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src/composeTyping.ts

@@ -0,0 +1,27 @@
+import { Func } from "./typing.ts";
+
+type Identity<T> = (x: T) => T;
+
+type UnwrapPromise<T> = T extends Promise<infer U> ? U : T;
+
+type UnwrapPromiseFn<T extends Func> = (
+  ...args: Parameters<T>
+) => UnwrapPromise<ReturnType<T>>;
+
+type SimpleCompose<F, G> = G extends (...args: infer GArgs) => infer GReturn
+  ? F extends (x: UnwrapPromise<GReturn>) => infer FReturn
+    ? (...args: GArgs) => UnwrapPromise<FReturn>
+  : never
+  : never;
+
+type Compose<F, G> = F extends Identity<infer _>
+  ? G extends Identity<infer _> ? UnwrapPromiseFn<G> : SimpleCompose<F, G>
+  : SimpleCompose<F, G>;
+
+export type ComposeMany<Fs extends Func[]> = Fs extends [] ? never
+  : Fs extends [infer F extends Func] ? F
+  : Fs extends
+    [infer F extends Func, infer G extends Func, ...infer rest extends Func[]]
+    ? rest extends [] ? Compose<G, F>
+    : Compose<ComposeMany<rest>, Compose<G, F>>
+  : never;