{}", n)?;
+        }
+        Ok(())
+    }
+}
+
+impl Display for X {
+    fn fmt(&self, _: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        match *self {}
+    }
+}
diff --git a/dhall_syntax/src/text.rs b/dhall_syntax/src/text.rs
new file mode 100644
index 0000000..83643d9
--- /dev/null
+++ b/dhall_syntax/src/text.rs
@@ -0,0 +1,116 @@
+use std::iter::FromIterator;
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct InterpolatedText<SubExpr> {
+    head: String,
+    tail: Vec<(SubExpr, String)>,
+}
+
+impl<SubExpr> From<(String, Vec<(SubExpr, String)>)>
+    for InterpolatedText<SubExpr>
+{
+    fn from(x: (String, Vec<(SubExpr, String)>)) -> Self {
+        InterpolatedText {
+            head: x.0,
+            tail: x.1,
+        }
+    }
+}
+
+impl<SubExpr> From<String> for InterpolatedText<SubExpr> {
+    fn from(s: String) -> Self {
+        InterpolatedText {
+            head: s,
+            tail: vec![],
+        }
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum InterpolatedTextContents<SubExpr> {
+    Text(String),
+    Expr(SubExpr),
+}
+
+impl<SubExpr> InterpolatedTextContents<SubExpr> {
+    pub fn is_empty(&self) -> bool {
+        use InterpolatedTextContents::{Expr, Text};
+        match self {
+            Expr(_) => false,
+            Text(s) => s.is_empty(),
+        }
+    }
+}
+
+impl<SubExpr> InterpolatedText<SubExpr> {
+    pub fn traverse_ref<'a, SubExpr2, E, F>(
+        &'a self,
+        mut f: F,
+    ) -> Result<InterpolatedText<SubExpr2>, E>
+    where
+        F: FnMut(&'a SubExpr) -> Result<SubExpr2, E>,
+    {
+        Ok(InterpolatedText {
+            head: self.head.clone(),
+            tail: self
+                .tail
+                .iter()
+                .map(|(e, s)| Ok((f(e)?, s.clone())))
+                .collect::<Result<_, _>>()?,
+        })
+    }
+
+    pub fn iter<'a>(
+        &'a self,
+    ) -> impl Iterator<Item = InterpolatedTextContents<SubExpr>> + 'a
+    where
+        SubExpr: Clone,
+    {
+        use std::iter::once;
+        use InterpolatedTextContents::{Expr, Text};
+        once(Text(self.head.clone()))
+            .chain(self.tail.iter().flat_map(|(e, s)| {
+                once(Expr(SubExpr::clone(e))).chain(once(Text(s.clone())))
+            }))
+            .filter(|c| !c.is_empty())
+    }
+
+    pub fn into_iter(
+        self,
+    ) -> impl Iterator<Item = InterpolatedTextContents<SubExpr>> {
+        use std::iter::once;
+        use InterpolatedTextContents::{Expr, Text};
+        once(Text(self.head))
+            .chain(
+                self.tail
+                    .into_iter()
+                    .flat_map(|(e, s)| once(Expr(e)).chain(once(Text(s)))),
+            )
+            .filter(|c| !c.is_empty())
+    }
+}
+
+impl<SubExpr> FromIterator<InterpolatedTextContents<SubExpr>>
+    for InterpolatedText<SubExpr>
+{
+    fn from_iter<T>(iter: T) -> Self
+    where
+        T: IntoIterator<Item = InterpolatedTextContents<SubExpr>>,
+    {
+        let mut res = InterpolatedText {
+            head: String::new(),
+            tail: Vec::new(),
+        };
+        let mut crnt_str = &mut res.head;
+        for x in iter.into_iter() {
+            match x {
+                InterpolatedTextContents::Text(s) => crnt_str.push_str(&s),
+                InterpolatedTextContents::Expr(e) => {
+                    res.tail.push((e, String::new()));
+                    crnt_str = &mut res.tail.last_mut().unwrap().1;
+                }
+            }
+        }
+        res
+    }
+}
diff --git a/dhall_syntax/src/visitor.rs b/dhall_syntax/src/visitor.rs
new file mode 100644
index 0000000..caaefce
--- /dev/null
+++ b/dhall_syntax/src/visitor.rs
@@ -0,0 +1,667 @@
+use std::collections::BTreeMap;
+
+use crate::*;
+
+/// A way too generic Visitor trait.
