Functional Programming - Simple For Loop For Incrementing Counter

Question

We dont use for loop in functional programming, instead, we use higher order functions like map, filter, reduce etc. These are fine for iterating through an array.

However, I wonder how do I do a simple counter loop.

let i = 0;
for( i; i < 10; i++) {
  console.log( "functional programming is a religion")
};

So, how would one do this in functional programming?

Solution 1

So, how would one do this in functional programming?

It is not doing much actually, you can still use forEach with a little workaround

Array.apply(null, Array(5)).forEach(function(){
 console.log( "funtional programming is a religion")
});

5 is the number of times you want to iterate.

Solution 2

The whole point would be to make most of your code testable. For you example I guess the best would be to create the text without printing it.

function unFold(fnStopPredicate, fnTerm, fnGenerate, aSeed) {
    var arr = [];
    while( ! fnStopPredicate(aSeed) ){
        arr.push(fnTerm(aSeed));
        aSeed = fnGenerate(aSeed);
    }
    return arr;
}

You might say this is not functional and that is true, BUT it has a functional interface. It does not mutate it's arguments and the value returned is always a direct result of it's initial arguments.

var strValues = unFold(x => x > 10,
                       x => "functional programming is a religion",
                       x => x+1,
                       0).join("\n");

// Real side effect goes here
console.log(strValues);

The main point here is that you may unit test the use of unFold as long as the function you provide does not do side effects themselves.

Solution 3

Why not build a Higher Order Function for Numbers.

Number.prototype.repeat = function (fn) {
    var i,
    n = Math.abs(Math.floor(this)) || 0;
    for (i = 0; i < n; i++) fn(i, this);
};

(10).repeat(function (i, n) { document.write(i + ' of ' + n + ': your claim<br>'); });
(NaN).repeat(function (i, n) { document.write(i + ' of ' + n + ': your claim<br>'); });

Comments

[sudo97]

functional programming does not mutate input, so the scnd function would return x+1

[Phil Tune]

I had the same thought to do a HOF, so +1. Not purely FP, but I'm a pragmatist, not a purist.

[Francois Carstens]

Array.apply is a pretty straightforward solution here. Nice.