diff --git a/compiler/forget/src/HIR/Globals.ts b/compiler/forget/src/HIR/Globals.ts
index f6d600772f..12e17b0f56 100644
--- a/compiler/forget/src/HIR/Globals.ts
+++ b/compiler/forget/src/HIR/Globals.ts
@@ -85,6 +85,7 @@ const TYPED_GLOBALS: Array<[string, BuiltInType]> = [
           restParam: null,
           returnType: { kind: "Primitive" },
           calleeEffect: Effect.Read,
+          returnValueKind: ValueKind.Immutable,
         }),
       ],
       // https://tc39.es/ecma262/multipage/indexed-collections.html#sec-array.from
@@ -104,6 +105,7 @@ const TYPED_GLOBALS: Array<[string, BuiltInType]> = [
           restParam: Effect.Read,
           returnType: { kind: "Object", shapeId: BuiltInArrayId },
           calleeEffect: Effect.Read,
+          returnValueKind: ValueKind.Mutable,
         }),
       ],
     ]),
@@ -122,6 +124,7 @@ const TYPED_GLOBALS: Array<[string, BuiltInType]> = [
           restParam: Effect.Read,
           returnType: { kind: "Primitive" },
           calleeEffect: Effect.Read,
+          returnValueKind: ValueKind.Immutable,
         }),
       ],
     ]),
@@ -138,6 +141,7 @@ const TYPED_GLOBALS: Array<[string, BuiltInType]> = [
           restParam: Effect.Read,
           returnType: { kind: "Primitive" },
           calleeEffect: Effect.Read,
+          returnValueKind: ValueKind.Immutable,
         }),
       ],
       [
@@ -147,6 +151,7 @@ const TYPED_GLOBALS: Array<[string, BuiltInType]> = [
           restParam: Effect.Read,
           returnType: { kind: "Primitive" },
           calleeEffect: Effect.Read,
+          returnValueKind: ValueKind.Immutable,
         }),
       ],
       [
@@ -156,6 +161,7 @@ const TYPED_GLOBALS: Array<[string, BuiltInType]> = [
           restParam: Effect.Read,
           returnType: { kind: "Primitive" },
           calleeEffect: Effect.Read,
+          returnValueKind: ValueKind.Immutable,
         }),
       ],
       [
@@ -165,6 +171,7 @@ const TYPED_GLOBALS: Array<[string, BuiltInType]> = [
           restParam: Effect.Read,
           returnType: { kind: "Primitive" },
           calleeEffect: Effect.Read,
+          returnValueKind: ValueKind.Immutable,
         }),
       ],
       [
@@ -174,6 +181,7 @@ const TYPED_GLOBALS: Array<[string, BuiltInType]> = [
           restParam: Effect.Read,
           returnType: { kind: "Primitive" },
           calleeEffect: Effect.Read,
+          returnValueKind: ValueKind.Immutable,
         }),
       ],
       [
@@ -183,6 +191,7 @@ const TYPED_GLOBALS: Array<[string, BuiltInType]> = [
           restParam: Effect.Read,
           returnType: { kind: "Primitive" },
           calleeEffect: Effect.Read,
+          returnValueKind: ValueKind.Immutable,
         }),
       ],
     ]),
@@ -194,6 +203,7 @@ const TYPED_GLOBALS: Array<[string, BuiltInType]> = [
       restParam: Effect.Read,
       returnType: { kind: "Primitive" },
       calleeEffect: Effect.Read,
+      returnValueKind: ValueKind.Immutable,
     }),
   ],
   [
@@ -203,6 +213,7 @@ const TYPED_GLOBALS: Array<[string, BuiltInType]> = [
       restParam: Effect.Read,
       returnType: { kind: "Primitive" },
       calleeEffect: Effect.Read,
+      returnValueKind: ValueKind.Immutable,
     }),
   ],
   [
@@ -212,6 +223,7 @@ const TYPED_GLOBALS: Array<[string, BuiltInType]> = [
       restParam: Effect.Read,
       returnType: { kind: "Primitive" },
       calleeEffect: Effect.Read,
+      returnValueKind: ValueKind.Immutable,
     }),
   ],
   // TODO: rest of Global objects
diff --git a/compiler/forget/src/HIR/ObjectShape.ts b/compiler/forget/src/HIR/ObjectShape.ts
index 43459357f0..873140f75c 100644
--- a/compiler/forget/src/HIR/ObjectShape.ts
+++ b/compiler/forget/src/HIR/ObjectShape.ts
@@ -6,7 +6,7 @@
  */
 
 import invariant from "invariant";
-import { Effect } from "./HIR";
+import { Effect, ValueKind } from "./HIR";
 import {
   BuiltInType,
   FunctionType,
@@ -100,6 +100,7 @@ export type FunctionSignature = {
   positionalParams: Array<Effect>;
   restParam: Effect | null;
   returnType: BuiltInType | PolyType;
+  returnValueKind: ValueKind;
   calleeEffect: Effect;
 };
 
@@ -138,6 +139,7 @@ addObject(BUILTIN_SHAPES, BuiltInArrayId, [
       restParam: null,
       returnType: { kind: "Poly" },
       calleeEffect: Effect.Read,
+      returnValueKind: ValueKind.Mutable,
     }),
   ],
   [
@@ -150,6 +152,7 @@ addObject(BUILTIN_SHAPES, BuiltInArrayId, [
         shapeId: BuiltInArrayId,
       },
       calleeEffect: Effect.Read,
+      returnValueKind: ValueKind.Mutable,
     }),
   ],
   ["length", PRIMITIVE_TYPE],
@@ -160,6 +163,7 @@ addObject(BUILTIN_SHAPES, BuiltInArrayId, [
       restParam: Effect.Capture,
       returnType: PRIMITIVE_TYPE,
       calleeEffect: Effect.Store,
+      returnValueKind: ValueKind.Immutable,
     }),
   ],
   // TODO: rest of Array properties
@@ -174,6 +178,7 @@ addObject(BUILTIN_SHAPES, BuiltInObjectId, [
       restParam: null,
       returnType: PRIMITIVE_TYPE,
       calleeEffect: Effect.Read,
+      returnValueKind: ValueKind.Immutable,
     }),
   ],
   // TODO:
diff --git a/compiler/forget/src/Inference/InferReferenceEffects.ts b/compiler/forget/src/Inference/InferReferenceEffects.ts
index c58b8fe872..edfc37dbc9 100644
--- a/compiler/forget/src/Inference/InferReferenceEffects.ts
+++ b/compiler/forget/src/Inference/InferReferenceEffects.ts
@@ -729,6 +729,8 @@ function inferBlock(
 
         const effects =
           signature !== null ? getFunctionEffects(instrValue, signature) : null;
+        const returnValueKind =
+          signature !== null ? signature.returnValueKind : ValueKind.Mutable;
         for (let i = 0; i < instrValue.args.length; i++) {
           const arg = instrValue.args[i];
           const place = arg.kind === "Identifier" ? arg : arg.place;
@@ -749,7 +751,7 @@ function inferBlock(
           state.reference(instrValue.callee, Effect.Mutate);
         }
 
-        state.initialize(instrValue, ValueKind.Mutable);
+        state.initialize(instrValue, returnValueKind);
         state.define(instr.lvalue, instrValue);
         instr.lvalue.effect = Effect.Mutate;
         continue;