const f: <A>(a: A) => A = a => a;  // Type of a is A

const arrayMap = <T, U>(f: (x: T) => U) => (a: T[]) => a.map(f);
const arrayFilter = <T>(f: (x: T) => boolean) => (a: T[]) => a.filter(f);

const f1: (a: string[]) => number[] = arrayMap(x => x.length);  // x: string
const f2: <A>(a: A[]) => A[][] = arrayMap(x => [x]);  // x: A
const f3: <A>(a: A[]) => { value: A }[] = arrayMap(value => ({ value }));  // value: A
const f4: (a: string[]) => string[] = arrayFilter(x => x.length > 10);  // x: string
const f5: <T extends { value: number }>(a: T[]) => T[] = arrayFilter(x => x.value > 10);  // x: T