polymorphic type: function type

[phadej]

The example above doesn't type check. Does all vars (and return types?) have to be monomorphic?

/* @flow */

function nth(n: number) : <T>(arr: Array<T>) => T {
  return function (arr) {
    return arr[n];
  }
}

var first: <T>(arr: Array<T>) => T = nth(0);

[samwgoldman]

This is because flow will not infer a generic type. If you specifically annotate the inner function, you can convince flow that the returned function fits the return type of nth.

Here is a smaller example showing that flow doesn't infer generics:

/* @flow */

function index(xs, i) {
  return xs[i];
}

// Either one of these individually type checks, but together it fails
var ex1: string = index(["hi"], 0);
var ex2: number = index([0], 0);

[ghost]

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