ramda.js

ramda.js

ramda.js 0.0.1
https://github.com/CrossEye/ramda
(c) 2013-2014 Scott Sauyet and Michael Hurley
Ramda may be freely distributed under the MIT license.

Ramda

¶

A practical functional library for Javascript programmers. Ramda is a collection of tools to make it easier to use Javascript as a functional programming language. (The name is just a silly play on lambda.)

Basic Setup

Uses a technique from the Universal Module Definition to wrap this up for use in Node.js or in the browser, with or without an AMD-style loader.

(function (root, factory) {if (typeof exports === 'object') {module.exports = factory(root);} else if (typeof define === 'function' && define.amd) {define(factory);} else {root.ramda = factory(root);}}(this, function (global) {

    return  (function() {

¶

This object is what is actually returned, with all the exposed functions attached as properties.
```
        var R = {};
```
¶

Internal Functions and Properties

        var undef = (function(){})(), EMPTY;

Makes a public alias for one of the public functions:

        var aliasFor = function(oldName) {
            var fn = function(newName) {R[newName] = R[oldName]; return fn;};
            fn.is = fn.are = fn.and = fn;
            return fn;
        };

(private) slice implemented iteratively for performance

        var _slice = function (args, from, to) {
            var i, arr = [];
            from = from || 0;
            to = to || args.length;
            for (i = from; i < to; i++) {
                arr[arr.length] = args[i];
            }
            return arr;
        };

(private)

        var isArray = function(val) {return Object.prototype.toString.call(val) === "[object Array]";};

Returns a curried version of the supplied function. For example:

 var discriminant = function(a, b, c) {
     return b * b - 4 * a * c;
 };
 var f = curry(discriminant);
 var g = f(3), h = f(3, 7) i = g(7);
 i(4) ≅ h(4) == g(7, 4) == f(3, 7, 4) == 1

Almost all exposed functions of more than one parameter already have curry applied to them.

        var _ = R.curry = function(fn) {
            var fnArity = fn.length;
            var f = function(args) {
                return arity(Math.max(fnArity - (args && args.length || 0), 0), function () {
                    var newArgs = (args || []).concat(_slice(arguments, 0));
                    if (newArgs.length >= fnArity) {
                        return fn.apply(this, newArgs);
                    }
                    else {return f(newArgs);}
                });
            };

            return f([]);
        };

(private) for dynamically dispatching Ramda method to non-Array objects

        var hasMethod = function(methodName, obj) {
            return obj && !isArray(obj) && typeof obj[methodName] === 'function'; 
        };

        var mkArgStr = function(n) {
            var arr = [], idx = -1;
            while(++idx < n) {
                arr[idx] = "arg" + idx;
            }
            return arr.join(", ");
        };

¶

Wraps a function that may be nullary, or may take fewer than or more than n parameters, in a function that specifically takes exactly n parameters. Any extraneous parameters will not be passed on to the function supplied
```
        var nAry = R.nAry = (function() {
            var cache = {};
```

For example:
cache[3] = function(func) {
    return function(arg0, arg1, arg2) {
        return func.call(this, arg0, arg1, arg2);
    }
};

            var makeN = function(n) {
                var fnArgs = mkArgStr(n);
                var body = [
                    "    return function(" + fnArgs + ") {",
                    "        return func.call(this" + (fnArgs ? ", " + fnArgs : "") + ");",
                    "    }"
                ].join("\n");
                return new Function("func", body);
            };

            return function(n, fn) {
                return (cache[n] || (cache[n] = makeN(n)))(fn);
            };
        }());

¶

Wraps a function that may be nullary, or may take fewer than or more than n parameters, in a function that specifically takes exactly n parameters. Note, though, that all parameters supplied will in fact be passed along, in contrast with nAry, which only passes along the exact number specified.
```
        var arity = R.arity = (function() {
            var cache = {};
```

For example:
cache[3] = function(func) {
    return function(arg0, arg1, arg2) {
        return func.apply(this, arguments);
    }
};

            var makeN = function(n) {
                var fnArgs = mkArgStr(n);
                var body = [
                    "    return function(" + fnArgs + ") {",
                    "        return func.apply(this, arguments);",
                    "    }"
                ].join("\n");
                return new Function("func", body);
            };

            return function(n, fn) {
                return (cache[n] || (cache[n] = makeN(n)))(fn);
            };
        }());

