Merge pull request #139 from Nadrieril/missing-features

Implement a bunch of missing features

15 files changed, 406 insertions, 227 deletions

diff --git a/dhall/src/error/mod.rs b/dhall/src/error/mod.rs
index 8829d47..e28b98b 100644
--- a/dhall/src/error/mod.rs
+++ b/dhall/src/error/mod.rs
@@ -1,6 +1,6 @@
 use std::io::Error as IOError;
 
-use crate::semantics::resolve::ImportStack;
+use crate::semantics::resolve::{ImportLocation, ImportStack};
 use crate::syntax::{Import, ParseError};
 
 mod builder;
@@ -26,8 +26,12 @@ pub(crate) enum ErrorKind {
 
 #[derive(Debug)]
 pub(crate) enum ImportError {
+    Missing,
+    MissingEnvVar,
+    SanityCheck,
     UnexpectedImport(Import<()>),
-    ImportCycle(ImportStack, Import<()>),
+    ImportCycle(ImportStack, ImportLocation),
+    Url(url::ParseError),
 }
 
 #[derive(Debug)]
@@ -109,6 +113,11 @@ impl From<ParseError> for Error {
         ErrorKind::Parse(err).into()
     }
 }
+impl From<url::ParseError> for Error {
+    fn from(err: url::ParseError) -> Error {
+        ErrorKind::Resolve(ImportError::Url(err)).into()
+    }
+}
 impl From<DecodeError> for Error {
     fn from(err: DecodeError) -> Error {
         ErrorKind::Decode(err).into()
diff --git a/dhall/src/lib.rs b/dhall/src/lib.rs
index 34d7bc3..d68ad6b 100644
--- a/dhall/src/lib.rs
+++ b/dhall/src/lib.rs
@@ -17,11 +17,12 @@ pub mod syntax;
 
 use std::fmt::Display;
 use std::path::Path;
+use url::Url;
 
 use crate::error::{EncodeError, Error, TypeError};
 use crate::semantics::parse;
 use crate::semantics::resolve;
-use crate::semantics::resolve::ImportRoot;
+use crate::semantics::resolve::ImportLocation;
 use crate::semantics::{
     typecheck, typecheck_with, Hir, Nir, NirKind, Tir, Type,
 };
@@ -34,7 +35,7 @@ pub type ResolvedExpr = Expr;
 pub type NormalizedExpr = Expr;
 
 #[derive(Debug, Clone)]
-pub struct Parsed(ParsedExpr, ImportRoot);
+pub struct Parsed(ParsedExpr, ImportLocation);
 
 /// An expression where all imports have been resolved
 ///
@@ -66,6 +67,9 @@ impl Parsed {
     pub fn parse_file(f: &Path) -> Result<Parsed, Error> {
         parse::parse_file(f)
     }
+    pub fn parse_remote(url: Url) -> Result<Parsed, Error> {
+        parse::parse_remote(url)
+    }
     pub fn parse_str(s: &str) -> Result<Parsed, Error> {
         parse::parse_str(s)
     }
diff --git a/dhall/src/semantics/nze/normalize.rs b/dhall/src/semantics/nze/normalize.rs
index dedd659..27862ee 100644
--- a/dhall/src/semantics/nze/normalize.rs
+++ b/dhall/src/semantics/nze/normalize.rs
@@ -233,7 +233,7 @@ pub(crate) fn normalize_one_layer(expr: ExprKind<Nir>, env: &NzEnv) -> NirKind {
         ExprKind::Builtin(b) => Ret::Nir(Nir::from_builtin_env(b, env)),
         ExprKind::Assert(_) => Ret::Expr(expr),
         ExprKind::App(v, a) => Ret::Nir(v.app(a)),
-        ExprKind::Lit(l) => Ret::NirKind(Lit(l.clone())),
+        ExprKind::Lit(l) => Ret::NirKind(Lit(l)),
         ExprKind::SomeLit(e) => Ret::NirKind(NEOptionalLit(e)),
         ExprKind::EmptyListLit(t) => {
             let arg = match t.kind() {
@@ -286,23 +286,6 @@ pub(crate) fn normalize_one_layer(expr: ExprKind<Nir>, env: &NzEnv) -> NirKind {
             None => Ret::Expr(expr),
         },
 
