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

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