Javascript Combinator examples

combinators.js

"use strict";

// compose :: (b -> c) -> (a -> b) -> (a -> c)
function compose(g, f) {
    return function() {
        return g(f.apply(f, arguments));
    };
}

// each :: [a] -> (a -> _) -> _
function each(xs, f) {
    var i;
    for (i = 0; i < xs.length; ++i) {
        f(xs[i], i);
    }
}

// map :: [a] -> (a -> b) -> [b]
function map(f, xs) {
    var i, result = [];
    each(xs, function(x, i) {
        result[i] = f(x);
    });
    return result;
}

// concatMap :: [a] -> (a -> [b]) -> [b]
function concatMap(f, xs) {
    var k=0, result = [];
    each(xs, function(x) {
        each(f(x), function(y) {
            result[k] = y;
            k += 1;
        });
    });
    return result;
}

// listCompose :: (b -> [c]) -> (a -> [b]) -> (a -> [c])
function listCompose(g, f) {
    return function() {
        return concatMap(g, f.apply(f, arguments));
    };
}        

// foldr :: (a -> b) -> b -> [a] -> [b]
function foldr(f, a, xs) {
    return foldr_(f, a, xs, xs.length - 1);
}

function foldr_(f, a, xs, i) {
    var x = xs[i];
        a = f(x, a);
    if (i === 0) { return a; }
    return foldr_(f, a, xs, i - 1);
}

// foldl :: (a -> b) -> b -> [a] -> [b]
function foldl(f, a, xs) {
    return foldl_(f, a, xs, 0);
}

function foldl_(f, a, xs, i) {
    var x = xs[i]
        a = f(x, a);
    if (i === xs.length - 1) { return a; }
    return foldl_(f, a, xs, i + 1);
}

// makePipe :: ((b -> c) -> (a -> b) -> (a -> c)) -> [(* -> *)] -> (* -> *)
function makePipe(compositor) {
    return function() {
	    var args = Array.prototype.slice.call(arguments),
		first = args.shift();
	    return foldl(compositor, first, args);
    };
}

var pipe = makePipe(compose);
var listPipe = makePipe(listCompose);

function add(rhs) {
    return function(lhs) {
        return lhs + rhs;
    };
}

function subtract(rhs) {
    return function(lhs) {
        return lhs - rhs;
    };
}

function multiply(rhs) {
    return function(lhs) {
       return lhs * rhs;
    };
}

var calc = pipe(
    add(5),
    multiply(7),
    subtract(4)
);

print(calc(8));

function sum(seq) {
    return foldr(function(a, b) { return a + b; }, 0, seq);
}

function avg(seq) {
   return sum(seq) / seq.length;
}


var tryAll = listPipe(
    function(a) {
        return [a, a + 1, a - 1];
    },
    function(b) {
        return [b, b * 2.0, b / 2.0];
    },
    function(c) {
        return [c, c + 1, c - 1];
    }
);

var getResult = pipe(
    tryAll,
    avg,
    print);

getResult(10);