Add named type arguments

Search Terms

generics, type parameters, named parameter, named type parameter, type argument, named type argument

Suggestion

It should be possible to pass type arguments to a generic by name rather than positionally, eg.

interface Foo<T = SomeDefaultType, U> { ... }

// Current syntax
const foo: Foo<SomeDefaultType, string> ...

// Proposed syntax
const foo: Foo<U = string> ...
// yields foo: Foo<SomeDefaultValue, string>

This is loosely inspired on python's named arguments:

def foo(bar = "I'm bar", baz):
    ...

foo(baz="I'm baz")

Use Cases

Generics only accept positional type arguments. If you have a generic accepting many type arguments, most or all of which having default values such as:

interface Handler<Type = string, TPayload = object, TOutput = void> {
  type: Type
  handle(payload: TPayload): TOutput
}

Let's say we have a class which implements the Handler interface but the defaults for Type and TPayload are fine for us and we only want to specify a type for TOuput, currently it is mandatory that we pass type arguments for Type and TPayload:

class Foo implements Handler<string, object, Promise<number>>

If it was possible to pass type arguments by name we could use the considerably terser form:

class Foo implements Handler<TOutput=Promise<number>>

Examples

Fastify exposes types generic over many parameters with default values, such as

  interface FastifyRequest<
    HttpRequest = http.IncomingMessage,
    Query = DefaultQuery,
    Params = DefaultParams,
    Headers = DefaultHeaders,
    Body = DefaultBody
  > { ...

With the proposed syntax we could create specialized interfaces with much less overhead such as

import * as fastify from 'fastify';

const app = fastify();
app.get('/users/:id', async (req: FastifyRequest<Params = {id: string }>, res: FastifyReply) => {
   // req.params is strongly typed now
})

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

L’issue descrive gli argomenti di tipo denominati per i generics di TypeScript e fornisce esempi di sintassi che usano valori predefiniti e tipi Fastify. Non sono indicati file di implementazione, test o punti di ingresso; inizia individuando le aree del compilatore che analizzano e verificano gli argomenti di tipo generici. Il lavoro è completato quando gli argomenti denominati possono selezionare parametri di tipo successivi mantenendo i valori predefiniti precedenti, con una copertura per gli esempi nell’issue.