Magic Flow Type 1: $FunctionXValue —— A magic flow type that takes a function type that takes one arg, the type of that arg, and returns the return value type of the function.

FunctionValue.ts

/* @flow */
type $Function1<A, B> = (arg: A) => B;
type _Function1Value<A, B, F: $Function1<A, B>> = B; // eslint-disable-line
type $Function1Value<F: $Function1<*, *>, A> = _Function1Value<A, *, F>;

var toString = (value) => String(value)
var toNumber = (value) => parseFloat(value) || 0

type NumberXString = ((value: number) => string) & ((value: string) => number)

// var numberXString: NumberXString = (value: number | string) => {
//     if (typeof value === 'string') {
//         return 1
//     }
//     return String(value)
// }

type ToStringRetVal = $Function1Value<typeof toString, any>
type ToNumberRetVal = $Function1Value<typeof toNumber, any>

;(toString: (val: any) => ToStringRetVal)

declare var x: ToStringRetVal
declare var y: ToNumberRetVal

;(x: string)
// $ExpectError
;(x: boolean)
// $ExpectError
;(x: number)

;(y: number)
// $ExpectError
;(y: string)

function fnValueReturner<V, F: $Function1<V, any>> (fn: F, v: V): $Function1Value<F, V> {
  return fn(v)
}

;(fnValueReturner(toString, 1234): string)
// $ExpectError
;(fnValueReturner(toString, 1234): number)

type ShouldBeNumber = $Function1Value<NumberXString, string>
declare var num: ShouldBeNumber

;(num: number)
// $ExpectError
;(num: string)

@freddi301 Sorry can't seem to make it work.

in 0.58.0 we have $Call now