-        ExprKind::Projection(_, ref ls) if ls.is_empty() => {
-            Ret::NirKind(RecordLit(HashMap::new()))
-        }
-        ExprKind::Projection(ref v, ref ls) => match v.kind() {
-            RecordLit(kvs) => Ret::NirKind(RecordLit(
-                ls.iter()
-                    .filter_map(|l| kvs.get(l).map(|x| (l.clone(), x.clone())))
-                    .collect(),
-            )),
-            PartialExpr(ExprKind::Projection(v2, _)) => {
-                return normalize_one_layer(
-                    ExprKind::Projection(v2.clone(), ls.clone()),
-                    env,
-                )
-            }
-            _ => Ret::Expr(expr),
-        },
         ExprKind::Field(ref v, ref l) => match v.kind() {
             RecordLit(kvs) => match kvs.get(l) {
                 Some(r) => Ret::Nir(r.clone()),
@@ -311,6 +294,12 @@ pub(crate) fn normalize_one_layer(expr: ExprKind<Nir>, env: &NzEnv) -> NirKind {
             UnionType(kts) => {
                 Ret::NirKind(UnionConstructor(l.clone(), kts.clone()))
             }
+            PartialExpr(ExprKind::Projection(x, _)) => {
+                return normalize_one_layer(
+                    ExprKind::Field(x.clone(), l.clone()),
+                    env,
+                )
+            }
             PartialExpr(ExprKind::BinOp(
                 BinOp::RightBiasedRecordMerge,
                 x,
@@ -329,11 +318,11 @@ pub(crate) fn normalize_one_layer(expr: ExprKind<Nir>, env: &NzEnv) -> NirKind {
                     Some(r) => Ret::Expr(ExprKind::Field(
                         Nir::from_kind(PartialExpr(ExprKind::BinOp(
                             BinOp::RightBiasedRecordMerge,
-                            Nir::from_kind(RecordLit({
-                                let mut kvs = HashMap::new();
-                                kvs.insert(l.clone(), r.clone());
-                                kvs
-                            })),
+                            Nir::from_kind(RecordLit(
+                                Some((l.clone(), r.clone()))
+                                    .into_iter()
+                                    .collect(),
+                            )),
                             y.clone(),
                         ))),
                         l.clone(),
@@ -347,66 +336,81 @@ pub(crate) fn normalize_one_layer(expr: ExprKind<Nir>, env: &NzEnv) -> NirKind {
                 },
                 _ => Ret::Expr(expr),
             },
-            PartialExpr(ExprKind::BinOp(
-                BinOp::RecursiveRecordTypeMerge,
-                x,
-                y,
-            )) => match (x.kind(), y.kind()) {
-                (RecordLit(kvs), _) => match kvs.get(l) {
-                    Some(_) => Ret::Expr(expr),
-                    None => {
-                        return normalize_one_layer(
-                            ExprKind::Field(y.clone(), l.clone()),
-                            env,
-                        )
-                    }
-                },
-                (_, RecordLit(kvs)) => match kvs.get(l) {
-                    Some(_) => Ret::Expr(expr),
-                    None => {
-                        return normalize_one_layer(
-                            ExprKind::Field(x.clone(), l.clone()),
-                            env,
-                        )
-                    }
-                },
-                _ => Ret::Expr(expr),
-            },
+            PartialExpr(ExprKind::BinOp(BinOp::RecursiveRecordMerge, x, y)) => {
+                match (x.kind(), y.kind()) {
+                    (RecordLit(kvs), _) => match kvs.get(l) {
+                        Some(r) => Ret::Expr(ExprKind::Field(
+                            Nir::from_kind(PartialExpr(ExprKind::BinOp(
+                                BinOp::RecursiveRecordMerge,
+                                Nir::from_kind(RecordLit(
+                                    Some((l.clone(), r.clone()))
+                                        .into_iter()
+                                        .collect(),
+                                )),
+                                y.clone(),
+                            ))),
+                            l.clone(),
+                        )),
+                        None => {
+                            return normalize_one_layer(
+                                ExprKind::Field(y.clone(), l.clone()),
+                                env,
+                            )
+                        }
+                    },
+                    (_, RecordLit(kvs)) => match kvs.get(l) {
+                        Some(r) => Ret::Expr(ExprKind::Field(
+                            Nir::from_kind(PartialExpr(ExprKind::BinOp(
+                                BinOp::RecursiveRecordMerge,
+                                x.clone(),
+                                Nir::from_kind(RecordLit(
+                                    Some((l.clone(), r.clone()))
+                                        .into_iter()
+                                        .collect(),
+                                )),
+                            ))),
+                            l.clone(),
+                        )),
+                        None => {
+                            return normalize_one_layer(
+                                ExprKind::Field(x.clone(), l.clone()),
+                                env,
+                            )
+                        }
+                    },
+                    _ => Ret::Expr(expr),
+                }
+            }
+            _ => Ret::Expr(expr),
+        },
+        ExprKind::Projection(_, ref ls) if ls.is_empty() => {
+            Ret::NirKind(RecordLit(HashMap::new()))
+        }
+        ExprKind::Projection(ref v, ref ls) => match v.kind() {
+            RecordLit(kvs) => Ret::NirKind(RecordLit(
+                ls.iter()
+                    .filter_map(|l| kvs.get(l).map(|x| (l.clone(), x.clone())))
+                    .collect(),
+            )),
             PartialExpr(ExprKind::Projection(v2, _)) => {
                 return normalize_one_layer(
-                    ExprKind::Field(v2.clone(), l.clone()),
+                    ExprKind::Projection(v2.clone(), ls.clone()),
                     env,
                 )
             }
             _ => Ret::Expr(expr),
         },
-
-        ExprKind::ProjectionByExpr(ref v, ref t) => match dbg!(v).kind() {
-            RecordLit(kvs) => match dbg!(t).kind() {
-                RecordType(kts) => Ret::NirKind(RecordLit(
-                    kts.iter()
-                        .filter_map(|(l, _)| {
-                            kvs.get(l).map(|x| (l.clone(), x.clone()))
-                        })
-                        .collect(),
-                )),
-                _ => Ret::Expr(expr),
-            },
-            _ => match dbg!(t).kind() {
-                RecordType(kts) => {
-                    use crate::syntax::map::DupTreeSet;
-                    use std::iter::FromIterator;
-
-                    let ts = DupTreeSet::from_iter(
-                        kts.iter().map(|(l, _)| l.clone()),
-                    );
-                    return normalize_one_layer(
-                        ExprKind::Projection(v.clone(), ts),
-                        env,
-                    );
-                }
-                _ => Ret::Expr(expr),
-            },
+        ExprKind::ProjectionByExpr(ref v, ref t) => match t.kind() {
+            RecordType(kts) => {
+                return normalize_one_layer(
+                    ExprKind::Projection(
+                        v.clone(),
+                        kts.keys().cloned().collect(),
+                    ),
+                    env,
+                )
+            }
+            _ => Ret::Expr(expr),
         },
 
