TomokiMiyauci
diff --git a/‎api.ts
+1 b/‎api.ts
+1
diff --git a/‎mod.ts
+2 b/‎mod.ts
+2
diff --git a/‎src/tap.ts
+25 b/‎src/tap.ts
+25
diff --git a/‎src/types/index.ts
+9 b/‎src/types/index.ts
+9
diff --git a/‎test/tag.test.ts
+27 b/‎test/tag.test.ts
+27
@@ -301,6 +301,7 @@ const api: Api = {
   // splitEvery: {
   //   fonction: undefined
   // },
+  tap: ['lodash', 'rambda', 'ramda'],
   take: ALL_MODULES,
   takeLast: {
     ramda: 'takeLast',
 
@@ -74,6 +74,7 @@ export { T } from './src/T.ts'
 export { tail } from './src/tail.ts'
 export { take } from './src/take.ts'
 export { takeLast } from './src/takeLast.ts'
+export { tap } from './src/tap.ts'
 export type { Trim } from './src/trim.ts'
 export { trim } from './src/trim.ts'
 export type { TrimLeft } from './src/trimLeft.ts'
@@ -83,6 +84,7 @@ export { trimRight } from './src/trimRight.ts'
 export { tryCatch } from './src/tryCatch.ts'
 export type {
   AnyFn,
+  Arity1Fn,
   Empty,
   Falsy,
   FalsyLike,
 
@@ -0,0 +1,25 @@
+// Copyright 2021-present the Fonction authors. All rights reserved. MIT license.
+import { Arity1Fn } from './types/index.ts'
+
+/**
+ * Runs the given function with the supplied value, then returns the value.
+ *
+ * @param fn - The function to call with `val`. The return value of fn will be thrown away.
+ * @returns The result of `(val) => fn(val)`
+ *
+ * @example
+ * ```ts
+ * tap(console.log)('hello') // hello
+ * // log: hello
+ * ```
+ *
+ * @beta
+ */
+const tap =
+  <T>(fn: Arity1Fn<T>) =>
+  <R extends T>(val: R): R => {
+    fn(val)
+    return val
+  }
+
+export { tap }
@@ -9,6 +9,15 @@
  */
 export type AnyFn<T = any, U = unknown> = (...args: T[]) => U
 
+/**
+ * Type of arity 1 function.
+ *
+ * @beta
+ *
+ * @typeParam T - Argument types
+ */
+export type Arity1Fn<T = any, U = unknown> = (args: T) => U
+
 /**
  * Alias for Primitive values types.
  *
 
@@ -0,0 +1,27 @@
+// Copyright 2021-present the Fonction authors. All rights reserved. MIT license.
+import { assertEquals } from '../dev_deps.ts'
+import { tap } from '../src/tap.ts'
+import { Arity1Fn } from '../src/types/index.ts'
+import { assertEqual } from './asserts.ts'
+
+Deno.test('tap', () => {
+  const table: [Arity1Fn, unknown][] = [
+    [console.log, 'hello'],
+    [() => console.log(1), ''],
+    [(a: string) => console.log(a), ''],
+    [(a: unknown[]) => console.log(a), ['hello', 'world']]
+  ]
+
+  table.forEach(([val, expected]) => {
+    assertEquals(tap(val)(expected), expected, `tap(${val}) -> ${expected}`)
+  })
+
+  assertEqual<''>(tap(console.log)(''))
+  assertEqual<''>(tap((_: string) => true)(''))
+  assertEqual<''>(tap(() => true)(''))
+  assertEqual<string[]>(tap((val: unknown[]) => val)(['hello']))
+  assertEqual<string[]>(tap((val: unknown[]) => val)(['hello']))
+  assertEqual<readonly ['hello']>(
+    tap((val: readonly unknown[]) => val)(['hello'] as const)
+  )
+})