From 77acfe571f60a6d00fa87cbd5a8a182474b08e18 Mon Sep 17 00:00:00 2001
From: Tim <hello@timsmart.co>
Date: Fri, 26 Apr 2024 15:45:07 +1200
Subject: [PATCH] add $is & $match helpers to Data.TaggedEnum constructors
 (#2620)

---
 .changeset/small-birds-beam.md    | 28 +++++++++++++++++++++
 packages/effect/src/Data.ts       | 42 ++++++++++++++++++++++++++++---
 packages/effect/test/Data.test.ts | 16 +++++++++++-
 3 files changed, 82 insertions(+), 4 deletions(-)
 create mode 100644 .changeset/small-birds-beam.md

diff --git a/.changeset/small-birds-beam.md b/.changeset/small-birds-beam.md
new file mode 100644
index 0000000000..683608e9ba
--- /dev/null
+++ b/.changeset/small-birds-beam.md
@@ -0,0 +1,28 @@
+---
+"effect": minor
+---
+
+add $is & $match helpers to Data.TaggedEnum constructors
+
+```ts
+import { Data } from "effect"
+
+type HttpError = Data.TaggedEnum<{
+  NotFound: {}
+  InternalServerError: { reason: string }
+}>
+const { $is, $match, InternalServerError, NotFound } =
+  Data.taggedEnum<HttpError>()
+
+// create a matcher
+const matcher = $match({
+  NotFound: () => 0,
+  InternalServerError: () => 1
+})
+
+// true
+$is("NotFound")(NotFound())
+
+// false
+$is("NotFound")(InternalServerError({ reason: "fail" }))
+```
diff --git a/packages/effect/src/Data.ts b/packages/effect/src/Data.ts
index ccc1f7d442..42dfb900c2 100644
--- a/packages/effect/src/Data.ts
+++ b/packages/effect/src/Data.ts
@@ -5,6 +5,7 @@ import type * as Cause from "./Cause.js"
 import * as core from "./internal/core.js"
 import * as internal from "./internal/data.js"
 import { StructuralPrototype } from "./internal/effectable.js"
+import * as Predicate from "./Predicate.js"
 import type * as Types from "./Types.js"
 
 /**
@@ -331,6 +332,23 @@ export declare namespace TaggedEnum {
     A extends { readonly _tag: string },
     K extends A["_tag"]
   > = Extract<A, { readonly _tag: K }>
+
+  /**
+   * @since 3.1.0
+   */
+  export type Constructor<A extends { readonly _tag: string }> = Types.Simplify<
+    & {
+      readonly [Tag in A["_tag"]]: Case.Constructor<Extract<A, { readonly _tag: Tag }>, "_tag">
+    }
+    & {
+      readonly $is: <Tag extends A["_tag"]>(tag: Tag) => (u: unknown) => u is Extract<A, { readonly _tag: Tag }>
+      readonly $match: <
+        Cases extends {
+          readonly [Tag in A["_tag"]]: (args: Extract<A, { readonly _tag: Tag }>) => any
+        }
+      >(cases: Cases) => (value: A) => ReturnType<Cases[A["_tag"]]>
+    }
+  >
 }
 
 /**
@@ -407,16 +425,34 @@ export const taggedEnum: {
     ) => TaggedEnum.Value<TaggedEnum.Kind<Z, A, B, C, D>, Tag>
   }
 
-  <A extends { readonly _tag: string }>(): {
-    readonly [Tag in A["_tag"]]: Case.Constructor<Extract<A, { readonly _tag: Tag }>, "_tag">
-  }
+  <A extends { readonly _tag: string }>(): TaggedEnum.Constructor<A>
 } = () =>
   new Proxy({}, {
     get(_target, tag, _receiver) {
+      if (tag === "$is") {
+        return taggedIs
+      } else if (tag === "$match") {
+        return taggedMatch
+      }
       return tagged(tag as string)
     }
   }) as any
 
+function taggedIs<A extends { readonly _tag: string }, Tag extends A["_tag"]>(tag: Tag) {
+  return Predicate.isTagged(tag)
+}
+
+function taggedMatch<
+  A extends { readonly _tag: string },
+  Cases extends {
+    readonly [K in A["_tag"]]: (args: Extract<A, { readonly _tag: K }>) => any
+  }
+>(cases: Cases) {
+  return function(value: A): ReturnType<Cases[A["_tag"]]> {
+    return cases[value._tag as A["_tag"]](value as any)
+  }
+}
+
 /**
  * Provides a constructor for a Case Class.
  *
diff --git a/packages/effect/test/Data.test.ts b/packages/effect/test/Data.test.ts
index dd57196e26..974c91bfe7 100644
--- a/packages/effect/test/Data.test.ts
+++ b/packages/effect/test/Data.test.ts
@@ -177,7 +177,12 @@ describe("Data", () => {
       NotFound: {}
       InternalServerError: { reason: string }
     }>
-    const { InternalServerError, NotFound } = Data.taggedEnum<HttpError>()
+    const {
+      $is,
+      $match,
+      InternalServerError,
+      NotFound
+    } = Data.taggedEnum<HttpError>()
 
     const a = NotFound()
     const b = InternalServerError({ reason: "test" })
@@ -191,6 +196,15 @@ describe("Data", () => {
 
     expect(Equal.equals(a, b)).toBe(false)
     expect(Equal.equals(b, c)).toBe(true)
+
+    expect($is("NotFound")(a)).toBe(true)
+    expect($is("InternalServerError")(a)).toBe(false)
+    const matcher = $match({
+      NotFound: () => 0,
+      InternalServerError: () => 1
+    })
+    expect(matcher(a)).toEqual(0)
+    expect(matcher(b)).toEqual(1)
   })
 
   it("taggedEnum - generics", () => {