stop unification when ext vars don't unify

compiler/reporting/src/type_error.rs

@@ -1025,7 +1025,7 @@ mod report_text {
         } else {
             let entry_to_text = |(tag_name, arguments)| concat(vec![tag_name, separate(arguments)]);
 
-            let starts = std::iter::once(plain_text("[")).chain(std::iter::repeat(plain_text(",")));
+            let starts = std::iter::once(plain_text("[ ")).chain(std::iter::repeat(plain_text(", ")));
 
             let mut lines: Vec<_> = entries
                 .into_iter()
@@ -1033,7 +1033,7 @@ mod report_text {
                 .map(|(entry, start)| concat(vec![start, entry_to_text(entry)]))
                 .collect();
 
-            lines.push(plain_text("]"));
+            lines.push(plain_text(" ]"));
             lines.push(ext_text);
 
             concat(lines)
@@ -1056,7 +1056,7 @@ mod report_text {
         } else {
             let entry_to_text = |(tag_name, arguments)| concat(vec![tag_name, separate(arguments)]);
 
-            let starts = std::iter::once(plain_text("[")).chain(std::iter::repeat(plain_text(",")));
+            let starts = std::iter::once(plain_text("[ ")).chain(std::iter::repeat(plain_text(", ")));
 
             let mut lines: Vec<_> = entries
                 .into_iter()
@@ -1064,7 +1064,7 @@ mod report_text {
                 .map(|(entry, start)| concat(vec![start, entry_to_text(entry)]))
                 .collect();
 
-            lines.push(plain_text("]"));
+            lines.push(plain_text(" ]"));
             lines.push(ext_text);
 
             lines.push(plain_text(" as "));

compiler/reporting/tests/test_reporting.rs

@@ -1190,6 +1190,39 @@ mod test_reporting {
 
                     { foo : Int }
 
+
+                "#
+            ),
+        )
+    }
+
+    #[test]
+    fn tag_mismatch() {
+        report_problem_as(
+            indoc!(
+                r#"
+                f : [ Red, Green ] -> Bool
+                f = \_ -> True
+
+                f Blue
+                "#
+            ),
+            indoc!(
+                r#"
+                The 1st argument to `f` is not what I expect:
+
+                4 ┆  f Blue
+                  ┆    ^^^^
+
+                This `Blue` global tag application produces:
+
+                    [ Blue, Green, Red ]
+
+                But `f` needs the 1st argument to be:
+
+                    [ Green, Red ]
+
+
                 "#
             ),
         )

compiler/unify/src/unify.rs

@@ -116,7 +116,7 @@ pub fn unify_pool(subs: &mut Subs, pool: &mut Pool, var1: Variable, var2: Variab
 }
 
 fn unify_context(subs: &mut Subs, pool: &mut Pool, ctx: Context) -> Outcome {
-    // println!( "{:?} {:?} ~ {:?} {:?}", ctx.first, ctx.first_desc.content, ctx.second, ctx.second_desc.content);
+    // println!( "{:?} {:?} ~ {:?} {:?}", ctx.first, ctx.first_desc.content, ctx.second, ctx.second_desc.content,);
     match &ctx.first_desc.content {
         FlexVar(opt_name) => unify_flex(subs, pool, &ctx, opt_name, &ctx.second_desc.content),
         RigidVar(name) => unify_rigid(subs, &ctx, name, &ctx.second_desc.content),
@@ -245,6 +245,11 @@ fn unify_record(
     if unique_fields1.is_empty() {
         if unique_fields2.is_empty() {
             let ext_problems = unify_pool(subs, pool, rec1.ext, rec2.ext);
+
+            if !ext_problems.is_empty() {
+                return ext_problems;
+            }
+
             let other_fields = MutMap::default();
             let mut field_problems =
                 unify_shared_fields(subs, pool, ctx, shared_fields, other_fields, rec1.ext);
@@ -256,6 +261,11 @@ fn unify_record(
             let flat_type = FlatType::Record(unique_fields2, rec2.ext);
             let sub_record = fresh(subs, pool, ctx, Structure(flat_type));
             let ext_problems = unify_pool(subs, pool, rec1.ext, sub_record);
+
+            if !ext_problems.is_empty() {
+                return ext_problems;
+            }
+
             let other_fields = MutMap::default();
             let mut field_problems =
                 unify_shared_fields(subs, pool, ctx, shared_fields, other_fields, sub_record);
@@ -268,6 +278,11 @@ fn unify_record(
         let flat_type = FlatType::Record(unique_fields1, rec1.ext);
         let sub_record = fresh(subs, pool, ctx, Structure(flat_type));
         let ext_problems = unify_pool(subs, pool, sub_record, rec2.ext);
+
+        if !ext_problems.is_empty() {
+            return ext_problems;
+        }
+
         let other_fields = MutMap::default();
         let mut field_problems =
             unify_shared_fields(subs, pool, ctx, shared_fields, other_fields, sub_record);
@@ -286,7 +301,14 @@ fn unify_record(
         let sub2 = fresh(subs, pool, ctx, Structure(flat_type2));
 
