From e51b89d717a1f1aaba7e59c79c9fb4b8e7031bab Mon Sep 17 00:00:00 2001
From: Fintan Halpenny
Date: Sat, 27 Jul 2019 15:09:31 +0100
Subject: Add case for RightBasedRecordMerge in the typechecking phase.

The implementation checks the types and kinds of the LHS and RHS.

In the happy path it unions the HashMap prefering keys on the RHS over
the LHS, and the result is the type of the resulting HashMap.

The error cases are:
  - If the kinds of the records differ it results in a RecordMismatch error.
  - If either the LHS or RHS are not records it results in a
MustCombineRecord error.
---
 dhall/src/error/mod.rs       |  2 ++
 dhall/src/phase/typecheck.rs | 51 +++++++++++++++++++++++++++++++++++++++-----
 2 files changed, 48 insertions(+), 5 deletions(-)

diff --git a/dhall/src/error/mod.rs b/dhall/src/error/mod.rs
index 125d013..20b8636 100644
--- a/dhall/src/error/mod.rs
+++ b/dhall/src/error/mod.rs
@@ -76,9 +76,11 @@ pub(crate) enum TypeMessage {
     MergeAnnotMismatch,
     MergeHandlerTypeMismatch,
     MergeHandlerReturnTypeMustNotBeDependent,
+    MustCombineRecord(Typed),
     ProjectionMustBeRecord,
     ProjectionMissingEntry,
     Sort,
+    RecordMismatch(Typed, Typed),
     RecordTypeDuplicateField,
     UnionTypeDuplicateField,
     Unimplemented,
diff --git a/dhall/src/phase/typecheck.rs b/dhall/src/phase/typecheck.rs
index 419b2e2..2315edb 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) => {
+            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 mut 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.
+            for (label, value) in kts_y {
+                kts_x.insert(label, value);
+            }
+
+            // Construct the final record type from the union
+            Ok(RetTypeOnly(tck_record_type(
+                ctx,
+                kts_x.iter()
+                     .map(|(x, v)| Ok((x.clone(), v.to_type()))),
+            )?
+            .into_type()))
+        }
         BinOp(o @ ListAppend, l, r) => {
             match l.get_type()?.to_value() {
                 Value::AppliedBuiltin(List, _) => {}
@@ -1135,11 +1176,11 @@ 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");
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