Reorganize dhall_syntax

7 files changed, 1226 insertions, 0 deletions

diff --git a/dhall_syntax/src/core/context.rs b/dhall_syntax/src/core/context.rs
new file mode 100644
index 0000000..55bfff5
--- /dev/null
+++ b/dhall_syntax/src/core/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/expr.rs b/dhall_syntax/src/core/expr.rs
new file mode 100644
index 0000000..2b76b9a
--- /dev/null
+++ b/dhall_syntax/src/core/expr.rs
@@ -0,0 +1,464 @@
+#![allow(non_snake_case)]
+use std::collections::BTreeMap;
+use std::rc::Rc;
+
+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;
+
+// Each node carries an annotation. In practice it's either X (no annotation) or a Span.
+#[derive(Debug)]
+pub struct SubExpr<Note, Embed>(Rc<(Expr<Note, Embed>, Option<Note>)>);
+
+impl<Note, Embed: PartialEq> std::cmp::PartialEq for SubExpr<Note, Embed> {
+    fn eq(&self, other: &Self) -> bool {
+        self.0.as_ref().0 == other.0.as_ref().0
+    }
+}
+
+impl<Note, Embed: Eq> std::cmp::Eq for SubExpr<Note, Embed> {}
+
+pub type Expr<Note, Embed> = ExprF<SubExpr<Note, Embed>, Label, 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, 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>),
+    /// Embeds an import or the result of resolving the import
+    Embed(Embed),
+}
+
+impl<SE, L, E> ExprF<SE, L, E> {
+    pub(crate) fn visit<'a, V, Return>(&'a self, v: V) -> Return
+    where
+        V: visitor::GenericVisitor<&'a ExprF<SE, L, E>, Return>,
+    {
+        v.visit(self)
+    }
+
+    fn traverse_ref_with_special_handling_of_binders<'a, SE2, L2, 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_embed: impl FnOnce(&'a E) -> Result<E2, Err>,
+        visit_label: impl FnMut(&'a L) -> Result<L2, Err>,
+    ) -> Result<ExprF<SE2, L2, E2>, Err>
+    where
+        L: Ord,
+        L2: Ord,
+    {
+        self.visit(visitor::TraverseRefWithBindersVisitor {
+            visit_subexpr,
+            visit_under_binder,
+            visit_embed,
+            visit_label,
+        })
+    }
+
+    fn traverse_ref<'a, SE2, L2, E2, Err>(
+        &'a self,
+        visit_subexpr: impl FnMut(&'a SE) -> Result<SE2, Err>,
+        visit_embed: impl FnOnce(&'a E) -> Result<E2, Err>,
+        visit_label: impl FnMut(&'a L) -> Result<L2, Err>,
+    ) -> Result<ExprF<SE2, L2, E2>, Err>
+    where
+        L: Ord,
+        L2: Ord,
+    {
+        self.visit(visitor::TraverseRefVisitor {
+            visit_subexpr,
+            visit_embed,
+            visit_label,
+        })
+    }
+
+    pub fn map_ref_with_special_handling_of_binders<'a, SE2, L2, E2>(
+        &'a self,
+        mut map_subexpr: impl FnMut(&'a SE) -> SE2,
+        mut map_under_binder: impl FnMut(&'a L, &'a SE) -> SE2,
+        map_embed: impl FnOnce(&'a E) -> E2,
+        mut map_label: impl FnMut(&'a L) -> L2,
+    ) -> ExprF<SE2, L2, 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_embed(x)),
+            |x| Ok(map_label(x)),
+        ))
+    }
+
+    pub fn map_ref<'a, SE2, L2, E2>(
+        &'a self,
+        mut map_subexpr: impl FnMut(&'a SE) -> SE2,
+        map_embed: impl FnOnce(&'a E) -> E2,
+        mut map_label: impl FnMut(&'a L) -> L2,
+    ) -> ExprF<SE2, L2, E2>
+    where
+        L: Ord,
+        L2: Ord,
+    {
+        trivial_result(self.traverse_ref(
+            |x| Ok(map_subexpr(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, E>, Err>
+    where
+        L: Ord + Clone,
+        E: Clone,
+    {
+        self.traverse_ref(
+            visit_subexpr,
+            |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, E>
+    where
+        L: Ord + Clone,
+        E: Clone,
+    {
+        self.map_ref(map_subexpr, 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))))
+    }
+}
+
+impl Expr<X, X> {
+    pub fn absurd<N, E>(&self) -> Expr<N, E> {
+        self.visit(&mut visitor::AbsurdVisitor)
+    }
+}
+
+impl<E: Clone> Expr<X, E> {
+    pub fn note_absurd<N>(&self) -> Expr<N, E> {
+        self.visit(&mut visitor::NoteAbsurdVisitor)
+    }
+}
+
+impl<N, E> SubExpr<N, E> {
