Add some new tests and implement import alternatives

5 files changed, 68 insertions, 4 deletions

diff --git a/dhall/src/error/mod.rs b/dhall/src/error/mod.rs
index aed6ccd..a0ee30a 100644
--- a/dhall/src/error/mod.rs
+++ b/dhall/src/error/mod.rs
@@ -86,7 +86,7 @@ pub(crate) enum TypeMessage {
     RecordTypeMergeRequiresRecordType(Type),
     RecordTypeMismatch(Type, Type, Type, Type),
     UnionTypeDuplicateField,
-    Unimplemented,
+    // Unimplemented,
 }
 
 impl TypeError {
diff --git a/dhall/src/phase/resolve.rs b/dhall/src/phase/resolve.rs
index f1329aa..0609694 100644
--- a/dhall/src/phase/resolve.rs
+++ b/dhall/src/phase/resolve.rs
@@ -86,7 +86,7 @@ fn do_resolve_expr(
             }
         }
     };
-    let expr = expr.traverse_embed(resolve)?;
+    let expr = expr.traverse_resolve(resolve)?;
     Ok(Resolved(expr))
 }
 
@@ -100,7 +100,7 @@ pub fn skip_resolve_expr(
     let resolve = |import: &Import| -> Result<Normalized, ImportError> {
         Err(ImportError::UnexpectedImport(import.clone()))
     };
-    let expr = expr.traverse_embed(resolve)?;
+    let expr = expr.traverse_resolve(resolve)?;
     Ok(Resolved(expr))
 }
 
diff --git a/dhall/src/phase/typecheck.rs b/dhall/src/phase/typecheck.rs
index 8551503..753d25e 100644
--- a/dhall/src/phase/typecheck.rs
+++ b/dhall/src/phase/typecheck.rs
@@ -823,7 +823,7 @@ fn type_last_layer(
                 RightBiasedRecordMerge => unreachable!(),
                 RecursiveRecordMerge => unreachable!(),
                 RecursiveRecordTypeMerge => unreachable!(),
-                _ => return Err(mkerr(Unimplemented)),
+                ImportAlt => unreachable!("There should remain no import alternatives in a resolved expression"),
             })?;
 
             ensure_equal!(
diff --git a/dhall_syntax/src/core/expr.rs b/dhall_syntax/src/core/expr.rs
index df2dc97..e33859b 100644
--- a/dhall_syntax/src/core/expr.rs
+++ b/dhall_syntax/src/core/expr.rs
@@ -307,6 +307,35 @@ impl<N, E> Expr<N, E> {
     {
         trivial_result(self.traverse_embed(|x| Ok(map_embed(x))))
     }
+
+    pub fn traverse_resolve<E2, Err>(
+        &self,
+        visit_embed: impl FnMut(&E) -> Result<E2, Err>,
+    ) -> Result<Expr<N, E2>, Err>
+    where
+        N: Clone,
+    {
+        self.traverse_resolve_with_visitor(&mut visitor::ResolveVisitor(
+            visit_embed,
+        ))
+    }
+
+    pub(crate) fn traverse_resolve_with_visitor<E2, Err, F1>(
+        &self,
+        visitor: &mut visitor::ResolveVisitor<F1>,
+    ) -> Result<Expr<N, E2>, Err>
+    where
+        N: Clone,
+        F1: FnMut(&E) -> Result<E2, Err>,
+    {
+        match self {
+            ExprF::BinOp(BinOp::ImportAlt, l, r) => l
+                .as_ref()
+                .traverse_resolve_with_visitor(visitor)
+                .or(r.as_ref().traverse_resolve_with_visitor(visitor)),
+            _ => self.visit(visitor),
+        }
+    }
 }
 
 impl Expr<X, X> {
@@ -383,6 +412,16 @@ impl<N, E> SubExpr<N, E> {
             )),
         }
     }
+
+    pub fn traverse_resolve<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_resolve(visit_embed)?))
+    }
 }
 
 impl SubExpr<X, X> {
diff --git a/dhall_syntax/src/core/visitor.rs b/dhall_syntax/src/core/visitor.rs
index 1745fdb..b02544f 100644
--- a/dhall_syntax/src/core/visitor.rs
+++ b/dhall_syntax/src/core/visitor.rs
@@ -358,6 +358,31 @@ where
     }
 }
 
+pub struct ResolveVisitor<F1>(pub F1);
+
+impl<'a, 'b, N, E, E2, Err, F1>
+    ExprFFallibleVisitor<'a, SubExpr<N, E>, SubExpr<N, E2>, E, E2>
+    for &'b mut ResolveVisitor<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()
+                .traverse_resolve_with_visitor(&mut **self)?,
+        ))
+    }
+    fn visit_embed(self, embed: &'a E) -> Result<E2, Self::Error> {
+        (self.0)(embed)
+    }
+}
 pub struct NoteAbsurdVisitor;
 
 impl<'a, 'b, N, E>