fantasy-states/example.js

State control structure.

var State = require('./'),
    Tuple2 = require('fantasy-tuples').Tuple2,

    // Tuple2 Number Number
    initial = Tuple2(0, 1),

    // State (Tuple2 Number Number) Number -> State (Tuple2 Number Number) Number
    next = discard(
        State.modify(function(t) {
            return Tuple2(t._1 + 1, (t._1 + 1) * t._2);
        }),
        State.get.map(snd)
    ),

    // Number
    result = factorial(5);

// Tuple2 a b -> b
function snd(t) {
    return t._2;
}

// (m a, m b) -> m b
function discard(a, b) {
    return a.chain(function(_) {
        return b;
    });
}

// Array (m a) -> m a
function sequence(actions) {
    var r = actions[0],
        i;
    for(i = 1; i < actions.length; i++) {
        r = discard(r, actions[i]);
    }
    return r;
}

// (Number, a) -> Array a
function replicate(n, x) {
    var r = [],
        i;
    for(i = 0; i < n; i++) {
        r.push(x);
    }
    return r;
}

// Number -> Number
function factorial(n) {
    return sequence(replicate(n, next)).evalState(initial);
}

// Side-effect!
console.log(result);