+    pub fn as_ref(&self) -> &Expr<N, E> {
+        &self.0.as_ref().0
+    }
+
+    pub fn new(x: Expr<N, E>, n: N) -> Self {
+        SubExpr(Rc::new((x, Some(n))))
+    }
+
+    pub fn from_expr_no_note(x: Expr<N, E>) -> Self {
+        SubExpr(Rc::new((x, None)))
+    }
+
+    pub fn rewrap<E2>(&self, x: Expr<N, E2>) -> SubExpr<N, E2>
+    where
+        N: Clone,
+    {
+        SubExpr(Rc::new((x, (self.0).1.clone())))
+    }
+
+    pub fn traverse_embed<E2, Err>(
+        &self,
+        visit_embed: impl FnMut(&E) -> Result<E2, Err>,
+    ) -> Result<SubExpr<N, E2>, Err>
+    where
+        N: Clone,
+    {
+        Ok(self.rewrap(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,
+    {
+        self.rewrap(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
+    where
+        N: Clone,
+    {
+        match self.as_ref() {
+            ExprF::Embed(_) => SubExpr::clone(self),
+            // This calls ExprF::map_ref
+            e => self.rewrap(e.map_ref_with_special_handling_of_binders(
+                map_expr,
+                map_under_binder,
+                |_| unreachable!(),
+                Label::clone,
+            )),
+        }
+    }
+}
+
+impl SubExpr<X, X> {
+    pub fn absurd<N: Clone, T>(&self) -> SubExpr<N, T> {
+        SubExpr::from_expr_no_note(self.as_ref().absurd())
+    }
+}
+
+impl<E: Clone> SubExpr<X, E> {
+    pub fn note_absurd<N>(&self) -> SubExpr<N, E> {
+        SubExpr::from_expr_no_note(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<E>(x: Expr<X, E>) -> SubExpr<X, E> {
+    SubExpr::from_expr_no_note(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)
+        }
+    }
+}
diff --git a/dhall_syntax/src/core/import.rs b/dhall_syntax/src/core/import.rs
new file mode 100644
index 0000000..00f293c
--- /dev/null
+++ b/dhall_syntax/src/core/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/core/label.rs b/dhall_syntax/src/core/label.rs
new file mode 100644
index 0000000..43c3f53
--- /dev/null
+++ b/dhall_syntax/src/core/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/core/mod.rs b/dhall_syntax/src/core/mod.rs
new file mode 100644
index 0000000..7762479
--- /dev/null
+++ b/dhall_syntax/src/core/mod.rs
@@ -0,0 +1,10 @@
+mod expr;
+pub use expr::*;
+mod import;
+pub use import::*;
+mod label;
+pub use label::*;
+mod text;
+pub use text::*;
+pub mod context;
+pub mod visitor;
diff --git a/dhall_syntax/src/core/text.rs b/dhall_syntax/src/core/text.rs
new file mode 100644
index 0000000..83643d9
--- /dev/null
+++ b/dhall_syntax/src/core/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/core/visitor.rs b/dhall_syntax/src/core/visitor.rs
new file mode 100644
index 0000000..20bfc72
--- /dev/null
+++ b/dhall_syntax/src/core/visitor.rs
@@ -0,0 +1,458 @@
+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, E1>: Sized {
+    type Error;
+    type SE2;
+    type L2;
+    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>;
+
+    // Called with the result of the map, in the non-embed 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::E2>,
+    ) -> Result<Ret, Self::Error>;
+}
+
+impl<'a, T, Ret, SE1, L1, E1>
+    GenericVisitor<&'a ExprF<SE1, L1, E1>, Result<Ret, T::Error>> for T
+where
+    L1: Ord,
+    T::L2: Ord,
+    T: ExprFVeryGenericVisitor<'a, Ret, SE1, L1, E1>,
+{
+    fn visit(self, input: &'a ExprF<SE1, L1, 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, 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, E>,
+        {
+            x.iter()
+                .map(|(k, x)| Ok((v.visit_label(k)?, v.visit_subexpr(x)?)))
+                .collect()
+        }
+        fn btoptmap<'a, V, Ret, SE, L, 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, 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))?)
+            }
+            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, 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_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, E2>, Self::Error> {
+        Ok(ExprF::Embed(self.visit_embed(embed)?))
