feat(partial): add partial function

README.md

@@ -26,6 +26,30 @@ curriedFn("", 0)(false);
 curriedFn("", 0, false);
 ```
 
+## Partial application
+
+Creates a new "bound function".
+
+It has the following characteristics:
+
+- The `length` property is 0.
+- The `name` property is `partial`.
+
+### partial
+
+Partially applies function arguments.
+
+```ts
+import { partial } from "https://deno.land/x/curry@$VERSION/mod.ts";
+
+declare const fn: (a: string, b: number, c: boolean) => void;
+
+const ternary = partial(fn);
+const binary = partial(fn, "");
+const unary = partial(fn, "", 0);
+const nullary = partial(fn, "", 0, false);
+```
+
 ## API
 
 See [deno doc](https://deno.land/x/curry/mod.ts) for all APIs.

_dev_deps.ts

@@ -1,4 +1,15 @@
 // Copyright 2023-latest Tomoki Miyauchi. All rights reserved. MIT license.
 
 export { assertEquals } from "https://deno.land/std@0.190.0/testing/asserts.ts";
+export {
+  afterEach,
+  beforeEach,
+  describe,
+  it,
+} from "https://deno.land/std@0.190.0/testing/bdd.ts";
+export {
+  assertSpyCallArgs,
+  type Spy,
+  spy,
+} from "https://deno.land/std@0.190.0/testing/mock.ts";
 export const assertEqualsTypes = <T, U extends T = T>(_actual?: U): void => {};

deno.lock

@@ -4,6 +4,9 @@
     "https://deno.land/std@0.190.0/fmt/colors.ts": "d67e3cd9f472535241a8e410d33423980bec45047e343577554d3356e1f0ef4e",
     "https://deno.land/std@0.190.0/testing/_diff.ts": "1a3c044aedf77647d6cac86b798c6417603361b66b54c53331b312caeb447aea",
     "https://deno.land/std@0.190.0/testing/_format.ts": "a69126e8a469009adf4cf2a50af889aca364c349797e63174884a52ff75cf4c7",
-    "https://deno.land/std@0.190.0/testing/asserts.ts": "e16d98b4d73ffc4ed498d717307a12500ae4f2cbe668f1a215632d19fcffc22f"
+    "https://deno.land/std@0.190.0/testing/_test_suite.ts": "30f018feeb3835f12ab198d8a518f9089b1bcb2e8c838a8b615ab10d5005465c",
+    "https://deno.land/std@0.190.0/testing/asserts.ts": "e16d98b4d73ffc4ed498d717307a12500ae4f2cbe668f1a215632d19fcffc22f",
+    "https://deno.land/std@0.190.0/testing/bdd.ts": "59f7f7503066d66a12e50ace81bfffae5b735b6be1208f5684b630ae6b4de1d0",
+    "https://deno.land/std@0.190.0/testing/mock.ts": "220ed9b8151cb2cac141043d4cfea7c47673fab5d18d1c1f0943297c8afb5d13"
   }
 }

mod.ts

@@ -2,3 +2,4 @@
 // This module is browser compatible.
 
 export { curry } from "./curry.ts";
+export { partial } from "./partial.ts";

partial.ts

@@ -0,0 +1,31 @@
+// Copyright 2023-latest Tomoki Miyauchi. All rights reserved. MIT license.
+// This module is browser compatible.
+
+/** Partially applies function arguments.
+ *
+ * @example
+ * ```ts
+ * import { partial } from "https://deno.land/x/curry@$VERSION/partial.ts";
+ *
+ * declare const fn: (a: string, b: number, c: boolean) => void;
+ *
+ * const ternary = partial(fn);
+ * const binary = partial(fn, "");
+ * const unary = partial(fn, "", 0);
+ * const nullary = partial(fn, "", 0, false);
+ * ```
+ */
+export function partial<
+  T extends readonly unknown[],
+  U extends readonly unknown[],
+  R,
+>(
+  fn: (...args: readonly [...T, ...U]) => R,
+  ...args: T
+): (...rest: U) => R {
+  function partial(...rest: U): R {
+    return fn(...args, ...rest);
+  }
+
+  return partial;
+}

partial_test.ts

@@ -0,0 +1,132 @@
+// Copyright 2023-latest Tomoki Miyauchi. All rights reserved. MIT license.