Turns a named method of an object (or object prototype) into a function that can be called directly. The object becomes the last parameter to the function, and the function is automatically curried. Passing the optional len parameter restricts the function to the initial len parameters of the method.

        var invoker = R.invoker = function(name, obj, len) {
            var method = obj[name];
            var length = len === undef ? method.length : len;
            return method && _(nAry(length + 1, function() {
                if(arguments.length) {
                    var target = Array.prototype.pop.call(arguments);
                    var targetMethod = target[name];
                    if (targetMethod == method) {
                        return targetMethod.apply(target, arguments);
                    }
                }
                return undef;
            }));
        };

Creates a new function that calls the function fn with parameters consisting of the result of the calling each supplied handler on successive arguments, followed by all unmatched arguments.

If there are extra expected arguments that don't need to be transformed, although you can ignore them, it might be best to pass in and identity function so that the new function correctly reports arity. See for example, the definition of project, below.

        var useWith = R.useWith = function(fn /*, transformers */) {
            var transformers = _slice(arguments, 1);
            var tlen = transformers.length;
            return _(arity(tlen, function() {
                var args = [], idx = -1;
                while (++idx < tlen) {
                    args.push(transformers[idx](arguments[idx]));
                }
                return fn.apply(this, args.concat(_slice(arguments, tlen)));
            }));
        };

A two-step version of the useWith function. This would allow us to write project, currently written as useWith(map, pickAll, identity), as, instead, use(map).over(pickAll, identity), which is a bit more explicit. TODO: One of these versions should be eliminated eventually. So not worrying about the duplication for now.

        R.use = function(fn) {
            return {
                over: function(/*transformers*/) {
                    var transformers = _slice(arguments, 0);
                    var tlen = transformers.length;
                    return _(arity(tlen, function() {
                        var args = [], idx = -1;
                        while (++idx < tlen) {
                            args.push(transformers[idx](arguments[idx]));
                        }
                        return fn.apply(this, args.concat(_slice(arguments, tlen)));
                    }));
                }
            };
        };

Fills out an array to the specified length. Internal private function.

        var expand = function(a, len) {
            var arr = a ? isArray(a) ? a : _slice(a) : [];
            while(arr.length < len) {arr[arr.length] = undef;}
            return arr;
        };

(Internal use only) The basic implementation of each.

        var internalEach = _(function(useIdx, fn, list) {
            var idx = -1, len = list.length;
            while (++idx < len) {
               if (useIdx) {
                 fn(list[idx], idx, list);
               } else {
                 fn(list[idx]);
               }
            }

¶

i can't bear not to return something
```
            return list;
        });
```

Loop over a list for side effects. Nasty, yes, but this is a practical library

        var each = R.each = internalEach(false);
        each.idx = internalEach(true);
        aliasFor("each").is("forEach");

Create a shallow copy of an array.

        var clone = R.clone = function(list) {
            return list.concat();
        };

Create a deep copy of an object.

        R.cloneDeep = function(obj) {
          return JSON.parse(JSON.stringify(obj));
        };

¶

Core Functions
¶

Prototypical (or only) empty list
```
        EMPTY = [];
```

Boolean function which reports whether a list is empty.

        var isEmpty = R.isEmpty = function(arr) {return !arr || !arr.length;};

Returns a new list with the new element at the front and the existing elements following

        var prepend = R.prepend = function(el, arr) {return [el].concat(arr);};
        aliasFor("prepend").is("cons");

Returns the first element of a list

        var head = R.head = function(arr) {
            arr = arr || EMPTY;
            return arr[0]; 
        };
        aliasFor("head").is("car").and("first");

Returns the last element of a list

        R.last = function(arr) {
            arr = arr || EMPTY;
            return arr[arr.length - 1];
        };

Returns the rest of the list after the first element. If the passed-in list is not annary, but is an object with a tail method, it will return object.tail().

        var tail = R.tail = function(arr) {
            arr = arr || EMPTY;
            if (hasMethod('tail', arr)) {
                return arr.tail();
            }
            return (arr.length > 1) ? _slice(arr, 1) : [];
        };
        aliasFor("tail").is("cdr");

Boolean function which is true for non-list, false for a list.

