fix flow typings and add tests

test/flow.test.ts

@@ -0,0 +1,32 @@
+import { expectError, expectType } from 'tsd';
+
+import { flow } from '../es';
+
+const strToNum = (str: string) => Number(str);
+const numToStr = (num: number) => String(num);
+
+expectType<number>(flow(1, []));
+expectType<string>(flow(1, [numToStr]));
+expectType<number>(flow(1, [numToStr, strToNum]));
+expectType<string>(flow(1, [numToStr, strToNum, numToStr]));
+expectType<number>(flow(1, [numToStr, strToNum, numToStr, strToNum]));
+expectType<string>(flow(1, [numToStr, strToNum, numToStr, strToNum, numToStr]));
+expectType<number>(flow(1, [numToStr, strToNum, numToStr, strToNum, numToStr, strToNum]));
+expectType<string>(flow(1, [numToStr, strToNum, numToStr, strToNum, numToStr, strToNum, numToStr]));
+expectType<number>(flow(1, [numToStr, strToNum, numToStr, strToNum, numToStr, strToNum, numToStr, strToNum]));
+expectType<string>(flow(1, [numToStr, strToNum, numToStr, strToNum, numToStr, strToNum, numToStr, strToNum, numToStr]));
+
+// with 10+, if you don't set the generic it returns unknown
+expectType<unknown>(flow(1, [numToStr, strToNum, numToStr, strToNum, numToStr, strToNum, numToStr, strToNum, numToStr, strToNum]));
+// setting the final return type has no typechecking against the actual chain of functions
+expectType<Date>(flow<number, Date>(1, [numToStr, strToNum, numToStr, strToNum, numToStr, strToNum, numToStr, strToNum, numToStr, strToNum]));
+// the Seed type has to match though, or it will error
+expectError(flow<string, Date>(1, [numToStr, strToNum, numToStr, strToNum, numToStr, strToNum, numToStr, strToNum, numToStr, strToNum]));
+
+// same applies if you have generic array of functions, even if the arg and return are typed
+const seriesOfFunctions: Array<(a: string) => string> = [];
+expectType<unknown>(flow(1, seriesOfFunctions));
+// setting the final return type has no typechecking against the actual chain of functions
+expectType<Date>(flow<number, Date>(1, seriesOfFunctions));
+// the Seed type has to match though, or it will error
+expectError(flow<string, Date>(1, seriesOfFunctions));

types/flow.d.ts

@@ -8,4 +8,4 @@ export function flow<S, R1, R2, R3, R4, R5, R6, R7>(seed: S, pipeline: [(a: S) =
 export function flow<S, R1, R2, R3, R4, R5, R6, R7, R8>(seed: S, pipeline: [(a: S) => R1, (a: R1) => R2, (a: R2) => R3, (a: R3) => R4, (a: R4) => R5, (a: R5) => R6, (a: R6) => R7, (a: R7) => R8]): R8;
 export function flow<S, R1, R2, R3, R4, R5, R6, R7, R8, R9>(seed: S, pipeline: [(a: S) => R1, (a: R1) => R2, (a: R2) => R3, (a: R3) => R4, (a: R4) => R5, (a: R5) => R6, (a: R6) => R7, (a: R7) => R8, (a: R8) => R9]): R9;
 // catch-all to larger than 9, or if you need to manually set seed type `S` and final return type `R`
-export function flow<S, R>(seed: S, pipeline: ReadonlyArray<(a: S) => any>): R;
+export function flow<S, R>(seed: S, pipeline: ReadonlyArray<(a: any) => any>): R;