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.

The issue describes named type arguments for TypeScript generics and gives syntax examples using defaults and Fastify types. No implementation files, tests, or entry points are named; begin by locating the compiler areas that parse and check generic type arguments. Done means named arguments can select later type parameters while preserving earlier defaults, with coverage for the examples in the issue.