special handling for Completion/NormalCompletion

tc39 · Sep 17, 2024 · 5a93027 · 5a93027
1 parent 0e610e6
commit 5a93027
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Showing 2 changed files with 41 additions and 7 deletions.
diff --git a/src/type-logic.ts b/src/type-logic.ts
@@ -207,11 +207,15 @@ export function meet(a: Type, b: Type): Type {
     // union is join. meet distributes over join.
     return a.of.map(t => meet(t, b)).reduce(join);
   }
-  if (
-    (a.kind === 'list' && b.kind === 'list') ||
-    (a.kind === 'normal completion' && b.kind === 'normal completion')
-  ) {
-    return { kind: a.kind, of: meet(a.of, b.of) };
+  if (a.kind === 'list' && b.kind === 'list') {
+    return { kind: 'list', of: meet(a.of, b.of) };
+  }
+  if (a.kind === 'normal completion' && b.kind === 'normal completion') {
+    const inner = meet(a.of, b.of);
+    if (inner.kind === 'never') {
+      return { kind: 'never' };
+    }
+    return { kind: 'normal completion', of: inner };
   }
   return { kind: 'never' };
 }
@@ -398,6 +402,29 @@ export function typeFromExpr(expr: Expr, biblio: Biblio): Type {
       }
       const calleeName = callee[0].contents;
 
+      // special case: `Completion` is identity on completions
+      if (expr.name === 'call' && calleeName === 'Completion' && expr.arguments.length === 1) {
+        const inner = typeFromExpr(expr.arguments[0], biblio);
+        if (!isCompletion(inner)) {
+          // probably unknown, we might as well refine to "some completion"
+          return {
+            kind: 'union',
+            of: [
+              { kind: 'normal completion', of: { kind: 'unknown' } },
+              { kind: 'abrupt completion' },
+            ],
+          };
+        }
+      }
+      // special case: `NormalCompletion` wraps its input
+      if (
+        expr.name === 'call' &&
+        calleeName === 'NormalCompletion' &&
+        expr.arguments.length === 1
+      ) {
+        return { kind: 'normal completion', of: typeFromExpr(expr.arguments[0], biblio) };
+      }
+
       const biblioEntry = biblio.byAoid(calleeName);
       if (biblioEntry?.signature?.return == null) {
         break;

diff --git a/test/typecheck.js b/test/typecheck.js
@@ -1511,7 +1511,7 @@ describe('type system', () => {
     await assertTypeError(
       'an ECMAScript language value',
       'NormalCompletion(42)',
-      'argument type (a normal completion) does not look plausibly assignable to parameter type (ECMAScript language value)',
+      'argument type (a normal completion containing 42) does not look plausibly assignable to parameter type (ECMAScript language value)',
       [completionBiblio],
     );
 
@@ -1535,7 +1535,14 @@ describe('type system', () => {
     await assertTypeError(
       'a Boolean',
       'NormalCompletion(*false*)',
-      'argument type (a normal completion) does not look plausibly assignable to parameter type (Boolean)',
+      'argument type (a normal completion containing false) does not look plausibly assignable to parameter type (Boolean)',
+      [completionBiblio],
+    );
+
+    await assertTypeError(
+      'a normal completion containing a Number',
+      'NormalCompletion(*false*)',
+      'argument type (a normal completion containing false) does not look plausibly assignable to parameter type (a normal completion containing Number)',
       [completionBiblio],
     );