+
+import { partial } from "./partial.ts";
+import {
+  assertEquals,
+  assertSpyCallArgs,
+  beforeEach,
+  describe,
+  it,
+  Spy,
+  spy,
+} from "./_dev_deps.ts";
+
+interface Arity1 {
+  (arg: string): unknown;
+}
+
+interface Arity2 {
+  (arg: string, arg2: number): unknown;
+}
+
+interface Arity3 {
+  (arg: string, arg2: number, arg3: boolean): unknown;
+}
+
+type Fn2Spy<T> = T extends (this: infer T, ...args: infer Args) => infer R
+  ? Spy<T, Args, R>
+  : unknown;
+
+describe("partial", () => {
+  describe("arity 1", function () {
+    interface Context {
+      fn: Fn2Spy<Arity1>;
+    }
+    beforeEach<Context>(function () {
+      this.fn = spy((arg1: string) => arg1);
+    });
+    it<Context>("should pass 0 argument", function () {
+      const arg = "";
+      const fn = partial(this.fn);
+
+      assertEquals(fn(arg), arg);
+      assertSpyCallArgs(this.fn, 0, [arg]);
+    });
+
+    it<Context>("should pass 1 argument", function () {
+      const arg = "";
+      const fn = partial(this.fn, arg);
+
+      assertEquals(fn(), arg);
+      assertSpyCallArgs(this.fn, 0, [arg]);
+    });
+  });
+
+  describe("arity 2", function () {
+    interface Context {
+      fn: Fn2Spy<Arity2>;
+    }
+
+    const arg = "";
+    const arg2 = 0;
+
+    beforeEach<Context>(function () {
+      this.fn = spy((arg1: string, arg2: number): string => arg1 + arg2);
+    });
+
+    it<Context>("should pass 0 argument", function () {
+      const fn = partial(this.fn);
+
+      assertEquals(fn(arg, arg2), arg + arg2);
+      assertSpyCallArgs(this.fn, 0, [arg, arg2]);
+    });
+
+    it<Context>("should pass 1 argument", function () {
+      const fn = partial(this.fn, arg);
+
+      assertEquals(fn(arg2), arg + arg2);
+      assertSpyCallArgs(this.fn, 0, [arg, arg2]);
+    });
+
+    it<Context>("should pass 2 argument", function () {
+      const fn = partial(this.fn, arg, arg2);
+
+      assertEquals(fn(), arg + arg2);
+      assertSpyCallArgs(this.fn, 0, [arg, arg2]);
+    });
+  });
+
+  describe("arity 3", function () {
+    interface Context {
+      fn: Fn2Spy<Arity3>;
+    }
+
+    const arg = "";
+    const arg2 = 0;
+    const arg3 = false;
+
+    beforeEach<Context>(function () {
+      this.fn = spy((arg1: string, arg2: number, arg3: boolean): string =>
+        arg1 + arg2 + arg3
+      );
+    });
+
+    it<Context>("should pass 0 argument", function () {
+      const fn = partial(this.fn);
+
+      assertEquals(fn(arg, arg2, arg3), arg + arg2 + arg3);
+      assertSpyCallArgs(this.fn, 0, [arg, arg2, arg3]);
+    });
+
+    it<Context>("should pass 1 argument", function () {
+      const fn = partial(this.fn, arg);
+
+      assertEquals(fn(arg2, arg3), arg + arg2 + arg3);
+      assertSpyCallArgs(this.fn, 0, [arg, arg2, arg3]);
+    });
+
+    it<Context>("should pass 2 argument", function () {
+      const fn = partial(this.fn, arg, arg2);
+
+      assertEquals(fn(arg3), arg + arg2 + arg3);
+      assertSpyCallArgs(this.fn, 0, [arg, arg2, arg3]);
+    });
+
+    it<Context>("should pass 3 argument", function () {
+      const fn = partial(this.fn, arg, arg2, arg3);
+
+      assertEquals(fn(), arg + arg2 + arg3);
+      assertSpyCallArgs(this.fn, 0, [arg, arg2, arg3]);
+    });
+  });
+});