type callbacks

search: https://www.google.com/search?q=type+functions+site%3Ahttps%3A%2F%2Fgithub.com%2FMicrosoft%2FTypeScript

The more I work with advance types the more apparent the following problems become:

No intermediate types

At the current stage the advanced types are represented via expressions. Quite often there is a need to reuse exactly the same part of such expression in more than one place. Currently there is no way to capture a part of it and reuse it, consider:

type A<X> = { x: B<X>, y: B<X> }

here I wish I could save B<X> into a local type C somehow to be able to write

// speculative syntax:
type A<X> = <
    type C = B<X>;
    { x: C, y: C };
>;

per @RyanCavanaugh intermediate types are covered by https://github.com/Microsoft/TypeScript/issues/23188

No way to specify a type callback

Currently type parameters are the only way to define a parametric type. There is no way to define a type based on a type callback:

interface Z { x: string; y: number; }
// speculative syntax:
type A<T, F: X =>...,  T> =  { [P in keyof T]: F<T[P]>; } // <-- wish could do this
type B = A<Z, X => X extends string ? true : false>;  // { x: true; y: false; }
type C = A<Z, X => { get: () => X }>;  // { x: { get: () => string; }; y: { get: () => number; }; }

I am not sure what a proposal would be, but these problems are definitely worth a discussion

Commencez par lire les exemples de type-callback proposés dans cette issue ainsi que l’issue liée #23188, qui traite des types intermédiaires. L’issue ne nomme aucun fichier d’implémentation ni test et demande une discussion plutôt qu’une modification définie. Un travail terminé nécessiterait une proposition approuvée pour exprimer et appliquer les type-callbacks.