         ExprKind::Merge(ref handlers, ref variant, _) => {
diff --git a/dhall/src/semantics/parse.rs b/dhall/src/semantics/parse.rs
index ee35536..45860d0 100644
--- a/dhall/src/semantics/parse.rs
+++ b/dhall/src/semantics/parse.rs
@@ -1,30 +1,37 @@
 use std::fs::File;
 use std::io::Read;
 use std::path::Path;
+use url::Url;
 
 use crate::error::Error;
-use crate::semantics::resolve::ImportRoot;
+use crate::semantics::resolve::ImportLocation;
 use crate::syntax::binary;
 use crate::syntax::parse_expr;
 use crate::Parsed;
 
 pub(crate) fn parse_file(f: &Path) -> Result<Parsed, Error> {
-    let mut buffer = String::new();
-    File::open(f)?.read_to_string(&mut buffer)?;
-    let expr = parse_expr(&*buffer)?;
-    let root = ImportRoot::LocalDir(f.parent().unwrap().to_owned());
+    let text = std::fs::read_to_string(f)?;
+    let expr = parse_expr(&text)?;
+    let root = ImportLocation::Local(f.to_owned());
+    Ok(Parsed(expr, root))
+}
+
+pub(crate) fn parse_remote(url: Url) -> Result<Parsed, Error> {
+    let body = reqwest::blocking::get(url.clone()).unwrap().text().unwrap();
+    let expr = parse_expr(&body)?;
+    let root = ImportLocation::Remote(url);
     Ok(Parsed(expr, root))
 }
 
 pub(crate) fn parse_str(s: &str) -> Result<Parsed, Error> {
     let expr = parse_expr(s)?;
-    let root = ImportRoot::LocalDir(std::env::current_dir()?);
+    let root = ImportLocation::Missing;
     Ok(Parsed(expr, root))
 }
 
 pub(crate) fn parse_binary(data: &[u8]) -> Result<Parsed, Error> {
     let expr = binary::decode(data)?;
-    let root = ImportRoot::LocalDir(std::env::current_dir()?);
+    let root = ImportLocation::Missing;
     Ok(Parsed(expr, root))
 }
 
@@ -32,6 +39,6 @@ pub(crate) fn parse_binary_file(f: &Path) -> Result<Parsed, Error> {
     let mut buffer = Vec::new();
     File::open(f)?.read_to_end(&mut buffer)?;
     let expr = binary::decode(&buffer)?;
-    let root = ImportRoot::LocalDir(f.parent().unwrap().to_owned());
+    let root = ImportLocation::Local(f.to_owned());
     Ok(Parsed(expr, root))
 }
diff --git a/dhall/src/semantics/resolve/env.rs b/dhall/src/semantics/resolve/env.rs
index 43676cc..5a7f139 100644
--- a/dhall/src/semantics/resolve/env.rs
+++ b/dhall/src/semantics/resolve/env.rs
@@ -1,7 +1,7 @@
 use std::collections::HashMap;
 
 use crate::error::{Error, ImportError};
-use crate::semantics::{AlphaVar, Import, TypedHir, VarEnv};
+use crate::semantics::{AlphaVar, ImportLocation, TypedHir, VarEnv};
 use crate::syntax::{Label, V};
 
 /// Environment for resolving names.
@@ -10,8 +10,8 @@ pub(crate) struct NameEnv {
     names: Vec<Label>,
 }
 
-pub(crate) type ImportCache = HashMap<Import, TypedHir>;
-pub(crate) type ImportStack = Vec<Import>;
+pub(crate) type ImportCache = HashMap<ImportLocation, TypedHir>;
+pub(crate) type ImportStack = Vec<ImportLocation>;
 
 /// Environment for resolving imports
 #[derive(Debug, Clone)]
@@ -74,28 +74,28 @@ impl ImportEnv {
 
     pub fn handle_import(
         &mut self,
-        import: Import,
-        mut do_resolve: impl FnMut(&mut Self, &Import) -> Result<TypedHir, Error>,
+        location: ImportLocation,
+        do_resolve: impl FnOnce(&mut Self) -> Result<TypedHir, Error>,
     ) -> Result<TypedHir, Error> {
-        if self.stack.contains(&import) {
+        if self.stack.contains(&location) {
             return Err(
-                ImportError::ImportCycle(self.stack.clone(), import).into()
+                ImportError::ImportCycle(self.stack.clone(), location).into()
             );
         }
-        Ok(match self.cache.get(&import) {
+        Ok(match self.cache.get(&location) {
             Some(expr) => expr.clone(),
             None => {
-                // Push the current import on the stack
-                self.stack.push(import.clone());
+                // Push the current location on the stack
+                self.stack.push(location);
 
                 // Resolve the import recursively
-                let expr = do_resolve(self, &import)?;
+                let expr = do_resolve(self)?;
 
-                // Remove import from the stack.
-                self.stack.pop();
+                // Remove location from the stack.
+                let location = self.stack.pop().unwrap();
 
-                // Add the import to the cache
-                self.cache.insert(import, expr.clone());
+                // Add the resolved import to the cache
+                self.cache.insert(location, expr.clone());
 
                 expr
             }
diff --git a/dhall/src/semantics/resolve/hir.rs b/dhall/src/semantics/resolve/hir.rs
index 2f3464a..317708a 100644
--- a/dhall/src/semantics/resolve/hir.rs
+++ b/dhall/src/semantics/resolve/hir.rs
@@ -72,6 +72,9 @@ impl Hir {
     ) -> Result<Tir<'hir>, TypeError> {
         type_with(env, self, None)
     }
+    pub fn typecheck_noenv<'hir>(&'hir self) -> Result<Tir<'hir>, TypeError> {
+        self.typecheck(&TyEnv::new())
+    }
 
