tco.md

inspired to understand this from:

• http://raganwald.com/2013/03/28/trampolines-in-javascript.html
• http://bilby.brianmckenna.org/#trampoline see http://en.wikipedia.org/wiki/Trampoline_(computing)

compare with https://github.com/Gozala/js-tail-call/blob/master/core.js for speed (after refactoring to work like his does)

http://jsperf.com/tail-call-head-to-head

tco0.js

//don't use native bind... it's too slow
function recur(fn) {
    return function () {
        var bounce = fn.apply(this, arguments);
        while (bounce.call) {
            bounce = bounce();
        }
        return bounce;
    };
}

function bind(fn, receiver) {
    return function () {
        var args = arguments;
        return function () {
            return fn.apply(receiver || this, args);
        };
    };
}
var sum = bind(function (x, y) {
        return y > 0 ? sum(x + 1, y - 1) :
            y < 0 ? sum(x - 1, y + 1) : 
            x + this.test;
}, {test: "done"});

var sum1 = recur(sum);
sum1(1, 50000);

//a cleaner approach?
function recur(fn) {
    return function () {
        var bounce = fn.apply(this, arguments);
        while (bounce.call) {
            bounce = bounce();
        }
        return bounce;
    };
}

var sum = Function.call.bind.bind(function (x, y) {
        return y > 0 ? sum(x + 1, y - 1) :
            y < 0 ? sum(x - 1, y + 1) : 
            x;
}, null);

var sum1 = recur(sum);
sum1(1, 50000);

tco1.js

function recur(fn) {
    return function () {
        var bounce = fn.apply(this, arguments);
        while (bounce.call) {
            bounce = bounce();
        }
        return bounce;
    };
}
var sum1 = recur(function sum(x, y) {
    return function() {
        return y > 0 ? sum(x + 1, y - 1) :
            y < 0 ? sum(x - 1, y + 1) : 
            x;
        };
});

sum1(1,50000);

tco2.js

// This works properly

var recur = function(fn) {
   return function () {
       var bounce = fn.apply(this, arguments);
       while (typeof bounce === "function") {
           bounce = bounce();
       }
       return bounce;
   };
};
 
var _$ = function(name, fn) {
    var lines = ("" + fn).split(/\r?\n/), first = (lines.pop(), lines.shift()),
        body = lines.join("\n    "), match = first.match(/\(([^)]*)\)/), args = match && match[1] || "";
    var newFnStr = ("" + function name(args) {
        return function () {
            return 'body';
        };
    }).replace("name", name).replace("args", args).replace("return 'body';", body).split("\n    ").join("\n");
    return recur(Function("return " + newFnStr)());
};
 
var sum = _$("sum", function(x, y) {
    return y > 0 ? sum(x + 1, y - 1) :
        y < 0 ? sum(x - 1, y + 1) :
        x;
});
 
console.log(sum(1, 5000));    // 5001
console.log(sum(1, 50000));   // 50001
console.log(sum(1, 500000));  // 500001
console.log(sum(1, 5000000)); // 5000001

tco3.js

// But the function constructor doesn't have access to local functions.

var lib = (function() {
    var recur = function(fn) {
       return function () {
           var bounce = fn.apply(this, arguments);
           while (typeof bounce === "function") {
               bounce = bounce();
           }
           return bounce;
       };
    };

    var _$ = function(name, fn) {
        var lines = ("" + fn).split(/\r?\n/), first = (lines.pop(), lines.shift()),
            body = lines.join("\n    "), match = first.match(/\(([^)]*)\)/), args = match && match[1] || "";
        var newFnStr = ("" + function name(args) {
            return function () {
                return 'body';
            };
        }).replace("name", name).replace("args", args).replace("return 'body';", body).split("\n    ").join("\n");
        return recur(Function("return " + newFnStr)());
    };

    var inc = function(n) {return n + 1;};
    var dec = function(n) {return n - 1;};

    var sum = _$("sum", function(x, y) {
        return y > 0 ? sum(inc(x), dec(y)) :
            y < 0 ? sum(dec(x), inc(y)) :
            x;
    });

    return {
        sum: sum
    };
}());

console.log(lib.sum(1, 5000));  // Uncaught ReferenceError: inc is not defined

tco4.js

// This does work, at the expense of requiring functions to use the namespaced access to otherwise local functions

var lib = (function() {
    var E = {};

    var recur = function(fn) {
       return function () {
           var bounce = fn.apply(this, arguments);
           while (typeof bounce === "function") {
               bounce = bounce();
           }
           return bounce;
       };
    };

    var _$ = function(name, fn) {
        var lines = ("" + fn).split(/\r?\n/), first = (lines.pop(), lines.shift()),
            body = lines.join("\n    "), match = first.match(/\(([^)]*)\)/), args = match && match[1] || "";
        var newFnStr = ("" + function name(args) {
            return function () {
                return 'body';
            };
        }).replace("name", name).replace("args", args).replace("return 'body';", body).split("\n    ").join("\n");
        return recur(Function("E", "return " + newFnStr)(E));
    };

    E.inc = function(n) {return n + 1;};
    E.dec = function(n) {return n - 1;};

    var sum = E.sum = _$("sum", function(x, y) {
        return y > 0 ? sum(E.inc(x), E.dec(y)) :
            y < 0 ? sum(E.dec(x), E.inc(y)) :
            x;
    });

    return E;
}());

console.log(lib.sum(1, 5000));    // 5001
console.log(lib.sum(1, 50000));   // 50001
console.log(lib.sum(1, 500000));  // 500001
console.log(lib.sum(1, 5000000)); // 5000001