        R.isAtom = function(x) {
            return (x !== null) && (x !== undef) && Object.prototype.toString.call(x) !== "[object Array]";
        };

Returns a new list with the new element at the end of a list following all the existing ones.

        var append = R.append = function(el, list) {
            var newList = clone(list);
            newList.push(el);
            return newList;
        };
        aliasFor("append").is("push");

Returns a new list consisting of the elements of the first list followed by the elements of the second.

        var merge = R.merge = _(function(list1, list2) {
            return (isEmpty(list1)) ? clone(list2) : list1.concat(list2);
        });
        aliasFor("merge").is("concat");

A surprisingly useful function that does nothing but return the parameter supplied to it.

        var identity = R.identity = function(x) {return x;};
        aliasFor("identity").is("I");

Returns a fixed list (of size n) of identical values.

        R.repeatN = _(function(value, n) {
            var arr = [];
            var i = -1;
            while(++i < n) {
                arr[i] = value;
            }
            return arr;
        });

¶

Function functions :-)
¶

These functions make new functions out of old ones.

Creates a new function that runs each of the functions supplied as parameters in turn, passing the output of each one to the next one, starting with whatever arguments were passed to the initial invocation. Note that if var h = compose(f, g), h(x) calls g(x) first, passing the result of that to f().

        var compose = R.compose = function() {  // TODO: type check of arguments?
            var fns = _slice(arguments);
            return function() {
                return foldr(function(args, fn) { return [fn.apply(this, args)]; }, _slice(arguments), fns)[0];
            };
        };

Similar to compose, but processes the functions in the reverse order so that if if var h = pipe(f, g), h(x) calls f(x) first, passing the result of that to g().

        R.pipe = function() { // TODO: type check of arguments?
            return compose.apply(this, _slice(arguments).reverse());
        };
        aliasFor("pipe").is("sequence");

Returns a new function much like the supplied one except that the first two arguments are inverted.

        var flip = R.flip = function(fn) {
            return _(function(a, b) {
                return fn.apply(this, [b, a].concat(_slice(arguments, 2)));
            });
        };

Creates a new function that acts like the supplied function except that the left-most parameters are pre-filled.

        R.lPartial = function (fn) {
            var args = _slice(arguments, 1);
            return arity(Math.max(fn.length - args.length, 0), function() {
                return fn.apply(this, args.concat(_slice(arguments)));
            });
        };
        aliasFor("lPartial").is("applyLeft");

Creates a new function that acts like the supplied function except that the right-most parameters are pre-filled.

        R.rPartial =function (fn) {
            var args = _slice(arguments, 1);
            return arity(Math.max(fn.length - args.length, 0), function() {
                return fn.apply(this, _slice(arguments).concat(args));
            });
        };
        aliasFor("rPartial").is("applyRight");

Creates a new function that stores the results of running the supplied function and returns those stored value when the same request is made. Note: this really only handles string and number parameters.

        R.memoize = function(fn) {
            var cache = {};
            return function() {
                var position = foldl(function(cache, arg) {return cache[arg] || (cache[arg] = {});}, cache,
                        _slice(arguments, 0, arguments.length - 1));
                var arg = arguments[arguments.length - 1];
                return (position[arg] || (position[arg] = fn.apply(this, arguments)));
            };
        };

Wraps a function up in one that will only call the internal one once, no matter how many times the outer one is called. Note: this is not really pure; it's mostly meant to keep side-effects from repeating.

        R.once = function(fn) {
            var called = false, result;
            return function() {
                if (called) {return result;}
                called = true;
                result = fn.apply(this, arguments);
                return result;
            };
        };

Wrap a function inside another to allow you to make adjustments to the parameters or do other processing either before the internal function is called or with its results.

        R.wrap = function(fn, wrapper) {
            return function() {
                return wrapper.apply(this, [fn].concat(_slice(arguments)));
            };
        };

Wraps a constructor function inside a (curried) plain function that can be called with the same arguments and returns the same type. Allows, for instance,

var Widget = function(config) { /* ... */ }; // Constructor
Widget.prototype = { /* ... */ }
map(construct(Widget), allConfigs); //=> list of Widgets

        R.construct = function(fn) {
            var f = function() {
                var obj = new fn();
                fn.apply(obj, arguments);
                return obj;
            };
            return fn.length > 1 ? _(nAry(fn.length, f)) : f;
        };

Runs two separate functions against a single one and then calls another function with the results of those initial calls.

