TypeScript types memo
TypeScriptの型
T1 extends T2 ? F<T1> : never
similar to f(t1 as T2)
circularly references
{0: T1}[T extends any ? 0 : 0]
{0: T1, 1: T2}[CONDITION ? 0 : 1]
CONDITION ? T1 : T2
code:ts
// type GET_LAST<X extends LIST> =
// X extends BIN, LIST ? CAR<X> : GET_LAST<CDR<X>>
// NG: Type alias 'GET_LAST' circularly references itself.
type GET_LAST<X extends LIST> = {
0: CAR<X>,
1: GET_LAST<CDR<X>>
}[X extends BIN, LIST ? 0 : 1]
// OK
CONDITION
N extends any
true
any extends any does not work
true: any, false: never
N extends M ? ... : ...の形でしか使えない