+pub trait GenericVisitor<Input, Return>: Sized {
+    fn visit(self, input: Input) -> Return;
+}
+
+/// A visitor trait that can be used to traverse `ExprF`s. We need this pattern
+/// so that Rust lets us have as much mutability as we can.
+/// For example, `traverse_embed` cannot be made using only `traverse_ref`, because
+/// `traverse_ref` takes a `FnMut` so we would need to pass multiple mutable
+/// reverences to this argument to `traverse_ref`. But Rust's ownership system
+/// is all about preventing exactly this ! So we have to be more clever.
+/// The visitor pattern allows us to have only one mutable thing the whole
+/// time: the visitor itself. The visitor can then carry around multiple closures
+/// or just one, and Rust is ok with either. See for example TraverseRefVisitor
+/// and TraverseEmbedVisitor.
+/// This is very generic. For a more legible trait, see ExprFInFallibleVisitor
+pub trait ExprFVeryGenericVisitor<'a, Ret, SE1, L1, N1, E1>: Sized {
+    type Error;
+    type SE2;
+    type L2;
+    type N2;
+    type E2;
+
+    fn visit_subexpr(
+        &mut self,
+        subexpr: &'a SE1,
+    ) -> Result<Self::SE2, Self::Error>;
+    fn visit_label(&mut self, label: &'a L1) -> Result<Self::L2, Self::Error>;
+
+    fn visit_binder(
+        self,
+        label: &'a L1,
+        subexpr: &'a SE1,
+    ) -> Result<(Self::L2, Self::SE2), Self::Error>;
+
+    fn visit_embed_squash(self, embed: &'a E1) -> Result<Ret, Self::Error>;
+
+    fn visit_note_squash(
+        self,
+        note: &'a N1,
+        subexpr: &'a SE1,
+    ) -> Result<Ret, Self::Error>;
+
+    // Called with the result of the map, in the non-embed/note case.
+    // Useful to change the result type, and/or avoid some loss of info
+    fn visit_resulting_exprf(
+        result: ExprF<Self::SE2, Self::L2, Self::N2, Self::E2>,
+    ) -> Result<Ret, Self::Error>;
+}
+
+impl<'a, T, Ret, SE1, L1, N1, E1>
+    GenericVisitor<&'a ExprF<SE1, L1, N1, E1>, Result<Ret, T::Error>> for T
+where
+    L1: Ord,
+    T::L2: Ord,
+    T: ExprFVeryGenericVisitor<'a, Ret, SE1, L1, N1, E1>,
+{
+    fn visit(self, input: &'a ExprF<SE1, L1, N1, E1>) -> Result<Ret, T::Error> {
+        fn vec<'a, T, U, Err, F: FnMut(&'a T) -> Result<U, Err>>(
+            x: &'a [T],
+            f: F,
+        ) -> Result<Vec<U>, Err> {
+            x.iter().map(f).collect()
+        }
+        fn opt<'a, T, U, Err, F: FnOnce(&'a T) -> Result<U, Err>>(
+            x: &'a Option<T>,
+            f: F,
+        ) -> Result<Option<U>, Err> {
+            Ok(match x {
+                Some(x) => Some(f(x)?),
+                None => None,
+            })
+        }
+        fn btmap<'a, V, Ret, SE, L, N, E>(
+            x: &'a BTreeMap<L, SE>,
+            mut v: V,
+        ) -> Result<BTreeMap<V::L2, V::SE2>, V::Error>
+        where
+            L: Ord,
+            V::L2: Ord,
+            V: ExprFVeryGenericVisitor<'a, Ret, SE, L, N, E>,
+        {
+            x.iter()
+                .map(|(k, x)| Ok((v.visit_label(k)?, v.visit_subexpr(x)?)))
