js lenses

lenses.js

var getter = function(obj, lens) {
    return _(lens).foldl(function(focused, element) {
        return focused[element];
    }, obj);
};

var modifier = function(obj, lens, mod) {
    if (lens.length === 0) {
        return mod(obj);
    } else {
        var monocle = lens[0];
        var newObj = _(obj).clone();
        newObj[monocle] = modifier(obj[monocle], lens.slice(1), mod);
        return newObj;
    }
};

// Lens must have length >= 1 or there would be nothing to return
var deleter = function(obj, lens) {
    var newObj = _(obj).clone();
    if (lens.length === 1) {
        delete newObj[lens[0]];
        return newObj;
    } else {
        var newSubObj = deleter(obj[lens[0]], lens.slice(1));
        return setter(newObj, lens[0], newSubObj);
    }
};

var setter = function(obj, lens, set) {
    return modifier(obj, lens, function() { return set; });
};

// Lens must point to a member of an array. We'll insert into that array.
var insertAt = function(obj, lens, toInsert) {
    var arrLens = lens.slice(0, -1);
    var arr = getter(obj, arrLens).slice(); // slice to copy

    var arrIdx = lens[lens.length-1];
    arr.splice(arrIdx, 0, toInsert);
    return setter(obj, arrLens, arr);
};

var insertBefore = insertAt;
var insertAfter = function(obj, lens, toInsert) {
    var newLens = lens.slice();
    newLens[newLens.length-1] += 1;
    return insertAt(obj, newLens, toInsert);
};

_.mixin({
    // > _([1,2,3,4]).scanl(function(l, r) { return l + r; }, 0)
    // [1, 3, 6, 10]
    scanl: function(obj, iterator, memo, context) {
        return _(obj).map(function(value, index, list) {
            memo = iterator.call(context, memo, value, index, list);
            return memo;
        });
    },
    lensGet: getter,
    lensMod: modifier,
    lensSet: setter,
    lensDel: deleter,
    insertAt: insertAt,
    insertBefore: insertBefore,
    insertAfter: insertAfter
});

// examples
//
// > _({ x: 1, y: 2 }).lensDel(['x'])
// { y: 2 }
//
// > _({ x: 1, y: 2 }).lensSet(['y'], 3)
// { x: 1, y: 3 }
//
// > _({ x: 1, y: 2 }).lensMod(['y'], function(y) { return y + 1; })
// { x: 1, y: 3 }
//
// > _({ x: 1, y: 2 }).lensGet(['x'])
// 1
//
// > _([0, 1, 2]).insertAt([3], 3)
// [0, 1, 2, 3]
//
// > _([0, 1, 2]).insertBefore([0], -1)
// [-1, 0, 1, 2]
//
// > _([0, 1, 2]).insertAfter([2], 3)
// [0, 1, 2, 3]