use ts-toolbelt for 'compose' types

Author: jedmao

package-lock.json

@@ -89,6 +89,7 @@
     "rollup-plugin-terser": "^5.1.1",
     "rollup-plugin-typescript2": "^0.24.0",
     "rxjs": "^6.5.2",
+    "ts-toolbelt": "^3.8.63",
     "typescript": "^3.5.3",
     "typings-tester": "^0.3.2"
   },

package.json

@@ -72,7 +72,7 @@ export default function applyMiddleware(
       dispatch: (action, ...args) => dispatch(action, ...args)
     }
     const chain = middlewares.map(middleware => middleware(middlewareAPI))
-    dispatch = compose<typeof dispatch>(...chain)(store.dispatch)
+    dispatch = compose(...chain)(store.dispatch as typeof dispatch)
 
     return {
       ...store,

src/applyMiddleware.ts

@@ -1,7 +1,4 @@
-type Func0<R> = () => R
-type Func1<T1, R> = (a1: T1) => R
-type Func2<T1, T2, R> = (a1: T1, a2: T2) => R
-type Func3<T1, T2, T3, R> = (a1: T1, a2: T2, a3: T3, ...args: any[]) => R
+import { F, T } from 'ts-toolbelt'
 
 /**
  * Composes single-argument functions from right to left. The rightmost
@@ -13,85 +10,15 @@ type Func3<T1, T2, T3, R> = (a1: T1, a2: T2, a3: T3, ...args: any[]) => R
  *   to left. For example, `compose(f, g, h)` is identical to doing
  *   `(...args) => f(g(h(...args)))`.
  */
-export default function compose(): <R>(a: R) => R
-
-export default function compose<F extends Function>(f: F): F
-
-/* two functions */
-export default function compose<A, R>(f1: (b: A) => R, f2: Func0<A>): Func0<R>
-export default function compose<A, T1, R>(
-  f1: (b: A) => R,
-  f2: Func1<T1, A>
-): Func1<T1, R>
-export default function compose<A, T1, T2, R>(
-  f1: (b: A) => R,
-  f2: Func2<T1, T2, A>
-): Func2<T1, T2, R>
-export default function compose<A, T1, T2, T3, R>(
-  f1: (b: A) => R,
-  f2: Func3<T1, T2, T3, A>
-): Func3<T1, T2, T3, R>
-
-/* three functions */
-export default function compose<A, B, R>(
-  f1: (b: B) => R,
-  f2: (a: A) => B,
-  f3: Func0<A>
-): Func0<R>
-export default function compose<A, B, T1, R>(
-  f1: (b: B) => R,
-  f2: (a: A) => B,
-  f3: Func1<T1, A>
-): Func1<T1, R>
-export default function compose<A, B, T1, T2, R>(
-  f1: (b: B) => R,
-  f2: (a: A) => B,
-  f3: Func2<T1, T2, A>
-): Func2<T1, T2, R>
-export default function compose<A, B, T1, T2, T3, R>(
-  f1: (b: B) => R,
-  f2: (a: A) => B,
-  f3: Func3<T1, T2, T3, A>
-): Func3<T1, T2, T3, R>
-
-/* four functions */
-export default function compose<A, B, C, R>(
-  f1: (b: C) => R,
-  f2: (a: B) => C,
-  f3: (a: A) => B,
-  f4: Func0<A>
-): Func0<R>
-export default function compose<A, B, C, T1, R>(
-  f1: (b: C) => R,
-  f2: (a: B) => C,
-  f3: (a: A) => B,
-  f4: Func1<T1, A>
-): Func1<T1, R>
-export default function compose<A, B, C, T1, T2, R>(
-  f1: (b: C) => R,
-  f2: (a: B) => C,
-  f3: (a: A) => B,
-  f4: Func2<T1, T2, A>
-): Func2<T1, T2, R>
-export default function compose<A, B, C, T1, T2, T3, R>(
-  f1: (b: C) => R,
-  f2: (a: B) => C,
-  f3: (a: A) => B,
-  f4: Func3<T1, T2, T3, A>
-): Func3<T1, T2, T3, R>
-
-/* rest */
-export default function compose<R>(
-  f1: (b: any) => R,
-  ...funcs: Function[]
-): (...args: any[]) => R
-
-export default function compose<R>(...funcs: Function[]): (...args: any[]) => R
-
-export default function compose(...funcs: Function[]) {
+const compose = <Fns extends F.Function<any[], any>[]>(
+  ...funcs: Fns
+): ((
+  ...args: Parameters<Fns[T.Tail<Fns>['length']]>
+) => F.Return<T.Head<Fns>>) => {
   if (funcs.length === 0) {
     // infer the argument type so it is usable in inference down the line
-    return <T>(arg: T) => arg
+    // TODO: should probably throw an error here instead
+    return (<T>(arg: T) => arg) as any
   }
 
   if (funcs.length === 1) {
@@ -100,3 +27,5 @@ export default function compose(...funcs: Function[]) {
 
   return funcs.reduce((a, b) => (...args: any) => a(b(...args)))
 }
+
+export default compose