+                .collect()
+        }
+        fn btoptmap<'a, V, Ret, SE, L, N, E>(
+            x: &'a BTreeMap<L, Option<SE>>,
+            mut v: V,
+        ) -> Result<BTreeMap<V::L2, Option<V::SE2>>, V::Error>
+        where
+            L: Ord,
+            V::L2: Ord,
+            V: ExprFVeryGenericVisitor<'a, Ret, SE, L, N, E>,
+        {
+            x.iter()
+                .map(|(k, x)| {
+                    Ok((
+                        v.visit_label(k)?,
+                        match x {
+                            Some(x) => Some(v.visit_subexpr(x)?),
+                            None => None,
+                        },
+                    ))
+                })
+                .collect()
+        }
+
+        let mut v = self;
+        use crate::ExprF::*;
+        T::visit_resulting_exprf(match input {
+            Var(V(l, n)) => Var(V(v.visit_label(l)?, *n)),
+            Lam(l, t, e) => {
+                let t = v.visit_subexpr(t)?;
+                let (l, e) = v.visit_binder(l, e)?;
+                Lam(l, t, e)
+            }
+            Pi(l, t, e) => {
+                let t = v.visit_subexpr(t)?;
+                let (l, e) = v.visit_binder(l, e)?;
+                Pi(l, t, e)
+            }
+            Let(l, t, a, e) => {
+                let t = opt(t, &mut |e| v.visit_subexpr(e))?;
+                let a = v.visit_subexpr(a)?;
+                let (l, e) = v.visit_binder(l, e)?;
+                Let(l, t, a, e)
+            }
+            App(f, a) => App(v.visit_subexpr(f)?, v.visit_subexpr(a)?),
+            Annot(x, t) => Annot(v.visit_subexpr(x)?, v.visit_subexpr(t)?),
+            Const(k) => Const(*k),
+            Builtin(v) => Builtin(*v),
+            BoolLit(b) => BoolLit(*b),
+            NaturalLit(n) => NaturalLit(*n),
+            IntegerLit(n) => IntegerLit(*n),
+            DoubleLit(n) => DoubleLit(*n),
+            TextLit(t) => TextLit(t.traverse_ref(|e| v.visit_subexpr(e))?),
+            BinOp(o, x, y) => {
+                BinOp(*o, v.visit_subexpr(x)?, v.visit_subexpr(y)?)
+            }
+            BoolIf(b, t, f) => BoolIf(
+                v.visit_subexpr(b)?,
+                v.visit_subexpr(t)?,
+                v.visit_subexpr(f)?,
+            ),
+            EmptyListLit(t) => EmptyListLit(v.visit_subexpr(t)?),
+            NEListLit(es) => NEListLit(vec(es, |e| v.visit_subexpr(e))?),
+            OldOptionalLit(x, t) => OldOptionalLit(
+                opt(x, |e| v.visit_subexpr(e))?,
+                v.visit_subexpr(t)?,
+            ),
+            SomeLit(e) => SomeLit(v.visit_subexpr(e)?),
+            RecordType(kts) => RecordType(btmap(kts, v)?),
+            RecordLit(kvs) => RecordLit(btmap(kvs, v)?),
+            UnionType(kts) => UnionType(btoptmap(kts, v)?),
+            UnionLit(k, x, kts) => UnionLit(
+                v.visit_label(k)?,
+                v.visit_subexpr(x)?,
+                btoptmap(kts, v)?,
+            ),
+            Merge(x, y, t) => Merge(
+                v.visit_subexpr(x)?,
+                v.visit_subexpr(y)?,
+                opt(t, |e| v.visit_subexpr(e))?,
+            ),
+            Field(e, l) => Field(v.visit_subexpr(e)?, v.visit_label(l)?),
+            Projection(e, ls) => {
+                Projection(v.visit_subexpr(e)?, vec(ls, |l| v.visit_label(l))?)
+            }
+            Note(n, e) => return v.visit_note_squash(n, e),
+            Embed(a) => return v.visit_embed_squash(a),
+        })
+    }
+}
+
+/// Like ExprFVeryGenericVisitor, but sets the return
+/// type to ExprF<_>
+pub trait ExprFFallibleVisitor<'a, SE1, SE2, L1, L2, N1, N2, E1, E2>:
+    Sized
+{
+    type Error;
+
+    fn visit_subexpr(&mut self, subexpr: &'a SE1) -> Result<SE2, Self::Error>;
+    fn visit_label(&mut self, label: &'a L1) -> Result<L2, Self::Error>;
+    fn visit_note(self, note: &'a N1) -> Result<N2, Self::Error>;
+    fn visit_embed(self, embed: &'a E1) -> Result<E2, Self::Error>;
+
+    fn visit_subexpr_under_binder(
+        mut self,
+        _label: &'a L1,
+        subexpr: &'a SE1,
+    ) -> Result<SE2, Self::Error> {
+        self.visit_subexpr(subexpr)
+    }
+
+    fn visit_binder(
+        mut self,
+        label: &'a L1,
+        subexpr: &'a SE1,
+    ) -> Result<(L2, SE2), Self::Error> {
+        Ok((
+            self.visit_label(label)?,
+            self.visit_subexpr_under_binder(label, subexpr)?,
+        ))
+    }
+
+    fn visit_embed_squash(
+        self,
+        embed: &'a E1,
+    ) -> Result<ExprF<SE2, L2, N2, E2>, Self::Error> {
+        Ok(ExprF::Embed(self.visit_embed(embed)?))
