Modifying function args in Typescript with spread syntax

Question

Is it possible to use the Tuple spreading syntax in Typescript to remove these function overloads? The complication is that I need to remap the function args to new types.

type Type = TString | TNumber
type TString = { tag: 'string' }
type TNumber = { tag: 'number' }

interface Mapping {
  string: string
  number: number
}

type Remap<T extends Type> = Mapping[T['tag']];

// can I collapse these function overloads in to one?
function checkFn<R extends Type, A extends Type, B extends Type, C extends Type>(argTypes: [A, B, C], retType: R, fn: (arg0: Remap<A>, arg1: Remap<B>, arg2: Remap<C>) => Remap<R>): void;
function checkFn<R extends Type, A extends Type, B extends Type>(argTypes: [A, B], retType: R, fn: (arg0: Remap<A>, arg1: Remap<B>) => Remap<R>): void;
function checkFn<R extends Type, A extends Type>(argTypes: [A], retType: R, fn: (arg: Remap<A>) => Remap<R>): void;
function checkFn(argTypes: any[], retType: any, fn: any): void {}

const stringType: TString = { tag: 'string' }
const numberType: TNumber = { tag: 'number' }

checkFn([numberType, stringType, numberType], numberType, (arg0, arg1, arg2) => 42)

TS playground

Sean Vieira · Accepted Answer · 2022-01-04 05:08:46Z

1

Using Variadic Tuple Types (see the docs for a less descriptive version) the answer is "yes":

type Type = TString | TNumber
type TString = { tag: 'string' }
type TNumber = { tag: 'number' }

interface Mapping {
  string: string
  number: number
}

type Remap<T extends Type> = Mapping[T['tag']];

function checkFn<P extends Type[], R extends Type>(
  argTypes: [...P],
  retType: R,
  fn: (...args: { [K in keyof P]: Remap<P[K] & Type> }) => Remap<R>
): void {}


const stringType: TString = { tag: 'string' }
const numberType: TNumber = { tag: 'number' }

checkFn(
  [numberType, stringType, numberType],
  numberType,
  (arg0: number, arg1: string, arg2: number) => 42);

edited Jan 4, 2022 at 5:08

answered Jan 4, 2022 at 4:04

Sean Vieira

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1 Comment

Daniil Loban Over a year ago

This is a perfectly appropriate answer. The only thing that it gives the runtime error checkFn is not defined, so I suggest removing declare and writing {} after void

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