From ae05f0eb0c78ac6a4a346d57b5cb0f05bfb8c87b Mon Sep 17 00:00:00 2001
From: Richard Feldman <oss@rtfeldman.com>
Date: Sat, 18 Jul 2020 12:50:14 -0400
Subject: [PATCH] Unify optional fields

---
 compiler/unify/src/unify.rs | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/compiler/unify/src/unify.rs b/compiler/unify/src/unify.rs
index 31b142cc56..7ff9edfd2d 100644
--- a/compiler/unify/src/unify.rs
+++ b/compiler/unify/src/unify.rs
@@ -4,7 +4,7 @@ use roc_module::symbol::Symbol;
 use roc_types::boolean_algebra::Bool;
 use roc_types::subs::Content::{self, *};
 use roc_types::subs::{Descriptor, FlatType, Mark, OptVariable, Subs, Variable};
-use roc_types::types::{gather_fields, ErrorType, Mismatch, RecordStructure};
+use roc_types::types::{gather_fields, ErrorType, Mismatch, RecordField, RecordStructure};
 
 macro_rules! mismatch {
     () => {{
@@ -319,17 +319,27 @@ fn unify_shared_fields(
     subs: &mut Subs,
     pool: &mut Pool,
     ctx: &Context,
-    shared_fields: MutMap<Lowercase, (Variable, Variable)>,
-    other_fields: MutMap<Lowercase, Variable>,
+    shared_fields: MutMap<Lowercase, (RecordField<Variable>, RecordField<Variable>)>,
+    other_fields: MutMap<Lowercase, RecordField<Variable>>,
     ext: Variable,
 ) -> Outcome {
     let mut matching_fields = MutMap::default();
     let num_shared_fields = shared_fields.len();
 
     for (name, (actual, expected)) in shared_fields {
-        let problems = unify_pool(subs, pool, actual, expected);
+        let problems = unify_pool(subs, pool, actual.into_inner(), expected.into_inner());
 
         if problems.is_empty() {
+            use RecordField::*;
+
+            // If either field is Required, both are Required.
+            let actual = match (actual, expected) {
+                (Required(val), Required(_)) => Required(val),
+                (Required(val), Optional(_)) => Required(val),
+                (Optional(val), Required(_)) => Required(val),
+                (Optional(val), Optional(_)) => Optional(val),
+            };
+
             matching_fields.insert(name, actual);
         }
     }