+    }
+
+    fn visit_note_squash(
+        mut self,
+        note: &'a N1,
+        subexpr: &'a SE1,
+    ) -> Result<ExprF<SE2, L2, N2, E2>, Self::Error> {
+        let subexpr = self.visit_subexpr(subexpr)?;
+        let note = self.visit_note(note)?;
+        Ok(ExprF::Note(note, subexpr))
+    }
+}
+
+impl<'a, T, SE1, SE2, L1, L2, N1, N2, E1, E2>
+    ExprFVeryGenericVisitor<'a, ExprF<SE2, L2, N2, E2>, SE1, L1, N1, E1> for T
+where
+    T: ExprFFallibleVisitor<'a, SE1, SE2, L1, L2, N1, N2, E1, E2>,
+{
+    type Error = T::Error;
+    type SE2 = SE2;
+    type L2 = L2;
+    type N2 = N2;
+    type E2 = E2;
+
+    fn visit_subexpr(
+        &mut self,
+        subexpr: &'a SE1,
+    ) -> Result<Self::SE2, Self::Error> {
+        self.visit_subexpr(subexpr)
+    }
+
+    fn visit_label(&mut self, label: &'a L1) -> Result<Self::L2, Self::Error> {
+        self.visit_label(label)
+    }
+
+    fn visit_binder(
+        self,
+        label: &'a L1,
+        subexpr: &'a SE1,
+    ) -> Result<(Self::L2, Self::SE2), Self::Error> {
+        self.visit_binder(label, subexpr)
+    }
+
+    fn visit_embed_squash(
+        self,
+        embed: &'a E1,
+    ) -> Result<ExprF<SE2, L2, N2, E2>, Self::Error> {
+        self.visit_embed_squash(embed)
+    }
+
+    fn visit_note_squash(
+        self,
+        note: &'a N1,
+        subexpr: &'a SE1,
+    ) -> Result<ExprF<SE2, L2, N2, E2>, Self::Error> {
+        self.visit_note_squash(note, subexpr)
+    }
+
+    // Called with the result of the map, in the non-embed/note case.
+    // Useful to change the result type, and/or avoid some loss of info
+    fn visit_resulting_exprf(
+        result: ExprF<Self::SE2, Self::L2, Self::N2, Self::E2>,
+    ) -> Result<ExprF<SE2, L2, N2, E2>, Self::Error> {
+        Ok(result)
+    }
+}
+
+/// Like ExprFFallibleVisitor, but without the error handling.
