3 files changed, 58 insertions, 15 deletions

diff --git a/dhall/src/error/mod.rs b/dhall/src/error/mod.rs
index 2345348..bc5322a 100644
--- a/dhall/src/error/mod.rs
+++ b/dhall/src/error/mod.rs
@@ -80,8 +80,8 @@ pub(crate) enum TypeMessage {
     MergeHandlerReturnTypeMustNotBeDependent,
     ProjectionMustBeRecord,
     ProjectionMissingEntry,
-    RecordMismatch(Typed, Typed),
     Sort,
+    RecordMismatch(Typed, Typed),
     RecordTypeDuplicateField,
     UnionTypeDuplicateField,
     Unimplemented,
diff --git a/dhall/src/phase/normalize.rs b/dhall/src/phase/normalize.rs
index e3c5d68..7d86833 100644
--- a/dhall/src/phase/normalize.rs
+++ b/dhall/src/phase/normalize.rs
@@ -382,13 +382,13 @@ enum Ret<'a> {
 /// * `fu` - Will convert the values of the second map
 ///          into the target value.
 ///
-/// * `ftu` - Will convert the key and values from both maps
+/// * `fktu` - Will convert the key and values from both maps
 ///           into the target type.
 ///
 /// # Description
 ///
 /// If the key is present in both maps then the final value for
-/// that key is computed via the `ftu` function. Otherwise, the
+/// that key is computed via the `fktu` function. Otherwise, the
 /// final value will be calculated by either the `ft` or `fu` value
 /// depending on which map the key is present in.
 ///
@@ -397,35 +397,36 @@ enum Ret<'a> {
 pub(crate) fn outer_join<K, T, U, V>(
     mut ft: impl FnMut(&T) -> V,
     mut fu: impl FnMut(&U) -> V,
-    mut ftu: impl FnMut(&K, &T, &U) -> V,
+    mut fktu: impl FnMut(&K, &T, &U) -> V,
     map1: &HashMap<K, T>,
     map2: &HashMap<K, U>,
 ) -> HashMap<K, V>
 where
     K: std::hash::Hash + Eq + Clone,
 {
-    let mut kus = HashMap::new();
+    let mut kvs = HashMap::new();
+
     for (k1, t) in map1 {
         let v = if let Some(u) = map2.get(k1) {
             // The key exists in both maps
             // so use all values for computation
-            ftu(k1, t, u)
+            fktu(k1, t, u)
         } else {
             // Key only exists in map1
             ft(t)
         };
-        kus.insert(k1.clone(), v);
+        kvs.insert(k1.clone(), v);
     }
 
     for (k1, u) in map2 {
         // Insert if key was missing in map1
-        kus.entry(k1.clone()).or_insert(fu(u));
+        kvs.entry(k1.clone()).or_insert(fu(u));
     }
 
-    kus
+    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 c927ae2..9bb0e32 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(RecursiveRecordMerge, l, r) => {
             // A recursive function to dig down into
             // records of records when merging.
@@ -687,6 +728,7 @@ fn type_last_layer(
                 NaturalTimes => Natural,
                 TextAppend => Text,
                 ListAppend => unreachable!(),
+                RightBiasedRecordMerge => unreachable!(),
                 RecursiveRecordMerge => unreachable!(),
                 _ => return Err(mkerr(Unimplemented)),
             })?;
@@ -1201,11 +1243,11 @@ 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");