3 files changed, 90 insertions, 14 deletions

diff --git a/dhall_syntax/src/core/import.rs b/dhall_syntax/src/core/import.rs
index d1f3fca..ef696b9 100644
--- a/dhall_syntax/src/core/import.rs
+++ b/dhall_syntax/src/core/import.rs
@@ -11,10 +11,15 @@ pub enum FilePrefix {
     Home,
 }
 
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub struct File {
+    pub file_path: Vec<String>,
+}
+
 /// The location of import (i.e. local vs. remote vs. environment)
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
 pub enum ImportLocation<SubExpr> {
-    Local(FilePrefix, Vec<String>),
+    Local(FilePrefix, File),
     Remote(URL<SubExpr>),
     Env(String),
     Missing,
@@ -24,7 +29,7 @@ pub enum ImportLocation<SubExpr> {
 pub struct URL<SubExpr> {
     pub scheme: Scheme,
     pub authority: String,
-    pub path: Vec<String>,
+    pub path: File,
     pub query: Option<String>,
     pub headers: Option<SubExpr>,
 }
@@ -99,3 +104,72 @@ impl<SE> Import<SE> {
         })
     }
 }
+
+pub trait Canonicalize {
+    fn canonicalize(&self) -> Self;
+}
+
+impl Canonicalize for File {
+    fn canonicalize(&self) -> File {
+        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,
+
+               // canonicalize(directory₀) = directory₁
+               // ───────────────────────────────────────
+               // canonicalize(directory₀/.) = directory₁
+               Some(c) if c == "." => continue,
+
+               Some(c) if c == ".." => match file_path_components.next() {
+                   // canonicalize(directory₀) = ε
+                   // ────────────────────────────
+                   // canonicalize(directory₀/..) = /..
+                   None => file_path.push("..".to_string()),
+
+                   // canonicalize(directory₀) = directory₁/..
+                   // ──────────────────────────────────────────────
+                   // canonicalize(directory₀/..) = directory₁/../..
+                   Some(ref c) if c == ".." => {
+                       file_path.push("..".to_string());
+                       file_path.push("..".to_string());
+                   },
+
+                   // canonicalize(directory₀) = directory₁/component
+                   // ───────────────────────────────────────────────  ; If "component" is not
+                   // canonicalize(directory₀/..) = directory₁         ; ".."
+                   Some(_) => continue,
+               },
+
+               // canonicalize(directory₀) = directory₁
+               // ─────────────────────────────────────────────────────────  ; If no other
+               // canonicalize(directory₀/component) = directory₁/component  ; rule matches
+               Some(c) => file_path.push(c.clone()),
+           }
+        }
+
+        File { file_path }
+    }
+}
+
+impl<SubExpr: Copy> Canonicalize for ImportLocation<SubExpr>  {
+    fn canonicalize(&self) -> ImportLocation<SubExpr> {
+        match self {
+            ImportLocation::Local(prefix, file) => ImportLocation::Local(*prefix, file.canonicalize()),
+            ImportLocation::Remote(url) => ImportLocation::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,
+        }
+    }
+}
diff --git a/dhall_syntax/src/parser.rs b/dhall_syntax/src/parser.rs
index 53fd68a..b70a236 100644
--- a/dhall_syntax/src/parser.rs
+++ b/dhall_syntax/src/parser.rs
@@ -726,19 +726,21 @@ make_parser! {
     });
 
     rule!(http_raw<URL<ParsedExpr>>; children!(
-        [scheme(sch), authority(auth), path(p)] => URL {
+        [scheme(sch), authority(auth), path(file_path)] => URL {
             scheme: sch,
             authority: auth,
-            path: p,
+            path: File { file_path },
             query: None,
             headers: None,
         },
-        [scheme(sch), authority(auth), path(p), query(q)] => URL {
-            scheme: sch,
-            authority: auth,
-            path: p,
-            query: Some(q),
-            headers: None,
+        [scheme(sch), authority(auth), path(file_path), query(q)] => {
+            URL {
+                scheme: sch,
+                authority: auth,
+                path: File { file_path },
+                query: Some(q),
+                headers: None,
+            }
         },
     ));
 
@@ -791,8 +793,8 @@ make_parser! {
         [http(url)] => {
             ImportLocation::Remote(url)
         },
-        [local((prefix, p))] => {
-            ImportLocation::Local(prefix, p)
+        [local((prefix, file_path))] => {
+            ImportLocation::Local(prefix, File { file_path })
         },
     ));
 
diff --git a/dhall_syntax/src/printer.rs b/dhall_syntax/src/printer.rs
index f3dc648..1e3b7f2 100644
--- a/dhall_syntax/src/printer.rs
+++ b/dhall_syntax/src/printer.rs
@@ -378,12 +378,12 @@ impl<SubExpr: Display> Display for Import<SubExpr> {
                 };
                 write!(f, "{}/", prefix)?;
                 let path: String =
-                    path.iter().map(|c| quote_if_needed(&*c)).join("/");
+                    path.file_path.iter().map(|c| quote_if_needed(&*c)).join("/");
                 f.write_str(&path)?;
             }
             Remote(url) => {
                 write!(f, "{}://{}/", url.scheme, url.authority,)?;
-                let path: String = url.path.iter().join("/");
+                let path: String = url.path.file_path.iter().join("/");
                 f.write_str(&path)?;
                 if let Some(q) = &url.query {
                     write!(f, "?{}", q)?