Javascript Flow generic functions syntax

Question

I'm facing some syntax errors when trying to write generic map function.

function mymap<A, B>(fun: (_: A) => B, array: Array<A>): Array<B> {
    let result: Array<B> = [];
    for (let i of array)
        result.push(fun(i));
    return result;
}

let x = mymap<number, number>(x => x, [1, 2, 3]);
                            ^ Unexpected token    

Flow says: Unexpected token >. What am I doing wrong? Thank you for your help.

Answer 1

Flow is a type checker only. While it doesn't allow type overloading of generic implementation, you shouldn't specify types while you call a function. Flow will just try to check arguments for compatibility with the signature of the function and deduct the output type. So you just need to write

let x = mymap(x => x, [1, 2, 3]);

and Flow will deduct the output type of the expression and apply it to the x variable.

Or you can explicitly specify a type of x:

let x: Array<number> = mymap(x => x, [1, 2, 3]);

In this case Flow can compare a deducted type with the required type, and if you erroneously will use an incorrect mapping function like x => x.toString(), then Flow will warn you about that:

   let x: Array<number> = mymap(x => x.toString(), [1, 2, 3]);
//              ^ string             ^ This type is incompatible with number