+pub trait ExprFInFallibleVisitor<'a, SE1, SE2, L1, L2, N1, N2, E1, E2>:
+    Sized
+{
+    fn visit_subexpr(&mut self, subexpr: &'a SE1) -> SE2;
+    fn visit_label(&mut self, label: &'a L1) -> L2;
+    fn visit_note(self, note: &'a N1) -> N2;
+    fn visit_embed(self, embed: &'a E1) -> E2;
+
+    fn visit_subexpr_under_binder(
+        mut self,
+        _label: &'a L1,
+        subexpr: &'a SE1,
+    ) -> SE2 {
+        self.visit_subexpr(subexpr)
+    }
+
+    fn visit_binder(mut self, label: &'a L1, subexpr: &'a SE1) -> (L2, SE2) {
+        (
+            self.visit_label(label),
+            self.visit_subexpr_under_binder(label, subexpr),
+        )
+    }
+
+    fn visit_embed_squash(self, embed: &'a E1) -> ExprF<SE2, L2, N2, E2> {
+        ExprF::Embed(self.visit_embed(embed))
+    }
+
+    fn visit_note_squash(
+        mut self,
+        note: &'a N1,
+        subexpr: &'a SE1,
+    ) -> ExprF<SE2, L2, N2, E2> {
+        let subexpr = self.visit_subexpr(subexpr);
+        let note = self.visit_note(note);
+        ExprF::Note(note, subexpr)
+    }
+}
+
+struct InfallibleWrapper<T>(T);
+
+impl<'a, T, SE1, SE2, L1, L2, N1, N2, E1, E2>
+    ExprFFallibleVisitor<'a, SE1, SE2, L1, L2, N1, N2, E1, E2>
+    for InfallibleWrapper<T>
+where
+    T: ExprFInFallibleVisitor<'a, SE1, SE2, L1, L2, N1, N2, E1, E2>,
+{
+    type Error = X;
+
+    fn visit_subexpr(&mut self, subexpr: &'a SE1) -> Result<SE2, Self::Error> {
+        Ok(self.0.visit_subexpr(subexpr))
+    }
+    fn visit_label(&mut self, label: &'a L1) -> Result<L2, Self::Error> {
+        Ok(self.0.visit_label(label))
+    }
+    fn visit_note(self, note: &'a N1) -> Result<N2, Self::Error> {
+        Ok(self.0.visit_note(note))
+    }
+    fn visit_embed(self, embed: &'a E1) -> Result<E2, Self::Error> {
+        Ok(self.0.visit_embed(embed))
+    }
+
+    fn visit_binder(
+        self,
+        label: &'a L1,
+        subexpr: &'a SE1,
+    ) -> Result<(L2, SE2), Self::Error> {
+        Ok(self.0.visit_binder(label, subexpr))
+    }
+
+    fn visit_embed_squash(
+        self,
+        embed: &'a E1,
+    ) -> Result<ExprF<SE2, L2, N2, E2>, Self::Error> {
+        Ok(self.0.visit_embed_squash(embed))
+    }
+
+    fn visit_note_squash(
+        self,
+        note: &'a N1,
+        subexpr: &'a SE1,
+    ) -> Result<ExprF<SE2, L2, N2, E2>, Self::Error> {
+        Ok(self.0.visit_note_squash(note, subexpr))
+    }
+}
+
+impl<'a, T, SE1, SE2, L1, L2, N1, N2, E1, E2>
+    GenericVisitor<&'a ExprF<SE1, L1, N1, E1>, ExprF<SE2, L2, N2, E2>> for T
+where
+    L1: Ord,
+    L2: Ord,
+    T: ExprFInFallibleVisitor<'a, SE1, SE2, L1, L2, N1, N2, E1, E2>,
+{
+    fn visit(
+        self,
+        input: &'a ExprF<SE1, L1, N1, E1>,
+    ) -> ExprF<SE2, L2, N2, E2> {
+        trivial_result(InfallibleWrapper(self).visit(input))
+    }
+}
+
+pub struct TraverseRefWithBindersVisitor<F1, F2, F3, F4, F5> {
+    pub visit_subexpr: F1,
+    pub visit_under_binder: F2,
+    pub visit_note: F3,
+    pub visit_embed: F4,
+    pub visit_label: F5,
+}
+
+impl<'a, SE, L, N, E, SE2, L2, N2, E2, Err, F1, F2, F3, F4, F5>
+    ExprFFallibleVisitor<'a, SE, SE2, L, L2, N, N2, E, E2>
+    for TraverseRefWithBindersVisitor<F1, F2, F3, F4, F5>
+where
+    SE: 'a,
+    L: 'a,
+    N: 'a,
+    E: 'a,
+    L: Ord,
+    L2: Ord,
+    F1: FnMut(&'a SE) -> Result<SE2, Err>,
