From fdd031604ffa7f6ef3b93ed0505b75277ceaaeac Mon Sep 17 00:00:00 2001
From: Giulio Canti <giulio.canti@gmail.com>
Date: Thu, 13 Feb 2025 15:16:19 +0100
Subject: [PATCH] Preserve function `length` property in `Effect.fn` /
 `Effect.fnUntraced`, closes #4435

---
 .changeset/nice-numbers-hammer.md      | 47 ++++++++++++++++++++++++++
 packages/effect/src/Effect.ts          | 41 +++++++++++++---------
 packages/effect/test/Effect/fn.test.ts | 26 ++++++++++++++
 packages/effect/vitest.config.ts       |  2 +-
 4 files changed, 99 insertions(+), 17 deletions(-)
 create mode 100644 .changeset/nice-numbers-hammer.md

diff --git a/.changeset/nice-numbers-hammer.md b/.changeset/nice-numbers-hammer.md
new file mode 100644
index 0000000000..7a36406a55
--- /dev/null
+++ b/.changeset/nice-numbers-hammer.md
@@ -0,0 +1,47 @@
+---
+"effect": patch
+---
+
+Preserve function `length` property in `Effect.fn` / `Effect.fnUntraced`, closes #4435
+
+Previously, functions created with `Effect.fn` and `Effect.fnUntraced` always had a `.length` of `0`, regardless of their actual number of parameters. This has been fixed so that the `length` property correctly reflects the expected number of arguments.
+
+**Before**
+
+```ts
+import { Effect } from "effect"
+
+const fn1 = Effect.fn("fn1")(function* (n: number) {
+  return n
+})
+
+console.log(fn1.length)
+// Output: 0 ❌ (incorrect)
+
+const fn2 = Effect.fnUntraced(function* (n: number) {
+  return n
+})
+
+console.log(fn2.length)
+// Output: 0 ❌ (incorrect)
+```
+
+**After**
+
+```ts
+import { Effect } from "effect"
+
+const fn1 = Effect.fn("fn1")(function* (n: number) {
+  return n
+})
+
+console.log(fn1.length)
+// Output: 1 ✅ (correct)
+
+const fn2 = Effect.fnUntraced(function* (n: number) {
+  return n
+})
+
+console.log(fn2.length)
+// Output: 1 ✅ (correct)
+```
diff --git a/packages/effect/src/Effect.ts b/packages/effect/src/Effect.ts
index 77dc89aefb..59ad3980c8 100644
--- a/packages/effect/src/Effect.ts
+++ b/packages/effect/src/Effect.ts
@@ -13742,7 +13742,7 @@ export const fn:
     const errorDef = new Error()
     Error.stackTraceLimit = limit
     if (typeof nameOrBody !== "string") {
-      return function(this: any, ...args: Array<any>) {
+      return defineLength(nameOrBody.length, function(this: any, ...args: Array<any>) {
         const limit = Error.stackTraceLimit
         Error.stackTraceLimit = 2
         const errorCall = new Error()
@@ -13759,12 +13759,12 @@ export const fn:
           errorDef,
           errorCall
         })
-      } as any
+      }) as any
     }
     const name = nameOrBody
     const options = pipeables[0]
-    return (body: Function, ...pipeables: Array<any>) => {
-      return function(this: any, ...args: Array<any>) {
+    return (body: Function, ...pipeables: Array<any>) =>
+      defineLength(body.length, function(this: any, ...args: Array<any>) {
         const limit = Error.stackTraceLimit
         Error.stackTraceLimit = 2
         const errorCall = new Error()
@@ -13779,10 +13779,16 @@ export const fn:
           errorDef,
           errorCall
         })
-      }
-    }
+      })
   }
 
+function defineLength<F extends Function>(length: number, fn: F) {
+  return Object.defineProperty(fn, "length", {
+    value: length,
+    configurable: true
+  })
+}
+
 function fnApply(options: {
   readonly self: any
   readonly body: Function
@@ -13847,14 +13853,17 @@ function fnApply(options: {
  * @category Tracing
  */
 export const fnUntraced: fn.Gen = (body: Function, ...pipeables: Array<any>) =>
-  pipeables.length === 0
-    ? function(this: any, ...args: Array<any>) {
-      return core.fromIterator(() => body.apply(this, args))
-    }
-    : function(this: any, ...args: Array<any>) {
-      let effect = core.fromIterator(() => body.apply(this, args))
-      for (const x of pipeables) {
-        effect = x(effect)
+  defineLength(
+    body.length,
+    pipeables.length === 0
+      ? function(this: any, ...args: Array<any>) {
+        return core.fromIterator(() => body.apply(this, args))
       }
-      return effect
-    }
+      : function(this: any, ...args: Array<any>) {
+        let effect = core.fromIterator(() => body.apply(this, args))
+        for (const x of pipeables) {
+          effect = x(effect)
+        }
+        return effect
+      }
+  )
diff --git a/packages/effect/test/Effect/fn.test.ts b/packages/effect/test/Effect/fn.test.ts
index e4299a7137..a2ec5d6aa5 100644
--- a/packages/effect/test/Effect/fn.test.ts
+++ b/packages/effect/test/Effect/fn.test.ts
@@ -64,4 +64,30 @@ describe("Effect.fn", () => {
       assertInstanceOf(cause.right.defect, Error)
       strictEqual(cause.right.defect.message, "test2")
     }))
+
+  it("should preserve the function length", () => {
+    const f = function*(n: number) {
+      return n
+    }
+    const fn1 = Effect.fn("fn1")(f)
+    strictEqual(fn1.length, 1)
+    strictEqual(Effect.runSync(fn1(2)), 2)
+    const fn2 = Effect.fn(f)
+    strictEqual(fn2.length, 1)
+    strictEqual(Effect.runSync(fn2(2)), 2)
+  })
+})
+
+describe("Effect.fnUntraced", () => {
+  it("should preserve the function length", () => {
+    const f = function*(n: number) {
+      return n
+    }
+    const fn1 = Effect.fnUntraced(f)
+    strictEqual(fn1.length, 1)
+    strictEqual(Effect.runSync(fn1(2)), 2)
+    const fn2 = Effect.fnUntraced(f, (x) => x)
+    strictEqual(fn2.length, 1)
+    strictEqual(Effect.runSync(fn2(2)), 2)
+  })
 })
diff --git a/packages/effect/vitest.config.ts b/packages/effect/vitest.config.ts
index c33fc3f516..ba4df2d008 100644
--- a/packages/effect/vitest.config.ts
+++ b/packages/effect/vitest.config.ts
@@ -5,7 +5,7 @@ const config: ViteUserConfig = {
   test: {
     coverage: {
       reporter: ["html"],
-      include: ["src/Match.ts", "src/internal/matcher.ts"]
+      include: ["src/Effect.ts"]
     }
   }
 }