feat: Add compose function for composing multiple operators

.github/FUNDING.yml

@@ -1,6 +1,8 @@
 # These are supported funding model platforms
 
-github: [lambdalisue] # Replace with up to 4 GitHub Sponsors-enabled usernames e.g., [user1, user2]
+github: [
+  lambdalisue,
+] # Replace with up to 4 GitHub Sponsors-enabled usernames e.g., [user1, user2]
 patreon: # Replace with a single Patreon username
 open_collective: # Replace with a single Open Collective username
 ko_fi: # Replace with a single Ko-fi username

README.md

@@ -21,7 +21,7 @@ inference and type checking of the operator functions.
 Pipe a value through a series of operator functions.
 
 ```ts
-import { pipe } from "@core/pipe";
+import { pipe } from "@core/pipe/pipe";
 
 const result = pipe(
   1,
@@ -36,7 +36,7 @@ Or use `async` module to pipe a value through a series of asynchronous operator
 functions.
 
 ```ts
-import { pipe } from "@core/pipe/async";
+import { pipe } from "@core/pipe/async/pipe";
 
 const result = pipe(
   1,
@@ -47,6 +47,65 @@ const result = pipe(
 console.log(result); // "4"
 ```
 
+If you want to create a new function that composes multiple operators, use
+`compose` like below.
+
+```ts
+import { compose } from "@core/pipe/compose";
+
+const operator = compose(
+  (v: number) => v + 1, // The first operator must be typed explicitly
+  (v) => v * 2, // inferred as (v: number) => number
+  (v) => v.toString(), // inferred as (v: number) => string
+);
+console.log(operator(1)); // "4"
+```
+
+Or use `async` module to compose multiple asynchronous operators.
+
+```ts
+import { compose } from "@core/pipe/async/compose";
+
+const operator = compose(
+  (v: number) => Promise.resolve(v + 1), // The first operator must be typed explicitly
+  (v) => Promise.resolve(v * 2), // inferred as (v: number) => number | Promise<number>
+  (v) => Promise.resolve(v.toString()), // inferred as (v: number) => string | Promise<string>
+);
+console.log(await operator(1)); // "4"
+```
+
+## Difference
+
+The `pipe` function in the root module is equivalent to function calls without
+`await` like below.
+
+```ts
+import { pipe } from "@core/pipe/pipe";
+
+const a = (v: unknown) => v;
+const b = (v: unknown) => v;
+const c = (v: unknown) => v;
+
+// Equivalent
+console.log(pipe(1, a, b, c)); // 1
+console.log(c(b(a(1)))); // 1
+```
+
+The `pipe` function in the `async` module is equivalent to function calls with
+`await` like below.
+
+```ts
+import { pipe } from "@core/pipe/async/pipe";
+
+const a = (v: unknown) => Promise.resolve(v);
+const b = (v: unknown) => Promise.resolve(v);
+const c = (v: unknown) => Promise.resolve(v);
+
+// Equivalent
+console.log(await pipe(1, a, b, c)); // 1
+console.log(await c(await b(await a(1)))); // 1
+```
+
 ## License
 
 The code follows MIT license written in [LICENSE](./LICENSE). Contributors need

_common.ts

@@ -0,0 +1,8 @@
+import type { Operator } from "./operator.ts";
+
+/**
+ * @internal
+ */
+export type LastOperatorReturn<T extends Operator<unknown, unknown>[]> =
+  T extends [...Operator<unknown, unknown>[], Operator<unknown, infer R>] ? R
+    : never;

async/_common.ts

@@ -0,0 +1,10 @@
+import type { AsyncOperator } from "./operator.ts";
+
+/**
+ * @internal
+ */
+export type LastAsyncOperatorReturn<
+  T extends AsyncOperator<unknown, unknown>[],
+> = T extends
+  [...AsyncOperator<unknown, unknown>[], AsyncOperator<unknown, infer R>] ? R
+  : never;