     /// Eval the Hir. It will actually get evaluated only as needed on demand.
     pub fn eval(&self, env: impl Into<NzEnv>) -> Nir {
diff --git a/dhall/src/semantics/resolve/resolve.rs b/dhall/src/semantics/resolve/resolve.rs
index 82800ec..d29271d 100644
--- a/dhall/src/semantics/resolve/resolve.rs
+++ b/dhall/src/semantics/resolve/resolve.rs
@@ -1,11 +1,18 @@
+use itertools::Itertools;
 use std::borrow::Cow;
-use std::path::{Path, PathBuf};
+use std::env;
+use std::path::PathBuf;
+use url::Url;
 
 use crate::error::ErrorBuilder;
 use crate::error::{Error, ImportError};
 use crate::semantics::{mkerr, Hir, HirKind, ImportEnv, NameEnv, Type};
 use crate::syntax;
-use crate::syntax::{BinOp, Expr, ExprKind, FilePath, ImportLocation, URL};
+use crate::syntax::map::DupTreeMap;
+use crate::syntax::{
+    BinOp, Builtin, Expr, ExprKind, FilePath, FilePrefix, ImportMode,
+    ImportTarget, Span, UnspannedExpr, URL,
+};
 use crate::{Parsed, ParsedExpr, Resolved};
 
 // TODO: evaluate import headers
@@ -14,42 +21,217 @@ pub(crate) type Import = syntax::Import<()>;
 /// Owned Hir with a type. Different from Tir because the Hir is owned.
 pub(crate) type TypedHir = (Hir, Type);
 
-/// A root from which to resolve relative imports.
-#[derive(Debug, Clone, PartialEq, Eq)]
-pub(crate) enum ImportRoot {
-    LocalDir(PathBuf),
+/// The location of some data, usually some dhall code.
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub(crate) enum ImportLocation {
+    /// Local file
+    Local(PathBuf),
+    /// Remote file
+    Remote(Url),
+    /// Environment variable
+    Env(String),
+    /// Data without a location
+    Missing,
+}
+
+impl ImportLocation {
+    /// Given an import pointing to `target` found in the current location, compute the next
+    /// location, or error if not allowed.
+    /// `sanity_check` indicates whether to check if that location is allowed to be referenced,
+    /// for example to prevent a remote file from reading an environment variable.
+    fn chain(
+        &self,
+        target: &ImportTarget<()>,
+        sanity_check: bool,
+    ) -> Result<ImportLocation, Error> {
+        Ok(match target {
+            ImportTarget::Local(prefix, path) => {
+                self.chain_local(prefix, path)?
+            }
+            ImportTarget::Remote(remote) => {
+                if sanity_check {
+                    if let ImportLocation::Remote(..) = self {
+                        // TODO: allow if CORS check passes
+                        Err(ImportError::SanityCheck)?
+                    }
+                }
+                let mut url = Url::parse(&format!(
+                    "{}://{}",
+                    remote.scheme, remote.authority
+                ))?;
+                url.set_path(&remote.path.file_path.iter().join("/"));
+                url.set_query(remote.query.as_ref().map(String::as_ref));
+                ImportLocation::Remote(url)
+            }
+            ImportTarget::Env(var_name) => {
+                if sanity_check {
+                    if let ImportLocation::Remote(..) = self {
+                        Err(ImportError::SanityCheck)?
+                    }
+                }
+                ImportLocation::Env(var_name.clone())
+            }
+            ImportTarget::Missing => ImportLocation::Missing,
+        })
+    }
+
+    fn chain_local(
+        &self,
+        prefix: &FilePrefix,
+        path: &FilePath,
+    ) -> Result<ImportLocation, Error> {
+        Ok(match self {
+            ImportLocation::Local(..)
+            | ImportLocation::Env(..)
+            | ImportLocation::Missing => {
+                let dir = match self {
+                    ImportLocation::Local(path) => {
+                        path.parent().unwrap().to_owned()
+                    }
+                    ImportLocation::Env(..) | ImportLocation::Missing => {
+                        std::env::current_dir()?
+                    }
+                    _ => unreachable!(),
+                };
+                let mut dir: Vec<String> = dir
+                    .components()
+                    .map(|component| {
+                        component.as_os_str().to_string_lossy().into_owned()
+                    })
+                    .collect();
+                let root = match prefix {
+                    FilePrefix::Here => dir,
+                    FilePrefix::Parent => {
+                        dir.push("..".to_string());
+                        dir
+                    }
+                    FilePrefix::Absolute => vec![],
+                    FilePrefix::Home => vec![],
+                };
+                let path: Vec<_> = root
+                    .into_iter()
+                    .chain(path.file_path.iter().cloned())
+                    .collect();
+                let path =
+                    (FilePath { file_path: path }).canonicalize().file_path;
+                let prefix = match prefix {
+                    FilePrefix::Here | FilePrefix::Parent => ".",
+                    FilePrefix::Absolute => "/",
+                    FilePrefix::Home => "~",
+                };
+                let path =
+                    Some(prefix.to_string()).into_iter().chain(path).collect();
+                ImportLocation::Local(path)
+            }
+            ImportLocation::Remote(url) => {
+                let mut url = url.clone();
+                match prefix {
+                    FilePrefix::Here => {}
+                    FilePrefix::Parent => {
+                        url = url.join("..")?;
+                    }
+                    FilePrefix::Absolute => panic!("error"),
+                    FilePrefix::Home => panic!("error"),
+                }
+                url = url.join(&path.file_path.join("/"))?;
+                ImportLocation::Remote(url)
+            }
+        })
+    }
+
+    fn fetch_dhall(self) -> Result<Parsed, Error> {
+        Ok(match self {
+            ImportLocation::Local(path) => Parsed::parse_file(&path)?,
+            ImportLocation::Remote(url) => Parsed::parse_remote(url)?,
+            ImportLocation::Env(var_name) => {
+                let val = match env::var(var_name) {
+                    Ok(val) => val,
+                    Err(_) => Err(ImportError::MissingEnvVar)?,
+                };
+                Parsed::parse_str(&val)?
+            }
+            ImportLocation::Missing => Err(ImportError::Missing)?,
+        })
+    }
+
+    fn fetch_text(self) -> Result<String, Error> {
+        Ok(match self {
+            ImportLocation::Local(path) => std::fs::read_to_string(&path)?,
+            ImportLocation::Remote(url) => {
+                reqwest::blocking::get(url).unwrap().text().unwrap()
+            }
+            ImportLocation::Env(var_name) => match env::var(var_name) {
+                Ok(val) => val,
+                Err(_) => Err(ImportError::MissingEnvVar)?,
+            },
+            ImportLocation::Missing => Err(ImportError::Missing)?,
+        })
+    }
+
+    fn into_location(self) -> Expr {
+        let (field_name, arg) = match self {
+            ImportLocation::Local(path) => {
+                ("Local", Some(path.to_string_lossy().into_owned()))
+            }
+            ImportLocation::Remote(url) => ("Remote", Some(url.into_string())),
+            ImportLocation::Env(name) => ("Environment", Some(name)),
+            ImportLocation::Missing => ("Missing", None),
+        };
+
+        let asloc_ty = make_aslocation_uniontype();
+        let expr = mkexpr(ExprKind::Field(asloc_ty, field_name.into()));
+        match arg {
+            Some(arg) => mkexpr(ExprKind::App(
+                expr,
+                mkexpr(ExprKind::TextLit(arg.into())),
+            )),
+            None => expr,
+        }
+    }
+}
+
+fn mkexpr(kind: UnspannedExpr) -> Expr {
+    Expr::new(kind, Span::Artificial)
+}
+
+fn make_aslocation_uniontype() -> Expr {
+    let text_type = mkexpr(ExprKind::Builtin(Builtin::Text));
+    let mut union = DupTreeMap::default();
+    union.insert("Local".into(), Some(text_type.clone()));
+    union.insert("Remote".into(), Some(text_type.clone()));
+    union.insert("Environment".into(), Some(text_type.clone()));
+    union.insert("Missing".into(), None);
+    mkexpr(ExprKind::UnionType(union))
 }
 
