评估顺序javascript

Question

I am tasked to write a function f in javascript, and

f(0)+f(1)//should return 0 if evaluated from left to right
         //should return 1 if evaluated from right to left

How can I implement the function f? I am thinking about keeping track of whether f is called before, but have no idea how.

The scheme solution is as following:

(define f (let ((set #f)) (define (fn n) (if set 0 (begin (set! set #t) n))) fn))

Answer 1

The Scheme solution seems to come down to

var f = (function() {
    var set = false;
    function fn(n) {
        if (set) {
            return 0;
        } else {
            set = true;
            return n;
        }
    }
    return fn;
}());

which is a bit boring but expected. Yes, you need to keep track of the state somewhere outside of f , declaring an extra variable is the simplest solution. It doesn't need to be global though, set is hidden in the scope of the IIFE.

Answer 2

Solution with a property of the function.

 function f(v) { return [f.value || 0, f.value = v][0]; } console.log(f(0) + f(1)); // 0 

 function f(v) { return [f.value || 0, f.value = v][0]; } console.log(f(1) + f(0)); // 1 

Answer 3

I'm sure there's a shorter version...

function f(arg) {
    if (!f.called && arg === 0) {
        f.called = true;
        return 0;
    } else if (f.called && arg === 1) {
        return 1;
    }

    return 0;
}