+    F2: FnOnce(&'a L, &'a SE) -> Result<SE2, Err>,
+    F3: FnOnce(&'a N) -> Result<N2, Err>,
+    F4: FnOnce(&'a E) -> Result<E2, Err>,
+    F5: FnMut(&'a L) -> Result<L2, Err>,
+{
+    type Error = Err;
+
+    fn visit_subexpr(&mut self, subexpr: &'a SE) -> Result<SE2, Self::Error> {
+        (self.visit_subexpr)(subexpr)
+    }
+    fn visit_subexpr_under_binder(
+        self,
+        label: &'a L,
+        subexpr: &'a SE,
+    ) -> Result<SE2, Self::Error> {
+        (self.visit_under_binder)(label, subexpr)
+    }
+    fn visit_note(self, note: &'a N) -> Result<N2, Self::Error> {
+        (self.visit_note)(note)
+    }
+    fn visit_embed(self, embed: &'a E) -> Result<E2, Self::Error> {
+        (self.visit_embed)(embed)
+    }
+    fn visit_label(&mut self, label: &'a L) -> Result<L2, Self::Error> {
+        (self.visit_label)(label)
+    }
+}
+
+pub struct TraverseRefVisitor<F1, F2, F3, F4> {
+    pub visit_subexpr: F1,
+    pub visit_note: F2,
+    pub visit_embed: F3,
+    pub visit_label: F4,
+}
+
+impl<'a, SE, L, N, E, SE2, L2, N2, E2, Err, F1, F2, F3, F4>
+    ExprFFallibleVisitor<'a, SE, SE2, L, L2, N, N2, E, E2>
+    for TraverseRefVisitor<F1, F2, F3, F4>
+where
+    SE: 'a,
+    L: 'a,
+    N: 'a,
+    E: 'a,
+    L: Ord,
+    L2: Ord,
+    F1: FnMut(&'a SE) -> Result<SE2, Err>,
+    F2: FnOnce(&'a N) -> Result<N2, Err>,
+    F3: FnOnce(&'a E) -> Result<E2, Err>,
+    F4: FnMut(&'a L) -> Result<L2, Err>,
+{
+    type Error = Err;
+
+    fn visit_subexpr(&mut self, subexpr: &'a SE) -> Result<SE2, Self::Error> {
+        (self.visit_subexpr)(subexpr)
+    }
+    fn visit_note(self, note: &'a N) -> Result<N2, Self::Error> {
+        (self.visit_note)(note)
+    }
+    fn visit_embed(self, embed: &'a E) -> Result<E2, Self::Error> {
+        (self.visit_embed)(embed)
+    }
+    fn visit_label(&mut self, label: &'a L) -> Result<L2, Self::Error> {
+        (self.visit_label)(label)
+    }
+}
+
+pub struct TraverseEmbedVisitor<F1>(pub F1);
+
+impl<'a, 'b, N, E, E2, Err, F1>
+    ExprFFallibleVisitor<
+        'a,
+        SubExpr<N, E>,
+        SubExpr<N, E2>,
+        Label,
+        Label,
+        N,
+        N,
+        E,
+        E2,
+    > for &'b mut TraverseEmbedVisitor<F1>
+where
+    N: Clone + 'a,
+    F1: FnMut(&E) -> Result<E2, Err>,
+{
+    type Error = Err;
+
+    fn visit_subexpr(
+        &mut self,
+        subexpr: &'a SubExpr<N, E>,
+    ) -> Result<SubExpr<N, E2>, Self::Error> {
+        Ok(rc(subexpr.as_ref().visit(&mut **self)?))
+    }
+    fn visit_note(self, note: &'a N) -> Result<N, Self::Error> {
+        Ok(N::clone(note))
+    }
+    fn visit_embed(self, embed: &'a E) -> Result<E2, Self::Error> {
+        (self.0)(embed)
+    }
+    fn visit_label(&mut self, label: &'a Label) -> Result<Label, Self::Error> {
+        Ok(Label::clone(label))
+    }
+}
+
+pub struct SquashEmbedVisitor<F1>(pub F1);
+
+impl<'a, 'b, N, E1, E2, F1>
+    ExprFVeryGenericVisitor<'a, SubExpr<N, E2>, SubExpr<N, E1>, Label, N, E1>
+    for &'b mut SquashEmbedVisitor<F1>
+where
+    N: Clone + 'a,
+    F1: FnMut(&E1) -> SubExpr<N, E2>,
+{
+    type Error = X;
+    type SE2 = SubExpr<N, E2>;
+    type L2 = Label;
+    type N2 = N;
+    type E2 = E2;
+
+    fn visit_subexpr(
+        &mut self,
+        subexpr: &'a SubExpr<N, E1>,
+    ) -> Result<Self::SE2, Self::Error> {
+        Ok(subexpr.as_ref().visit(&mut **self)?)