 fn resolve_one_import(
     env: &mut ImportEnv,
     import: &Import,
-    root: &ImportRoot,
+    location: &ImportLocation,
 ) -> Result<TypedHir, Error> {
-    use self::ImportRoot::*;
-    use syntax::FilePrefix::*;
-    use syntax::ImportLocation::*;
-    let cwd = match root {
-        LocalDir(cwd) => cwd,
-    };
-    match &import.location {
-        Local(prefix, path) => {
-            let path_buf: PathBuf = path.file_path.iter().collect();
-            let path_buf = match prefix {
-                // TODO: fail gracefully
-                Parent => cwd.parent().unwrap().join(path_buf),
-                Here => cwd.join(path_buf),
-                _ => unimplemented!("{:?}", import),
-            };
-            Ok(load_import(env, &path_buf)?)
+    let do_sanity_check = import.mode != ImportMode::Location;
+    let location = location.chain(&import.location, do_sanity_check)?;
+    env.handle_import(location.clone(), |env| match import.mode {
+        ImportMode::Code => {
+            let parsed = location.fetch_dhall()?;
+            let typed = resolve_with_env(env, parsed)?.typecheck()?;
+            Ok((typed.normalize().to_hir(), typed.ty().clone()))
         }
-        _ => unimplemented!("{:?}", import),
-    }
-}
-
-fn load_import(env: &mut ImportEnv, f: &Path) -> Result<TypedHir, Error> {
-    let parsed = Parsed::parse_file(f)?;
-    let typed = resolve_with_env(env, parsed)?.typecheck()?;
-    Ok((typed.normalize().to_hir(), typed.ty().clone()))
+        ImportMode::RawText => {
+            let text = location.fetch_text()?;
+            let hir = Hir::new(
+                HirKind::Expr(ExprKind::TextLit(text.into())),
+                Span::Artificial,
+            );
+            Ok((hir, Type::from_builtin(Builtin::Text)))
+        }
+        ImportMode::Location => {
+            let expr = location.into_location();
+            let hir = skip_resolve(&expr)?;
+            let ty = hir.typecheck_noenv()?.ty().clone();
+            Ok((hir, ty))
+        }
+    })
 }
 
 /// Desugar the first level of the expression.
@@ -139,12 +321,10 @@ fn resolve_with_env(
     env: &mut ImportEnv,
     parsed: Parsed,
 ) -> Result<Resolved, Error> {
-    let Parsed(expr, root) = parsed;
+    let Parsed(expr, location) = parsed;
     let resolved =
         traverse_resolve_expr(&mut NameEnv::new(), &expr, &mut |import| {
-            env.handle_import(import, |env, import| {
-                resolve_one_import(env, import, &root)
-            })
+            resolve_one_import(env, &import, &location)
         })?;
     Ok(Resolved(resolved))
 }
@@ -166,21 +346,15 @@ pub trait Canonicalize {
 impl Canonicalize for FilePath {
     fn canonicalize(&self) -> FilePath {
         let mut file_path = Vec::new();
-        let mut file_path_components = self.file_path.clone().into_iter();
-
-        loop {
-            let component = file_path_components.next();
-            match component.as_ref() {
-                // ───────────────────
-                // canonicalize(ε) = ε
-                None => break,
 
+        for c in &self.file_path {
+            match c.as_ref() {
                 // canonicalize(directory₀) = directory₁
                 // ───────────────────────────────────────
                 // canonicalize(directory₀/.) = directory₁
-                Some(c) if c == "." => continue,
+                "." => continue,
 
