3 files changed, 50 insertions, 6 deletions
diff --git a/dhall/src/error/mod.rs b/dhall/src/error/mod.rs
index 2f65443..aef84a9 100644
--- a/dhall/src/error/mod.rs
+++ b/dhall/src/error/mod.rs
@@ -77,9 +77,11 @@ pub(crate) enum TypeMessage {
     MergeAnnotMismatch,
     MergeHandlerTypeMismatch,
     MergeHandlerReturnTypeMustNotBeDependent,
+    MustCombineRecord(Typed),
     ProjectionMustBeRecord,
     ProjectionMissingEntry,
     Sort,
+    RecordMismatch(Typed, Typed),
     RecordTypeDuplicateField,
     RecordTypeMergeRequiresRecordType(Type),
     RecordTypeMismatch(Type, Type, Type, Type),
diff --git a/dhall/src/phase/normalize.rs b/dhall/src/phase/normalize.rs
index 653f415..4a25ed9 100644
--- a/dhall/src/phase/normalize.rs
+++ b/dhall/src/phase/normalize.rs
@@ -404,7 +404,7 @@ where
     kvs
 }
 
-fn merge_maps<K, V>(
+pub(crate) fn merge_maps<K, V>(
     map1: &HashMap<K, V>,
     map2: &HashMap<K, V>,
     mut f: impl FnMut(&V, &V) -> V,
diff --git a/dhall/src/phase/typecheck.rs b/dhall/src/phase/typecheck.rs
index bd8e1bb..3bff7ca 100644
--- a/dhall/src/phase/typecheck.rs
+++ b/dhall/src/phase/typecheck.rs
@@ -598,6 +598,47 @@ fn type_last_layer(
             }
             Ok(RetTypeOnly(text_type))
         }
+        BinOp(RightBiasedRecordMerge, l, r) => {
+            use crate::phase::normalize::merge_maps;
+
+            let l_type = l.get_type()?;
+            let l_kind = l_type.get_type()?;
+            let r_type = r.get_type()?;
+            let r_kind = r_type.get_type()?;
+
+            // Check the equality of kinds.
+            // This is to disallow expression such as:
+            // "{ x = Text } // { y = 1 }"
+            ensure_equal!(
+                l_kind,
+                r_kind,
+                mkerr(RecordMismatch(l.clone(), r.clone())),
+            );
+
+            // Extract the LHS record type
+            let kts_x = match l_type.to_value() {
+                Value::RecordType(kts) => kts,
+                _ => return Err(mkerr(MustCombineRecord(l.clone()))),
+            };
+
+            // Extract the RHS record type
+            let kts_y = match r_type.to_value() {
+                Value::RecordType(kts) => kts,
+                _ => return Err(mkerr(MustCombineRecord(r.clone()))),
+            };
+
+            // Union the two records, prefering
+            // the values found in the RHS.
+            let kts = merge_maps(&kts_x, &kts_y, |_, r_t| r_t.clone());
+
+            // Construct the final record type from the union
+            Ok(RetTypeOnly(tck_record_type(
+                ctx,
+                kts.iter()
+                   .map(|(x, v)| Ok((x.clone(), v.to_type()))),
+            )?
+            .into_type()))
+        }
         BinOp(RecursiveRecordTypeMerge, l, r) => {
             // A recursive function to dig down into
             // records of records when merging.
@@ -699,6 +740,7 @@ fn type_last_layer(
                 NaturalTimes => Natural,
                 TextAppend => Text,
                 ListAppend => unreachable!(),
+                RightBiasedRecordMerge => unreachable!(),
                 RecursiveRecordTypeMerge => unreachable!(),
                 _ => return Err(mkerr(Unimplemented)),
             })?;
@@ -1213,11 +1255,11 @@ mod spec_tests {
     ti_success!(ti_success_unit_RecursiveRecordTypeMergeTwo, "unit/RecursiveRecordTypeMergeTwo");
     ti_success!(ti_success_unit_RecursiveRecordTypeMergeTwoKinds, "unit/RecursiveRecordTypeMergeTwoKinds");
     ti_success!(ti_success_unit_RecursiveRecordTypeMergeTwoTypes, "unit/RecursiveRecordTypeMergeTwoTypes");
-    // ti_success!(ti_success_unit_RightBiasedRecordMergeRhsEmpty, "unit/RightBiasedRecordMergeRhsEmpty");
-    // ti_success!(ti_success_unit_RightBiasedRecordMergeTwo, "unit/RightBiasedRecordMergeTwo");
-    // ti_success!(ti_success_unit_RightBiasedRecordMergeTwoDifferent, "unit/RightBiasedRecordMergeTwoDifferent");
-    // ti_success!(ti_success_unit_RightBiasedRecordMergeTwoKinds, "unit/RightBiasedRecordMergeTwoKinds");
-    // ti_success!(ti_success_unit_RightBiasedRecordMergeTwoTypes, "unit/RightBiasedRecordMergeTwoTypes");
+    ti_success!(ti_success_unit_RightBiasedRecordMergeRhsEmpty, "unit/RightBiasedRecordMergeRhsEmpty");
+    ti_success!(ti_success_unit_RightBiasedRecordMergeTwo, "unit/RightBiasedRecordMergeTwo");
+    ti_success!(ti_success_unit_RightBiasedRecordMergeTwoDifferent, "unit/RightBiasedRecordMergeTwoDifferent");
+    ti_success!(ti_success_unit_RightBiasedRecordMergeTwoKinds, "unit/RightBiasedRecordMergeTwoKinds");
+    ti_success!(ti_success_unit_RightBiasedRecordMergeTwoTypes, "unit/RightBiasedRecordMergeTwoTypes");
     ti_success!(ti_success_unit_SomeTrue, "unit/SomeTrue");
     ti_success!(ti_success_unit_Text, "unit/Text");
     ti_success!(ti_success_unit_TextLiteral, "unit/TextLiteral");