Extend merge to work on Optionals

2 files changed, 36 insertions, 4 deletions

diff --git a/dhall/src/semantics/phase/normalize.rs b/dhall/src/semantics/phase/normalize.rs
index 5a0f566..459eaf1 100644
--- a/dhall/src/semantics/phase/normalize.rs
+++ b/dhall/src/semantics/phase/normalize.rs
@@ -590,8 +590,8 @@ pub(crate) fn normalize_one_layer(
     ty: &Value,
 ) -> ValueKind {
     use ValueKind::{
-        AppliedBuiltin, BoolLit, DoubleLit, EmptyListLit, IntegerLit,
-        NEListLit, NEOptionalLit, NaturalLit, RecordLit, TextLit,
+        AppliedBuiltin, BoolLit, DoubleLit, EmptyListLit, EmptyOptionalLit,
+        IntegerLit, NEListLit, NEOptionalLit, NaturalLit, RecordLit, TextLit,
         UnionConstructor, UnionLit, UnionType,
     };
 
@@ -739,6 +739,26 @@ pub(crate) fn normalize_one_layer(
                         Ret::Expr(expr)
                     }
                 },
+                (RecordLit(kvs), EmptyOptionalLit(_)) => {
+                    match kvs.get(&"None".into()) {
+                        Some(h) => Ret::Value(h.clone()),
+                        None => {
+                            drop(handlers_borrow);
+                            drop(variant_borrow);
+                            Ret::Expr(expr)
+                        }
+                    }
+                }
+                (RecordLit(kvs), NEOptionalLit(v)) => {
+                    match kvs.get(&"Some".into()) {
+                        Some(h) => Ret::Value(h.app(v.clone())),
+                        None => {
+                            drop(handlers_borrow);
+                            drop(variant_borrow);
+                            Ret::Expr(expr)
+                        }
+                    }
+                }
                 _ => {
                     drop(handlers_borrow);
                     drop(variant_borrow);
diff --git a/dhall/src/semantics/phase/typecheck.rs b/dhall/src/semantics/phase/typecheck.rs
index 7ea4951..97502d4 100644
--- a/dhall/src/semantics/phase/typecheck.rs
+++ b/dhall/src/semantics/phase/typecheck.rs
@@ -1,3 +1,4 @@
+use std::borrow::Cow;
 use std::cmp::max;
 use std::collections::HashMap;
 
@@ -695,8 +696,19 @@ fn type_last_layer(
             let union_type = union.get_type()?;
             let union_borrow = union_type.as_whnf();
             let variants = match &*union_borrow {
-                ValueKind::UnionType(kts) => kts,
-                _ => return mkerr(Merge2ArgMustBeUnion(union.clone())),
+                ValueKind::UnionType(kts) => Cow::Borrowed(kts),
+                ValueKind::AppliedBuiltin(syntax::Builtin::Optional, args)
+                    if args.len() == 1 =>
+                {
+                    let ty = &args[0];
+                    let mut kts = HashMap::new();
+                    kts.insert("None".into(), None);
+                    kts.insert("Some".into(), Some(ty.clone()));
+                    Cow::Owned(kts)
+                }
+                _ => {
+                    return mkerr(Merge2ArgMustBeUnionOrOptional(union.clone()))
+                }
             };
 
             let mut inferred_type = None;