-                Some(c) if c == ".." => match file_path_components.next() {
+                ".." => match file_path.last() {
                     // canonicalize(directory₀) = ε
                     // ────────────────────────────
                     // canonicalize(directory₀/..) = /..
@@ -189,21 +363,20 @@ impl Canonicalize for FilePath {
                     // canonicalize(directory₀) = directory₁/..
                     // ──────────────────────────────────────────────
                     // canonicalize(directory₀/..) = directory₁/../..
-                    Some(ref c) if c == ".." => {
-                        file_path.push("..".to_string());
-                        file_path.push("..".to_string());
-                    }
+                    Some(c) if c == ".." => file_path.push("..".to_string()),
 
                     // canonicalize(directory₀) = directory₁/component
                     // ───────────────────────────────────────────────  ; If "component" is not
                     // canonicalize(directory₀/..) = directory₁         ; ".."
-                    Some(_) => continue,
+                    Some(_) => {
+                        file_path.pop();
+                    }
                 },
 
                 // canonicalize(directory₀) = directory₁
                 // ─────────────────────────────────────────────────────────  ; If no other
                 // canonicalize(directory₀/component) = directory₁/component  ; rule matches
-                Some(c) => file_path.push(c.clone()),
+                _ => file_path.push(c.clone()),
             }
         }
 
@@ -211,21 +384,21 @@ impl Canonicalize for FilePath {
     }
 }
 
-impl<SE: Copy> Canonicalize for ImportLocation<SE> {
-    fn canonicalize(&self) -> ImportLocation<SE> {
+impl<SE: Copy> Canonicalize for ImportTarget<SE> {
+    fn canonicalize(&self) -> ImportTarget<SE> {
         match self {
-            ImportLocation::Local(prefix, file) => {
-                ImportLocation::Local(*prefix, file.canonicalize())
+            ImportTarget::Local(prefix, file) => {
+                ImportTarget::Local(*prefix, file.canonicalize())
             }
-            ImportLocation::Remote(url) => ImportLocation::Remote(URL {
+            ImportTarget::Remote(url) => ImportTarget::Remote(URL {
                 scheme: url.scheme,
                 authority: url.authority.clone(),
                 path: url.path.canonicalize(),
                 query: url.query.clone(),
                 headers: url.headers.clone(),
             }),
-            ImportLocation::Env(name) => ImportLocation::Env(name.to_string()),
-            ImportLocation::Missing => ImportLocation::Missing,
+            ImportTarget::Env(name) => ImportTarget::Env(name.to_string()),
+            ImportTarget::Missing => ImportTarget::Missing,
         }
     }
 }
diff --git a/dhall/src/syntax/ast/expr.rs b/dhall/src/syntax/ast/expr.rs
index bb1a5b3..ce0a3d2 100644
--- a/dhall/src/syntax/ast/expr.rs
+++ b/dhall/src/syntax/ast/expr.rs
@@ -178,7 +178,7 @@ pub enum ExprKind<SubExpr> {
     Field(SubExpr, Label),
     ///  `e.{ x, y, z }`
     Projection(SubExpr, DupTreeSet<Label>),
-    ///  `e.(s)`
+    ///  `e.(t)`
     ProjectionByExpr(SubExpr, SubExpr),
     ///  `x::y`
     Completion(SubExpr, SubExpr),
diff --git a/dhall/src/syntax/ast/import.rs b/dhall/src/syntax/ast/import.rs
index 7bde6e0..75d7946 100644
--- a/dhall/src/syntax/ast/import.rs
+++ b/dhall/src/syntax/ast/import.rs
@@ -18,7 +18,7 @@ pub struct FilePath {
 
 /// The location of import (i.e. local vs. remote vs. environment)
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
-pub enum ImportLocation<SubExpr> {
+pub enum ImportTarget<SubExpr> {
     Local(FilePrefix, FilePath),
     Remote(URL<SubExpr>),
     Env(String),
@@ -57,7 +57,7 @@ pub enum Hash {
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
 pub struct Import<SubExpr> {
     pub mode: ImportMode,
-    pub location: ImportLocation<SubExpr>,
+    pub location: ImportTarget<SubExpr>,
     pub hash: Option<Hash>,
 }
 
@@ -77,12 +77,12 @@ impl<SE> URL<SE> {
     }
 }
 
-impl<SE> ImportLocation<SE> {
+impl<SE> ImportTarget<SE> {
     pub fn traverse_ref<'a, Err, SE2>(
         &'a self,
         f: impl FnOnce(&'a SE) -> Result<SE2, Err>,
-    ) -> Result<ImportLocation<SE2>, Err> {
-        use ImportLocation::*;
+    ) -> Result<ImportTarget<SE2>, Err> {
+        use ImportTarget::*;
         Ok(match self {
             Local(prefix, path) => Local(*prefix, path.clone()),
             Remote(url) => Remote(url.traverse_ref(f)?),
diff --git a/dhall/src/syntax/ast/text.rs b/dhall/src/syntax/ast/text.rs
index 83aaf9a..c40f4a1 100644
--- a/dhall/src/syntax/ast/text.rs
+++ b/dhall/src/syntax/ast/text.rs
@@ -54,16 +54,6 @@ impl<SubExpr> InterpolatedTextContents<SubExpr> {
             Text(s) => Text(s.clone()),
         })
     }
-    pub fn traverse_mut<'a, E, F>(&'a mut self, mut f: F) -> Result<(), E>
-    where
-        F: FnMut(&'a mut SubExpr) -> Result<(), E>,
-    {
-        use InterpolatedTextContents::Expr;
-        if let Expr(e) = self {
-            f(e)?;
-        }
-        Ok(())
-    }
     pub fn map_ref<'a, SubExpr2, F>(
         &'a self,
         mut f: F,
@@ -77,15 +67,6 @@ impl<SubExpr> InterpolatedTextContents<SubExpr> {
             Text(s) => Text(s.clone()),
         }
     }
-    pub fn map_mut<'a, F>(&'a mut self, mut f: F)
-    where
-        F: FnMut(&'a mut SubExpr),
-    {
-        use InterpolatedTextContents::Expr;
-        if let Expr(e) = self {
-            f(e);
-        }
-    }
 }
 
 impl<SubExpr> InterpolatedText<SubExpr> {
@@ -126,16 +107,6 @@ impl<SubExpr> InterpolatedText<SubExpr> {
         })
     }
 
