proposal for Effect.Tag conundrum (#2266)

.changeset/tasty-seals-film.md

@@ -0,0 +1,5 @@
+---
+"effect": patch
+---
+
+fix Effect.Tag generated proxy functions to work with andThen/tap, or others that do function/isEffect checks

packages/effect/src/Effect.ts

@@ -5336,8 +5336,18 @@ export const Tag: <const Id extends string>(id: Id) => <Self, Type>() =>
         if (cache.has(prop)) {
           return cache.get(prop)
         }
-        // @ts-expect-error
-        const fn = (...args: Array<any>) => core.andThen(TagClass, (s: any) => s[prop](...args))
+        const fn = (...args: Array<any>) =>
+          // @ts-expect-error
+          core.andThen(TagClass, (s: any) => {
+            if (typeof s[prop] === "function") {
+              // @ts-expect-error
+              cache.set(prop, (...args: Array<any>) => core.andThen(TagClass, (s: any) => s[prop](...args)))
+              return s[prop](...args)
+            }
+            // @ts-expect-error
+            cache.set(prop, core.andThen(TagClass, (s) => s[prop]))
+            return s[prop]
+          })
         // @ts-expect-error
         const cn = core.andThen(TagClass, (s) => s[prop])
         Object.assign(fn, cn)

packages/effect/test/Effect/environment.test.ts

@@ -45,6 +45,24 @@ class NumberTag extends Effect.Tag("NumberTag")<NumberTag, number>() {
 }
 
 describe("Effect", () => {
+  describe("and Then", () => {
+    it.effect("effect tag", () =>
+      Effect.gen(function*($) {
+        const [n, s, z] = yield* $(Effect.all([
+          Effect.andThen(Effect.unit, DemoTag.getNumbers),
+          Effect.andThen(Effect.succeed("a"), DemoTag.strings),
+          Effect.andThen(Effect.succeed("a"), DemoTag.fn)
+        ]))
+        expect(n).toEqual([0, 1])
+        expect(s).toEqual(["a", "b"])
+        expect(z).toEqual(["a"])
+      }).pipe(Effect.provideService(DemoTag, {
+        getNumbers: () => [0, 1],
+        strings: ["a", "b"],
+        fn: (...args) => Array.from(args),
+        fnGen: (s) => [s]
+      })))
+  })
   it.effect("effect tag", () =>
     Effect.gen(function*($) {
       const [n, s, z] = yield* $(Effect.all([