Encountering a Typescript error when defining a curried function after an uncurried function

Upon placing the uncurried definition of a method above the curried definition, I encountered an error stating:

Expected 1 arguments, but got 2.ts(2554)
.

The dtslint test that failed reads as follows:

function match(regExpression: RegExp, str: string): string[]; 
function match(regExpression: RegExp): (str: string) => string[]; 

function throttle<T, U>(fn: (input: T) => U, ms: number): (input: T) => U;
function throttle<T, Q, U>(
  fn: (input1: T, input2: Q) => U, ms: number
): (input1: T, input2: Q) => U;

it('match', () => {
    const fn = throttle(match, 1000)
    fn(/foo/,'foo bar')  // line of error
})

I noticed that if I move the curried match definition above the uncurried one, the error disappears.

My inquiry is whether it is preferable to place the uncurried version after the curried one, or if there might be an issue within the throttle definition?


This question pertains to crafting Typescript definitions for a library akin to ramda.

Answer №1

Typescript prioritizes overloaded signatures (internally intersection types) from top to bottom order. When deciding which signature to use for inferring generic parameters in throttle (similar to conditional types), it always chooses the last signature.

A closely related feature of TS type system is explained here:

When inferring from a type with multiple call signatures (like an overloaded function), inferences are based on the last signature, which is usually the most inclusive catch-all case. Overload resolution cannot be done based on a list of argument types (as proposed in #6606) without supporting typeof for arbitrary expressions or similar features.

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