From 36266c607900b8fe096e8b122d485cd8c1d63998 Mon Sep 17 00:00:00 2001
From: Mark Dawson <mark.dawson@polecat.com>
Date: Wed, 3 Jun 2026 17:47:07 +0100
Subject: [PATCH] fix(Effect): handle transpiled generator bodies in Effect.fn

Effect.fn(name)(body) crashes at runtime with
"RuntimeException: Not a valid effect: {}" when body is a generator
function that has been lowered by a bundler/transpiler into a plain
function returning an iterator IIFE (e.g. babel-preset-expo on React
Native / Hermes). isGeneratorFunction returns false for such bodies,
so the iterator was passed through as if it were an Effect.

Detect the iterator-shape post-apply and re-wrap it with
core.fromIterator. The first iterator is consumed immediately; subsequent
invocations re-apply body to produce fresh iterators, preserving
Effect.fn's reusable-wrapper contract.

fnUntraced is unaffected: it always wraps with fromIterator already.
---
 .changeset/fix-fn-iterator-result.md   |  7 +++++++
 packages/effect/src/Effect.ts          | 25 ++++++++++++++++++++++++-
 packages/effect/test/Effect/fn.test.ts | 17 +++++++++++++++++
 3 files changed, 48 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/fix-fn-iterator-result.md

diff --git a/.changeset/fix-fn-iterator-result.md b/.changeset/fix-fn-iterator-result.md
new file mode 100644
index 00000000000..162bcfd388a
--- /dev/null
+++ b/.changeset/fix-fn-iterator-result.md
@@ -0,0 +1,7 @@
+---
+"effect": patch
+---
+
+fix(Effect): handle transpiled generator bodies in `Effect.fn`
+
+`Effect.fn(name)(body)` crashed at runtime with `RuntimeException: Not a valid effect: {}` when `body` was a generator function lowered by a compiler (e.g. `babel-preset-expo` on React Native / Hermes) into a plain function returning an iterator IIFE. Such a body fails the `isGeneratorFunction` check, so its return value was passed through as if it were an `Effect`. We now duck-type the result and re-wrap it with `fromIterator` when it is an iterator.
diff --git a/packages/effect/src/Effect.ts b/packages/effect/src/Effect.ts
index d9143c239b6..52136152c41 100644
--- a/packages/effect/src/Effect.ts
+++ b/packages/effect/src/Effect.ts
@@ -14707,7 +14707,30 @@ function fnApply(options: {
     effect = core.fromIterator(() => options.body.apply(options.self, options.args))
   } else {
     try {
-      effect = options.body.apply(options.self, options.args)
+      const result = options.body.apply(options.self, options.args)
+      // Some compilers (e.g. `babel-preset-expo` on React Native / Hermes)
+      // lower destructured-param generators into a plain function that returns
+      // an iterator IIFE. Such a body fails `isGeneratorFunction` but its
+      // return value is an iterator, not an Effect. Detect and re-wrap.
+      if (
+        result !== null &&
+        typeof result === "object" &&
+        (result as any)[EffectTypeId] === undefined &&
+        typeof (result as any).next === "function" &&
+        typeof (result as any)[Symbol.iterator] === "function"
+      ) {
+        let firstIterator: any = result
+        effect = core.fromIterator(() => {
+          if (firstIterator !== null) {
+            const it = firstIterator
+            firstIterator = null
+            return it
+          }
+          return options.body.apply(options.self, options.args)
+        })
+      } else {
+        effect = result
+      }
     } catch (error) {
       fnError = error
       effect = die(error)
diff --git a/packages/effect/test/Effect/fn.test.ts b/packages/effect/test/Effect/fn.test.ts
index a5416625a5a..2bc053b5d18 100644
--- a/packages/effect/test/Effect/fn.test.ts
+++ b/packages/effect/test/Effect/fn.test.ts
@@ -76,6 +76,23 @@ describe("Effect.fn", () => {
     strictEqual(fn2.length, 1)
     strictEqual(Effect.runSync(fn2(2)), 2)
   })
+
+  it.effect("handles a non-generator body that returns an iterator (transpiled generator)", () =>
+    Effect.gen(function*() {
+      // Mimics `babel-preset-expo` lowering `function*({a}) { return a }` into
+      // a plain function that returns a generator IIFE iterator.
+      const body = function(arg: { a: number }) {
+        const a = arg.a
+        return (function*() {
+          return a
+        })()
+      }
+      const fn = Effect.fn("test")(body as any)
+      const v = yield* fn({ a: 42 }) as Effect.Effect<number>
+      strictEqual(v, 42)
+      const v2 = yield* fn({ a: 7 }) as Effect.Effect<number>
+      strictEqual(v2, 7)
+    }))
 })
 
 describe("Effect.fnUntraced", () => {