         let rec1_problems = unify_pool(subs, pool, rec1.ext, sub2);
+        if !rec1_problems.is_empty() {
+            return rec1_problems;
+        }
+
         let rec2_problems = unify_pool(subs, pool, sub1, rec2.ext);
+        if !rec2_problems.is_empty() {
+            return rec2_problems;
+        }
 
         let mut field_problems =
             unify_shared_fields(subs, pool, ctx, shared_fields, other_fields, ext);
@@ -362,6 +384,11 @@ fn unify_tag_union(
     if unique_tags1.is_empty() {
         if unique_tags2.is_empty() {
             let ext_problems = unify_pool(subs, pool, rec1.ext, rec2.ext);
+
+            if !ext_problems.is_empty() {
+                return ext_problems;
+            }
+
             let mut tag_problems = unify_shared_tags(
                 subs,
                 pool,
@@ -379,6 +406,11 @@ fn unify_tag_union(
             let flat_type = FlatType::TagUnion(unique_tags2, rec2.ext);
             let sub_record = fresh(subs, pool, ctx, Structure(flat_type));
             let ext_problems = unify_pool(subs, pool, rec1.ext, sub_record);
+
+            if !ext_problems.is_empty() {
+                return ext_problems;
+            }
+
             let mut tag_problems = unify_shared_tags(
                 subs,
                 pool,
@@ -397,6 +429,11 @@ fn unify_tag_union(
         let flat_type = FlatType::TagUnion(unique_tags1, rec1.ext);
         let sub_record = fresh(subs, pool, ctx, Structure(flat_type));
         let ext_problems = unify_pool(subs, pool, sub_record, rec2.ext);
+
+        if !ext_problems.is_empty() {
+            return ext_problems;
+        }
+
         let mut tag_problems = unify_shared_tags(
             subs,
             pool,
@@ -420,15 +457,22 @@ fn unify_tag_union(
         let sub1 = fresh(subs, pool, ctx, Structure(flat_type1));
         let sub2 = fresh(subs, pool, ctx, Structure(flat_type2));
 
-        let rec1_problems = unify_pool(subs, pool, rec1.ext, sub2);
+        let ext1_problems = unify_pool(subs, pool, rec1.ext, sub2);
-        let rec2_problems = unify_pool(subs, pool, sub1, rec2.ext);
+        if !ext1_problems.is_empty() {
+            return ext1_problems;
+        }
+
+        let ext2_problems = unify_pool(subs, pool, sub1, rec2.ext);
+        if !ext2_problems.is_empty() {
+            return ext2_problems;
+        }
 
         let mut tag_problems =
             unify_shared_tags(subs, pool, ctx, shared_tags, other_tags, ext, recursion_var);
 
-        tag_problems.reserve(rec1_problems.len() + rec2_problems.len());
+        tag_problems.reserve(ext1_problems.len() + ext2_problems.len());
-        tag_problems.extend(rec1_problems);
+        tag_problems.extend(ext1_problems);
-        tag_problems.extend(rec2_problems);
+        tag_problems.extend(ext2_problems);
 
         tag_problems
     }
@@ -610,7 +654,7 @@ fn unify_flat_type(
             }
         }
         (other1, other2) => mismatch!(
-            "Trying to two flat types that are incompatible: {:?} ~ {:?}",
+            "Trying to unify two flat types that are incompatible: {:?} ~ {:?}",
             other1,
             other2
         ),