CODEWITHSUNDEEP
genericsflowtype
aij
aij

Reputation: 6531

How to explicitly pass type arguments to generic functions in flow?

I'm trying to explicitly specify the type when calling a generic function.

For example:

export function gen<T>(a: string): { eat: T => string } {
  return {
    eat: (v: T): string => a
  };
}

Of course, using C++-like syntax doesn't work, because flow extends Javascript syntax, and this is already a valid JS expression (a comparison):

const { eat } = gen<number>("str")

What is the correct syntax?

I want to pass a type explicitly, because otherwise this sort of code won't produce an error:

const { eat } = gen("str")
const a = eat(5)
// I want this to be an error, but it is not
const b = eat("foo")

I can of course annotate the assignment, causing the desired generic type to be inferred, but that can be cumbersome. Try it here

Upvotes: 3

Views: 1570

Answers (2)

re-gor
re-gor

Reputation: 1352

It is able since v0.72. CHANGELOG. Example

// @flow

declare function makeFoo<T>(): T;

const t = makeFoo<number>();

(t: number);

// $ExpectError
(t: string);

Upvotes: 2

Kirill Glazunov
Kirill Glazunov

Reputation: 494

Generics can be used for specifying a relation between arguments or arguments and a result. You can`t use it at "call time", only on declaration.
It makes you write more unmistakable code - it`s better to have 2 functions than one universal (from point of understanding and typing):

function eatNumbers(v: number): string {return "" + v;}
function eatSomething<T>(v: T): T {return v;}

Upvotes: 1

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