From 9d97015236b8f19bbf313e98065aaeb4d0322cbc Mon Sep 17 00:00:00 2001
From: mofeiZ <34200447+mofeiZ@users.noreply.github.com>
Date: Wed, 5 Apr 2023 12:53:39 -0400
Subject: [PATCH] [globals] Remove global shape for Array.from

Type inference currently assumes that a `FunctionSignature`'s effects have no
false positives. If a `mutate` effect is observed on a read-only place, Forget
currently assumes this is an user error and
[throws](https://github.com/facebook/react-forget/blob/207595e04e2be08b8f62bf21dac9d846b9651e43/forget/src/Inference/InferReferenceEffects.ts#L275-L281).

Array.from is polymorphic -- its effects are dependent on the type of its
parameters
---
 compiler/forget/src/HIR/Globals.ts | 19 +++++++++----------
 1 file changed, 9 insertions(+), 10 deletions(-)

diff --git a/compiler/forget/src/HIR/Globals.ts b/compiler/forget/src/HIR/Globals.ts
index 8770cc0ecb..a886a041d9 100644
--- a/compiler/forget/src/HIR/Globals.ts
+++ b/compiler/forget/src/HIR/Globals.ts
@@ -87,16 +87,15 @@ const TYPED_GLOBALS: Array<[string, BuiltInType]> = [
           calleeEffect: Effect.Read,
         }),
       ],
-      [
-        "from",
-        // Array.from(arrayLike, optionalFn, optionalThis)
-        addFunction(DEFAULT_SHAPES, [], {
-          positionalParams: [Effect.Mutate],
-          restParam: Effect.Read,
-          returnType: { kind: "Object", shapeId: BuiltInArrayId },
-          calleeEffect: Effect.Read,
-        }),
-      ],
+      // https://tc39.es/ecma262/multipage/indexed-collections.html#sec-array.from
+      // Array.from(arrayLike, optionalFn, optionalThis) not added because
+      // the Effect of `arrayLike` is polymorphic i.e.
+      //  - Effect.read if
+      //     - it does not have an @iterator property and is array-like
+      //       (i.e. has a length property)
+      ///    - it is an iterable object whose iterator does not mutate itself
+      //  - Effect.mutate if it is a self-mutative iterator (e.g. a generator
+      //    function)
       [
         "of",
         // Array.of(element0, ..., elementN)