CODEWITHSUNDEEP
javascriptflowtype
Herrington Darkholme
Herrington Darkholme

Reputation: 6315

Flow-type does not retain generic function parameter when returning function

In my code, I want to return a high order function and flow my argument type to the returning function. The minimal reduced code looks like below.

function curry<A, B: A>(a: A): (b: B) => void {
  return () => {}
}

curry(123)("123") // expected error but not

I wonder why B does not flow to the returning function. It seems like the returning function has the type (b: any) => void.

I know in this example I can change the type bound to a signature like (a: A) => (b: A) => void. But my real scenario is more complicated and needs a phantom type as a bound, which looks like the B above.

So the question is, what type is B instantiated to? Can I make a type parameter flow to the argument position of the returning function? Can the type in argument position influence the type inference of an actual argument?

Upvotes: 1

Views: 882

Answers (1)

gcanti
gcanti

Reputation: 1462

The returned function has type (b: string) => void as you can see running the type-at-pos command

// @flow

function curry<A, B: A>(a: A): (b: B) => void {
  return () => {}
}

const f = curry(123)
f("123")

running flow type-at-pos index.js 7 7 you get:

(b: string) => void

Keep in mind that, due to how type inference works, the type A (and thus B) will change accordingly to the following calls

const f = curry(123) // <= f now has type (b: string | boolean) => void
f("123")
f(true)

Upvotes: 4

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