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

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

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)
;(y: number)

// Clearly, this is not  the best use-case for this type.
function fnValueReturner<V, F: $Function1<V, any>>(fn: F, v: V): $Function1Value<F, V> {
    return fn(v)
}

;(fnValueReturner(toString, 1234): string)