2 files changed, 42 insertions, 20 deletions

diff --git a/dhall/src/imports.rs b/dhall/src/imports.rs
index ff28ab2..0078040 100644
--- a/dhall/src/imports.rs
+++ b/dhall/src/imports.rs
@@ -1,6 +1,7 @@
 use crate::error::Error;
 use crate::expr::*;
 use dhall_core::*;
+use std::collections::HashMap;
 use std::fs::File;
 use std::io::Read;
 use std::path::Path;
@@ -18,9 +19,12 @@ pub enum ImportRoot {
     LocalDir(PathBuf),
 }
 
+type ImportCache = HashMap<Import, Normalized<'static>>;
+
 fn resolve_import(
     import: &Import,
     root: &ImportRoot,
+    import_cache: &mut ImportCache,
 ) -> Result<Normalized<'static>, ImportError> {
     use self::ImportRoot::*;
     use dhall_core::FilePrefix::*;
@@ -36,7 +40,7 @@ fn resolve_import(
                 Here => cwd.join(path),
                 _ => unimplemented!("{:?}", import),
             };
-            Ok(load_import(&path).map_err(|e| {
+            Ok(load_import(&path, import_cache).map_err(|e| {
                 ImportError::Recursive(import.clone(), Box::new(e))
             })?)
         }
@@ -44,23 +48,41 @@ fn resolve_import(
     }
 }
 
-fn load_import(f: &Path) -> Result<Normalized<'static>, Error> {
-    Ok(Parsed::parse_file(f)?.resolve()?.typecheck()?.normalize())
+fn load_import(
+    f: &Path,
+    import_cache: &mut ImportCache,
+) -> Result<Normalized<'static>, Error> {
+    Ok(resolve_expr_imports(Parsed::parse_file(f)?, import_cache)?
+        .typecheck()?
+        .normalize())
 }
 
-fn resolve_expr<'a>(
+fn resolve_expr_imports<'a>(
     Parsed(expr, root): Parsed<'a>,
-    allow_imports: bool,
+    import_cache: &mut ImportCache,
 ) -> Result<Resolved<'a>, ImportError> {
     let resolve =
         |import: &Import| -> Result<Normalized<'static>, ImportError> {
-            if allow_imports {
-                let expr = resolve_import(import, &root)?;
-                Ok(expr)
-            } else {
-                Err(ImportError::UnexpectedImport(import.clone()))
+            match import_cache.get(import) {
+                Some(expr) => Ok(expr.clone()),
+                None => {
+                    let expr = resolve_import(import, &root, import_cache)?;
+                    import_cache.insert(import.clone(), expr.clone());
+                    Ok(expr)
+                }
             }
         };
+    let expr = expr.as_ref().traverse_embed(resolve)?;
+    Ok(Resolved(rc(expr)))
+}
+
+fn resolve_expr<'a>(
+    Parsed(expr, _root): Parsed<'a>,
+) -> Result<Resolved<'a>, ImportError> {
+    let resolve =
+        |import: &Import| -> Result<Normalized<'static>, ImportError> {
+            Err(ImportError::UnexpectedImport(import.clone()))
+        };
     let expr = expr.as_ref().traverse_embed(&resolve)?;
     Ok(Resolved(rc(expr)))
 }
@@ -90,11 +112,11 @@ impl<'a> Parsed<'a> {
     }
 
     pub fn resolve(self) -> Result<Resolved<'a>, ImportError> {
-        crate::imports::resolve_expr(self, true)
+        crate::imports::resolve_expr_imports(self, &mut HashMap::new())
     }
 
     #[allow(dead_code)]
     pub fn skip_resolve(self) -> Result<Resolved<'a>, ImportError> {
-        crate::imports::resolve_expr(self, false)
+        crate::imports::resolve_expr(self)
     }
 }
diff --git a/dhall_core/src/import.rs b/dhall_core/src/import.rs
index f039953..00f293c 100644
--- a/dhall_core/src/import.rs
+++ b/dhall_core/src/import.rs
@@ -1,7 +1,7 @@
 use std::path::PathBuf;
 
 /// The beginning of a file path which anchors subsequent path components
-#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
 pub enum FilePrefix {
     /// Absolute path
     Absolute,
@@ -14,7 +14,7 @@ pub enum FilePrefix {
 }
 
 /// The location of import (i.e. local vs. remote vs. environment)
-#[derive(Debug, Clone, PartialEq, Eq)]
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
 pub enum ImportLocation {
     Local(FilePrefix, PathBuf),
     Remote(URL),
@@ -22,7 +22,7 @@ pub enum ImportLocation {
     Missing,
 }
 
-#[derive(Debug, Clone, PartialEq, Eq)]
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
 pub struct URL {
     pub scheme: Scheme,
     pub authority: String,
@@ -31,33 +31,33 @@ pub struct URL {
     pub headers: Option<Box<ImportHashed>>,
 }
 
-#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+#[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)]
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
 pub enum ImportMode {
     Code,
     RawText,
 }
 
-#[derive(Debug, Clone, PartialEq, Eq)]
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
 pub struct Hash {
     pub protocol: String,
     pub hash: String,
 }
 
-#[derive(Debug, Clone, PartialEq, Eq)]
+#[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)]
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
 pub struct Import {
     pub mode: ImportMode,
     pub location_hashed: ImportHashed,