Add tap for Identity (#1943)

src/Identity.ts

@@ -4,7 +4,7 @@
 import { Alt1 } from './Alt'
 import { Applicative as ApplicativeHKT, Applicative1 } from './Applicative'
 import { apFirst as apFirst_, Apply1, apS as apS_, apSecond as apSecond_ } from './Apply'
-import { bind as bind_, Chain1, chainFirst as chainFirst_ } from './Chain'
+import { bind as bind_, Chain1, tap as tap_ } from './Chain'
 import { ChainRec1, tailRec } from './ChainRec'
 import { Comonad1 } from './Comonad'
 import { Eq } from './Eq'
@@ -275,10 +275,21 @@ export const Monad: Monad1<URI> = {
  * Composes computations in sequence, using the return value of one computation to determine the next computation and
  * keeping only the result of the first.
  *
- * @category sequencing
+ * @category combinators
+ * @since 2.16.7
+ */
+export const tap: {
+  <A, _>(self: Identity<A>, f: (a: A) => Identity<_>): Identity<A>
+  <A, _>(f: (a: A) => Identity<_>): (self: Identity<A>) => Identity<A>
+} = /*#__PURE__*/ dual(2, tap_(Chain))
+
+/**
+ * Alias of `tap`
+ *
+ * @category legacy
  * @since 2.0.0
  */
-export const chainFirst: <A, B>(f: (a: A) => B) => (first: A) => A = /*#__PURE__*/ chainFirst_(Chain)
+export const chainFirst: <A, B>(f: (a: A) => B) => (first: A) => A = tap
 
 /**
  * @category instances

test/Identity.ts

@@ -36,6 +36,11 @@ describe.concurrent('Identity', () => {
       U.deepStrictEqual(pipe(1, _.chain(f)), 2)
     })
 
+    it('tap', () => {
+      const f = (n: number) => n * 2
+      U.deepStrictEqual(pipe(1, _.tap(f)), 1)
+    })
+
     it('chainFirst', () => {
       const f = (n: number) => n * 2
       U.deepStrictEqual(pipe(1, _.chainFirst(f)), 1)