-    pub fn traverse_mut<'a, E, F>(&'a mut self, mut f: F) -> Result<(), E>
-    where
-        F: FnMut(&'a mut SubExpr) -> Result<(), E>,
-    {
-        for (e, _) in &mut self.tail {
-            f(e)?
-        }
-        Ok(())
-    }
-
     pub fn iter<'a>(
         &'a self,
     ) -> impl Iterator<Item = InterpolatedTextContents<&'a SubExpr>> + 'a {
diff --git a/dhall/src/syntax/binary/decode.rs b/dhall/src/syntax/binary/decode.rs
index 2e50d61..bebc800 100644
--- a/dhall/src/syntax/binary/decode.rs
+++ b/dhall/src/syntax/binary/decode.rs
@@ -5,7 +5,7 @@ use std::iter::FromIterator;
 use crate::error::DecodeError;
 use crate::syntax;
 use crate::syntax::{
-    Expr, ExprKind, FilePath, FilePrefix, Hash, ImportLocation, ImportMode,
+    Expr, ExprKind, FilePath, FilePrefix, Hash, ImportMode, ImportTarget,
     Integer, InterpolatedText, Label, LitKind, Natural, Scheme, Span,
     UnspannedExpr, URL, V,
 };
@@ -305,7 +305,7 @@ fn cbor_value_to_dhall(data: &cbor::Value) -> Result<DecodedExpr, DecodeError> {
                             })
                             .collect::<Result<_, _>>()?;
                         let path = FilePath { file_path };
-                        ImportLocation::Remote(URL {
+                        ImportTarget::Remote(URL {
                             scheme,
                             authority,
                             path,
@@ -332,7 +332,7 @@ fn cbor_value_to_dhall(data: &cbor::Value) -> Result<DecodedExpr, DecodeError> {
                             })
                             .collect::<Result<_, _>>()?;
                         let path = FilePath { file_path };
-                        ImportLocation::Local(prefix, path)
+                        ImportTarget::Local(prefix, path)
                     }
                     6 => {
                         let env = match rest.next() {
@@ -341,9 +341,9 @@ fn cbor_value_to_dhall(data: &cbor::Value) -> Result<DecodedExpr, DecodeError> {
                                 "import/env".to_owned(),
                             ))?,
                         };
-                        ImportLocation::Env(env)
+                        ImportTarget::Env(env)
                     }
-                    7 => ImportLocation::Missing,
+                    7 => ImportTarget::Missing,
                     _ => Err(DecodeError::WrongFormatError(
                         "import/type".to_owned(),
                     ))?,
diff --git a/dhall/src/syntax/binary/encode.rs b/dhall/src/syntax/binary/encode.rs
index d2aa240..2484d8d 100644
--- a/dhall/src/syntax/binary/encode.rs
+++ b/dhall/src/syntax/binary/encode.rs
@@ -6,8 +6,8 @@ use crate::error::EncodeError;
 use crate::syntax;
 use crate::syntax::map::DupTreeMap;
 use crate::syntax::{
-    Expr, ExprKind, FilePrefix, Hash, Import, ImportLocation, ImportMode,
-    Label, Scheme, V,
+    Expr, ExprKind, FilePrefix, Hash, Import, ImportMode, ImportTarget, Label,
+    Scheme, V,
 };
 
 pub(crate) fn encode(expr: &Expr) -> Result<Vec<u8>, EncodeError> {
@@ -179,10 +179,10 @@ where
     use serde::ser::SerializeSeq;
 
     let count = 4 + match &import.location {
-        ImportLocation::Remote(url) => 3 + url.path.file_path.len(),
-        ImportLocation::Local(_, path) => path.file_path.len(),
-        ImportLocation::Env(_) => 1,
-        ImportLocation::Missing => 0,
+        ImportTarget::Remote(url) => 3 + url.path.file_path.len(),
+        ImportTarget::Local(_, path) => path.file_path.len(),
+        ImportTarget::Env(_) => 1,
+        ImportTarget::Missing => 0,
     };
     let mut ser_seq = ser.serialize_seq(Some(count))?;
 
@@ -206,23 +206,23 @@ where
     ser_seq.serialize_element(&U64(mode))?;
 
     let scheme = match &import.location {
-        ImportLocation::Remote(url) => match url.scheme {
+        ImportTarget::Remote(url) => match url.scheme {
             Scheme::HTTP => 0,
             Scheme::HTTPS => 1,
         },
-        ImportLocation::Local(prefix, _) => match prefix {
+        ImportTarget::Local(prefix, _) => match prefix {
             FilePrefix::Absolute => 2,
             FilePrefix::Here => 3,
             FilePrefix::Parent => 4,
             FilePrefix::Home => 5,
         },
-        ImportLocation::Env(_) => 6,
-        ImportLocation::Missing => 7,
+        ImportTarget::Env(_) => 6,
+        ImportTarget::Missing => 7,
     };
     ser_seq.serialize_element(&U64(scheme))?;
 
