2 files changed, 128 insertions, 8 deletions

diff --git a/dhall/src/phase/normalize.rs b/dhall/src/phase/normalize.rs
index da19dc1..7d86833 100644
--- a/dhall/src/phase/normalize.rs
+++ b/dhall/src/phase/normalize.rs
@@ -372,6 +372,60 @@ enum Ret<'a> {
     Expr(ExprF<Thunk, X>),
 }
 
+/// Performs an outer join of two HashMaps.
+///
+/// # Arguments
+///
+/// * `ft` - Will convert the values of the first map
+///          into the target value.
+///
+/// * `fu` - Will convert the values of the second map
+///          into the target value.
+///
+/// * `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 `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.
+///
+/// The final map will contain all keys from the two input maps with
+/// also values computed as per above.
+pub(crate) fn outer_join<K, T, U, V>(
+    mut ft: impl FnMut(&T) -> V,
+    mut fu: impl FnMut(&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 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
+            fktu(k1, t, u)
+        } else {
+            // Key only exists in map1
+            ft(t)
+        };
+        kvs.insert(k1.clone(), v);
+    }
+
+    for (k1, u) in map2 {
+        // Insert if key was missing in map1
+        kvs.entry(k1.clone()).or_insert(fu(u));
+    }
+
+    kvs
+}
+
 pub(crate) fn merge_maps<K, V>(
     map1: &HashMap<K, V>,
     map2: &HashMap<K, V>,
diff --git a/dhall/src/phase/typecheck.rs b/dhall/src/phase/typecheck.rs
index ecd0a69..9bb0e32 100644
--- a/dhall/src/phase/typecheck.rs
+++ b/dhall/src/phase/typecheck.rs
@@ -639,6 +639,71 @@ fn type_last_layer(
             )?
             .into_type()))
         }
+        BinOp(RecursiveRecordMerge, l, r) => {
+            // A recursive function to dig down into
+            // records of records when merging.
+            fn combine_record_types(
+                ctx: &TypecheckContext,
+                kts_l: HashMap<Label, TypeThunk>,
+                kts_r: HashMap<Label, TypeThunk>,
+            ) -> Result<Typed, TypeError> {
+                use crate::phase::normalize::outer_join;
+
+                // If the Label exists for both records and Type for the values
+                // are records themselves, then we hit the recursive case.
+                // Otherwise we have a field collision.
+                let combine = |k: &Label, inner_l: &TypeThunk, inner_r: &TypeThunk|
+                    -> Result<Typed, TypeError> {
+                    match (inner_l.to_value(), inner_r.to_value()) {
+                        (Value::RecordType(inner_l_kvs), Value::RecordType(inner_r_kvs)) =>
+                            combine_record_types(ctx, inner_l_kvs, inner_r_kvs),
+                        (_, _) => Err(TypeError::new(ctx, FieldCollision(k.clone()))),
+                    }
+                };
+
+                let kts: HashMap<Label, Result<Typed, TypeError>> = outer_join(
+                    |l| Ok(l.to_type()),
+                    |r| Ok(r.to_type()),
+                    |k: &Label, l: &TypeThunk, r: &TypeThunk| combine(k, l, r),
+                    &kts_l,
+                    &kts_r,
+                );
+
+                Ok(tck_record_type(
+                    ctx,
+                    kts.into_iter().map(|(x, v)| v.map(|r| (x.clone(), r)))
+                )?
+                .into_type())
+            };
+
+            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()))),
+            };
+
+            combine_record_types(ctx, kts_x, kts_y).map(|r| RetTypeOnly(r))
+        }
         BinOp(o @ ListAppend, l, r) => {
             match l.get_type()?.to_value() {
                 Value::AppliedBuiltin(List, _) => {}
@@ -664,6 +729,7 @@ fn type_last_layer(
                 TextAppend => Text,
                 ListAppend => unreachable!(),
                 RightBiasedRecordMerge => unreachable!(),
+                RecursiveRecordMerge => unreachable!(),
                 _ => return Err(mkerr(Unimplemented)),
             })?;
 
@@ -1162,14 +1228,14 @@ mod spec_tests {
     ti_success!(ti_success_unit_RecordTypeNestedKind, "unit/RecordTypeNestedKind");
     ti_success!(ti_success_unit_RecordTypeNestedKindLike, "unit/RecordTypeNestedKindLike");
     ti_success!(ti_success_unit_RecordTypeType, "unit/RecordTypeType");
-    // ti_success!(ti_success_unit_RecursiveRecordMergeLhsEmpty, "unit/RecursiveRecordMergeLhsEmpty");
-    // ti_success!(ti_success_unit_RecursiveRecordMergeRecursively, "unit/RecursiveRecordMergeRecursively");
-    // ti_success!(ti_success_unit_RecursiveRecordMergeRecursivelyKinds, "unit/RecursiveRecordMergeRecursivelyKinds");
-    // ti_success!(ti_success_unit_RecursiveRecordMergeRecursivelyTypes, "unit/RecursiveRecordMergeRecursivelyTypes");
-    // ti_success!(ti_success_unit_RecursiveRecordMergeRhsEmpty, "unit/RecursiveRecordMergeRhsEmpty");
-    // ti_success!(ti_success_unit_RecursiveRecordMergeTwo, "unit/RecursiveRecordMergeTwo");
-    // ti_success!(ti_success_unit_RecursiveRecordMergeTwoKinds, "unit/RecursiveRecordMergeTwoKinds");
-    // ti_success!(ti_success_unit_RecursiveRecordMergeTwoTypes, "unit/RecursiveRecordMergeTwoTypes");
+    ti_success!(ti_success_unit_RecursiveRecordMergeLhsEmpty, "unit/RecursiveRecordMergeLhsEmpty");
+    ti_success!(ti_success_unit_RecursiveRecordMergeRecursively, "unit/RecursiveRecordMergeRecursively");
+    ti_success!(ti_success_unit_RecursiveRecordMergeRecursivelyKinds, "unit/RecursiveRecordMergeRecursivelyKinds");
+    ti_success!(ti_success_unit_RecursiveRecordMergeRecursivelyTypes, "unit/RecursiveRecordMergeRecursivelyTypes");
+    ti_success!(ti_success_unit_RecursiveRecordMergeRhsEmpty, "unit/RecursiveRecordMergeRhsEmpty");
+    ti_success!(ti_success_unit_RecursiveRecordMergeTwo, "unit/RecursiveRecordMergeTwo");
+    ti_success!(ti_success_unit_RecursiveRecordMergeTwoKinds, "unit/RecursiveRecordMergeTwoKinds");
+    ti_success!(ti_success_unit_RecursiveRecordMergeTwoTypes, "unit/RecursiveRecordMergeTwoTypes");
     // ti_success!(ti_success_unit_RecursiveRecordTypeMergeRecursively, "unit/RecursiveRecordTypeMergeRecursively");
     // ti_success!(ti_success_unit_RecursiveRecordTypeMergeRecursivelyKinds, "unit/RecursiveRecordTypeMergeRecursivelyKinds");
     // ti_success!(ti_success_unit_RecursiveRecordTypeMergeRecursivelyTypes, "unit/RecursiveRecordTypeMergeRecursivelyTypes");