+    }
+
+    fn visit_label(
+        &mut self,
+        label: &'a Label,
+    ) -> Result<Self::L2, Self::Error> {
+        Ok(Label::clone(label))
+    }
+
+    fn visit_binder(
+        mut self,
+        label: &'a Label,
+        subexpr: &'a SubExpr<N, E1>,
+    ) -> Result<(Self::L2, Self::SE2), Self::Error> {
+        Ok((self.visit_label(label)?, self.visit_subexpr(subexpr)?))
+    }
+
+    fn visit_embed_squash(
+        self,
+        embed: &'a E1,
+    ) -> Result<SubExpr<N, E2>, Self::Error> {
+        Ok((self.0)(embed))
+    }
+
+    fn visit_note_squash(
+        mut self,
+        note: &'a N,
+        subexpr: &'a SubExpr<N, E1>,
+    ) -> Result<SubExpr<N, E2>, Self::Error> {
+        let subexpr = self.visit_subexpr(subexpr)?;
+        let note = N::clone(note);
+        Ok(rc(ExprF::Note(note, subexpr)))
+    }
+
+    // Called with the result of the map, in the non-embed/note case.
+    // Useful to change the result type, and/or avoid some loss of info
+    fn visit_resulting_exprf(
+        result: ExprF<Self::SE2, Self::L2, Self::N2, Self::E2>,
+    ) -> Result<SubExpr<N, E2>, Self::Error> {
+        Ok(rc(result))
+    }
+}
+
+pub struct UnNoteVisitor;
+
+impl<'a, 'b, N, E>
+    ExprFInFallibleVisitor<
+        'a,
+        SubExpr<N, E>,
+        SubExpr<X, E>,
+        Label,
+        Label,
+        N,
+        X,
+        E,
+        E,
+    > for &'b mut UnNoteVisitor
+where
+    E: Clone + 'a,
+{
+    fn visit_subexpr(&mut self, subexpr: &'a SubExpr<N, E>) -> SubExpr<X, E> {
+        rc(subexpr.as_ref().visit(&mut **self))
+    }
+    fn visit_note(self, _: &'a N) -> X {
+        unreachable!()
+    }
+    fn visit_note_squash(
+        self,
+        _: &'a N,
+        subexpr: &'a SubExpr<N, E>,
+    ) -> Expr<X, E> {
+        subexpr.as_ref().visit(self)
+    }
+    fn visit_embed(self, embed: &'a E) -> E {
+        E::clone(embed)
+    }
+    fn visit_label(&mut self, label: &'a Label) -> Label {
+        Label::clone(label)
+    }
+}
+
+pub struct NoteAbsurdVisitor;
+
+impl<'a, 'b, N, E>
+    ExprFInFallibleVisitor<
+        'a,
+        SubExpr<X, E>,
+        SubExpr<N, E>,
+        Label,
+        Label,
+        X,
+        N,
+        E,
+        E,
+    > for &'b mut NoteAbsurdVisitor
+where
+    E: Clone + 'a,
+{
+    fn visit_subexpr(&mut self, subexpr: &'a SubExpr<X, E>) -> SubExpr<N, E> {
+        rc(subexpr.as_ref().visit(&mut **self))
+    }
+    fn visit_note(self, note: &'a X) -> N {
+        match *note {}
+    }
+    fn visit_embed(self, embed: &'a E) -> E {
+        E::clone(embed)
+    }
+    fn visit_label(&mut self, label: &'a Label) -> Label {
+        Label::clone(label)
+    }
+}
+
+pub struct EmbedAbsurdVisitor;
+
+impl<'a, 'b, N, E>
+    ExprFInFallibleVisitor<
+        'a,
+        SubExpr<N, X>,
+        SubExpr<N, E>,
+        Label,
+        Label,
+        N,
+        N,
+        X,
+        E,
+    > for &'b mut EmbedAbsurdVisitor
+where
+    N: Clone + 'a,
+{
+    fn visit_subexpr(&mut self, subexpr: &'a SubExpr<N, X>) -> SubExpr<N, E> {
+        rc(subexpr.as_ref().visit(&mut **self))
+    }
+    fn visit_note(self, note: &'a N) -> N {
+        N::clone(note)
+    }
+    fn visit_embed(self, embed: &'a X) -> E {
+        match *embed {}
+    }
+    fn visit_label(&mut self, label: &'a Label) -> Label {
+        Label::clone(label)
+    }
+}