type-checking function signatures with less or more arguments

Question

This is about Flow typechecking pronouncements when a function type is defined and the function expression we're trying to typecheck has: (a) more or (b) fewer arguments than the defined type.

The following typechecks as it should, no questions here.

declare type TFunctionNumberToBoolean = (n: number) => boolean;
const f: TFunctionNumberToBoolean = function isEven(n: number) {return n%2==0;}

The following case (case A) where the normative "number to boolean" function type is changed as follows:

(n: number) => boolean

… doesn't typecheck:

declare type TFunctionNumberToBoolean = () => boolean;

const f: TFunctionNumberToBoolean = function isEven(n: number) {return n%2==0;}

The following case (case B) where the normative "number to boolean" function type is changed as follows:

(n: number, foo :number) => boolean

… typechecks:

declare type TFunctionNumberToBoolean = (n: number, foo: number) => boolean;

const f: TFunctionNumberToBoolean = function isEven(n: number) {return n%2==0;}

What is the reasoning behind Case A (more arguments than the defined function type) not typechecking but Case B (less arguments than the defined function type) typechecking ? I naively think that an argument could be made for the opposite behavior to have been more intuitive.

I am using Flow 0.35.

Answer 1

Let's say you have a function that takes two properties of an object and sums them:

function sum(obj) {
  return obj.a + obj.b;
}

What happens if you pass additional arguments? Nothing bad, it will be ignored.

What happens if you pass no arguments? You'll get TypeError: Cannot read property 'a' of undefined.

Passing more arguments is safe, passing less arguments is unsafe.