     match &import.location {
-        ImportLocation::Remote(url) => {
+        ImportTarget::Remote(url) => {
             match &url.headers {
                 None => ser_seq.serialize_element(&Null)?,
                 Some(e) => {
@@ -238,15 +238,15 @@ where
                 Some(x) => ser_seq.serialize_element(x)?,
             };
         }
-        ImportLocation::Local(_, path) => {
+        ImportTarget::Local(_, path) => {
             for p in path.file_path.iter() {
                 ser_seq.serialize_element(&p)?;
             }
         }
-        ImportLocation::Env(env) => {
+        ImportTarget::Env(env) => {
             ser_seq.serialize_element(env)?;
         }
-        ImportLocation::Missing => {}
+        ImportTarget::Missing => {}
     }
 
     ser_seq.end()
diff --git a/dhall/src/syntax/text/parser.rs b/dhall/src/syntax/text/parser.rs
index ba64a75..7140332 100644
--- a/dhall/src/syntax/text/parser.rs
+++ b/dhall/src/syntax/text/parser.rs
@@ -10,7 +10,7 @@ use crate::syntax::map::{DupTreeMap, DupTreeSet};
 use crate::syntax::ExprKind::*;
 use crate::syntax::LitKind::*;
 use crate::syntax::{
-    Double, Expr, FilePath, FilePrefix, Hash, ImportLocation, ImportMode,
+    Double, Expr, FilePath, FilePrefix, Hash, ImportMode, ImportTarget,
     Integer, InterpolatedText, InterpolatedTextContents, Label, NaiveDouble,
     Natural, Scheme, Span, UnspannedExpr, URL, V,
 };
@@ -483,9 +483,9 @@ impl DhallParser {
     }
 
     #[alias(import_type)]
-    fn local(input: ParseInput) -> ParseResult<ImportLocation<Expr>> {
+    fn local(input: ParseInput) -> ParseResult<ImportTarget<Expr>> {
         Ok(match_nodes!(input.into_children();
-            [local_path((prefix, p))] => ImportLocation::Local(prefix, p),
+            [local_path((prefix, p))] => ImportTarget::Local(prefix, p),
         ))
     }
 
@@ -550,17 +550,17 @@ impl DhallParser {
     }
 
     #[alias(import_type)]
-    fn http(input: ParseInput) -> ParseResult<ImportLocation<Expr>> {
-        Ok(ImportLocation::Remote(match_nodes!(input.into_children();
+    fn http(input: ParseInput) -> ParseResult<ImportTarget<Expr>> {
+        Ok(ImportTarget::Remote(match_nodes!(input.into_children();
             [http_raw(url)] => url,
             [http_raw(url), expression(e)] => URL { headers: Some(e), ..url },
         )))
     }
 
     #[alias(import_type)]
-    fn env(input: ParseInput) -> ParseResult<ImportLocation<Expr>> {
+    fn env(input: ParseInput) -> ParseResult<ImportTarget<Expr>> {
         Ok(match_nodes!(input.into_children();
-            [environment_variable(v)] => ImportLocation::Env(v),
+            [environment_variable(v)] => ImportTarget::Env(v),
         ))
     }
     #[alias(environment_variable)]
@@ -593,8 +593,8 @@ impl DhallParser {
     }
 
     #[alias(import_type)]
-    fn missing(_input: ParseInput) -> ParseResult<ImportLocation<Expr>> {
-        Ok(ImportLocation::Missing)
+    fn missing(_input: ParseInput) -> ParseResult<ImportTarget<Expr>> {
+        Ok(ImportTarget::Missing)
     }
 
     fn hash(input: ParseInput) -> ParseResult<Hash> {
diff --git a/dhall/src/syntax/text/printer.rs b/dhall/src/syntax/text/printer.rs
index 8891d41..5bb987b 100644
--- a/dhall/src/syntax/text/printer.rs
+++ b/dhall/src/syntax/text/printer.rs
@@ -379,8 +379,8 @@ impl Display for Hash {
 impl<SubExpr: Display> Display for Import<SubExpr> {
     fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
         use FilePrefix::*;
-        use ImportLocation::*;
         use ImportMode::*;
+        use ImportTarget::*;
         let quote_if_needed = |s: &str| -> String {
             if s.chars().all(|c| c.is_ascii_alphanumeric()) {
                 s.to_string()
@@ -406,7 +406,7 @@ impl<SubExpr: Display> Display for Import<SubExpr> {
                 f.write_str(&path)?;
             }
             Remote(url) => {
-                write!(f, "{}://{}/", url.scheme, url.authority,)?;
+                write!(f, "{}://{}/", url.scheme, url.authority)?;
                 let path: String = url.path.file_path.iter().join("/");
                 f.write_str(&path)?;
                 if let Some(q) = &url.query {
diff --git a/dhall/src/tests.rs b/dhall/src/tests.rs
index 482f9ae..369c6cc 100644
--- a/dhall/src/tests.rs
+++ b/dhall/src/tests.rs
@@ -80,7 +80,7 @@ impl TestFile {
         match self {
             TestFile::Source(path)
             | TestFile::Binary(path)
-            | TestFile::UI(path) => PathBuf::from(path),
+            | TestFile::UI(path) => PathBuf::from("dhall").join(path),
         }
     }
 
@@ -243,6 +243,13 @@ fn run_test_stringy_error(test: Test) -> std::result::Result<(), String> {
 
 fn run_test(test: Test) -> Result<()> {
     use self::Test::*;
+    // Setup current directory to the root of the repository. Important for `as Location` tests.
+    env::set_current_dir(
+        PathBuf::from(env!("CARGO_MANIFEST_DIR")).parent().unwrap(),
+    )?;
+    // Set environment variable for import tests.
+    env::set_var("DHALL_TEST_VAR", "6 * 7");
+
     match test {
         ParserSuccess(expr, expected) => {
             let expr = expr.parse()?;
@@ -302,6 +309,7 @@ fn run_test(test: Test) -> Result<()> {
             expected.compare(expr)?;
         }
     }
+
     Ok(())
 }