+    }
+}
+
+impl<'a, T, SE1, SE2, L1, L2, E1, E2>
+    ExprFVeryGenericVisitor<'a, ExprF<SE2, L2, E2>, SE1, L1, E1> for T
+where
+    T: ExprFFallibleVisitor<'a, SE1, SE2, L1, L2, E1, E2>,
+{
+    type Error = T::Error;
+    type SE2 = SE2;
+    type L2 = L2;
+    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, E2>, Self::Error> {
+        self.visit_embed_squash(embed)
+    }
+
+    // Called with the result of the map, in the non-embed 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::E2>,
+    ) -> Result<ExprF<SE2, L2, E2>, Self::Error> {
+        Ok(result)
+    }
+}
+
+/// Like ExprFFallibleVisitor, but without the error handling.
+pub trait ExprFInFallibleVisitor<'a, SE1, SE2, L1, L2, E1, E2>: Sized {
+    fn visit_subexpr(&mut self, subexpr: &'a SE1) -> SE2;
+    fn visit_label(&mut self, label: &'a L1) -> L2;
+    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, E2> {
+        ExprF::Embed(self.visit_embed(embed))
+    }
+}
+
+struct InfallibleWrapper<T>(T);
+
+impl<'a, T, SE1, SE2, L1, L2, E1, E2>
+    ExprFFallibleVisitor<'a, SE1, SE2, L1, L2, E1, E2> for InfallibleWrapper<T>
+where
+    T: ExprFInFallibleVisitor<'a, SE1, SE2, L1, L2, 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_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, E2>, Self::Error> {
+        Ok(self.0.visit_embed_squash(embed))
+    }
+}
+
+impl<'a, T, SE1, SE2, L1, L2, E1, E2>
+    GenericVisitor<&'a ExprF<SE1, L1, E1>, ExprF<SE2, L2, E2>> for T
+where
+    L1: Ord,
+    L2: Ord,
+    T: ExprFInFallibleVisitor<'a, SE1, SE2, L1, L2, E1, E2>,
+{
+    fn visit(self, input: &'a ExprF<SE1, L1, E1>) -> ExprF<SE2, L2, E2> {
+        trivial_result(InfallibleWrapper(self).visit(input))
+    }
+}
+
+pub struct TraverseRefWithBindersVisitor<F1, F2, F4, F5> {
+    pub visit_subexpr: F1,
+    pub visit_under_binder: F2,
+    pub visit_embed: F4,
+    pub visit_label: F5,
+}
+
+impl<'a, SE, L, E, SE2, L2, E2, Err, F1, F2, F4, F5>
+    ExprFFallibleVisitor<'a, SE, SE2, L, L2, E, E2>
+    for TraverseRefWithBindersVisitor<F1, F2, F4, F5>
+where
+    SE: 'a,
+    L: 'a,
+    E: 'a,
+    L: Ord,
+    L2: Ord,
+    F1: FnMut(&'a SE) -> Result<SE2, Err>,
+    F2: FnOnce(&'a L, &'a SE) -> Result<SE2, 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_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, F3, F4> {
+    pub visit_subexpr: F1,
+    pub visit_embed: F3,
+    pub visit_label: F4,
+}
+
+impl<'a, SE, L, E, SE2, L2, E2, Err, F1, F3, F4>
+    ExprFFallibleVisitor<'a, SE, SE2, L, L2, E, E2>
+    for TraverseRefVisitor<F1, F3, F4>
+where
+    SE: 'a,
+    L: 'a,
+    E: 'a,
+    L: Ord,
+    L2: Ord,
+    F1: FnMut(&'a SE) -> Result<SE2, 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_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, 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(subexpr.rewrap(subexpr.as_ref().visit(&mut **self)?))
+    }
+    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 NoteAbsurdVisitor;
+
+impl<'a, 'b, N, E>
+    ExprFInFallibleVisitor<'a, SubExpr<X, E>, SubExpr<N, E>, Label, Label, E, E>
+    for &'b mut NoteAbsurdVisitor
+where
+    E: Clone + 'a,
+{
+    fn visit_subexpr(&mut self, subexpr: &'a SubExpr<X, E>) -> SubExpr<N, E> {
+        SubExpr::from_expr_no_note(subexpr.as_ref().visit(&mut **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 AbsurdVisitor;
+
+impl<'a, 'b, N, E>
+    ExprFInFallibleVisitor<'a, SubExpr<X, X>, SubExpr<N, E>, Label, Label, X, E>
+    for &'b mut AbsurdVisitor
+{
+    fn visit_subexpr(&mut self, subexpr: &'a SubExpr<X, X>) -> SubExpr<N, E> {
+        SubExpr::from_expr_no_note(subexpr.as_ref().visit(&mut **self))
+    }
+    fn visit_embed(self, embed: &'a X) -> E {
+        match *embed {}
+    }
+    fn visit_label(&mut self, label: &'a Label) -> Label {
+        Label::clone(label)
+    }
+}