        R.fork = function(after) {
            var fns = _slice(arguments, 1);
            return function() {
                var args = arguments;
                return after.apply(this, map(function(fn) {return fn.apply(this, args);}, fns));
            };
        };

¶

List Functions

These functions operate on logical lists, here plain arrays. Almost all of these are curried, and the list parameter comes last, so you can create a new function by supplying the preceding arguments, leaving the list parameter off. For instance:

// skip third parameter
var checkAllPredicates = reduce(andFn, alwaysTrue);
// ... given suitable definitions of odd, lt20, gt5
var test = checkAllPredicates([odd, lt20, gt5]);
// test(7) => true, test(9) => true, test(10) => false,
// test(3) => false, test(21) => false,

(Internal use only) The basic implementation of filter.

        var internalFoldl = _(function(useIdx, fn, acc, list) {
            if (hasMethod('foldl', list)) {
                return list.foldl(fn, acc); // TODO: figure out useIdx
            }
            var idx = -1, len = list.length, result = [];
            while (++idx < len) {
                acc = (useIdx) ? fn(acc, list[idx], idx, list) : fn(acc, list[idx]);
            }
            return acc;
        });

¶

Returns a single item, by successively calling the function with the current element and the the next element of the list, passing the result to the next call. We start with the acc parameter to get things going. The function supplied should accept this running value and the latest element of the list, and return an updated value.
```
        var foldl = R.foldl = internalFoldl(false);
        aliasFor("foldl").is("reduce");
        R.foldl.idx = internalFoldl(true);
```

Much like foldl/reduce, except that this takes as its starting value the first element in the list.

        R.foldl1 = _(function (fn, list) {
            if (isEmpty(list)) {
                throw new Error("foldl1 does not work on empty lists");
            }
            return foldl(fn, head(list), tail(list));
        });

(Internal use only) The basic implementation of foldr.

        var internalFoldr= _(function(useIdx, fn, acc, list) {
            if (hasMethod('foldr', list)) {
                return list.foldr(fn, acc); // TODO: figure out useIdx
            }
            var idx = list.length;
            while (idx--) {
                acc = (useIdx) ? fn(acc, list[idx], idx, list) : fn(acc, list[idx]);
            }
            return acc;
        });

¶

Returns a single item, by successively calling the function with the current element and the the next Similar to foldl/reduce except that it moves from right to left on the list.
```
        var foldr = R.foldr = internalFoldr(false);
        aliasFor("foldr").is("reduceRight");
        R.foldr.idx = internalFoldr(true);
```

Much like foldr/reduceRight, except that this takes as its starting value the last element in the list.

        R.foldr1 = _(function (fn, list) {
            if (isEmpty(list)) {
                throw new Error("foldr1 does not work on empty lists");
            }
            var newList = clone(list), acc = newList.pop();
            return foldr(fn, acc, newList);
        });

Builds a list from a seed value, using a function that returns falsy to quit and a pair otherwise, consisting of the current value and the seed to be used for the next value.

        R.unfoldr = _(function(fn, seed) {
            var pair = fn(seed), result = [];
            while (pair && pair.length) {
                result.push(pair[0]);
                pair = fn(pair[1]);
            }
            return result;
        });

(Internal use only) The basic implementation of map.

        var internalMap = _(function(useIdx, fn, list) {
            if (hasMethod('map', list)) {
                return list.map(fn);
            }
            var idx = -1, len = list.length, result = new Array(len);
            if (useIdx) {
                while (++idx < len) {
                    result[idx] = fn(list[idx], idx, list);
                }
            } else {
                while (++idx < len) {
                    result[idx] = fn(list[idx]);
                }
            }
            return result;
        });

¶

Returns a new list constructed by applying the function to every element of the list supplied.
```
        var map = R.map = internalMap(false);
```

Like map, but passes additional parameters to the predicate function. Parameters are list item, index of item in list, entire list.

Example:

var squareEnds = function(x, idx, list) {
    return (idx === 0 || idx === list.length - 1) ? x * x : x;
};

map(squareEnds, [8, 6, 7, 5, 3, 0, 9];
//=> [64, 6, 7, 5, 3, 0, 81]

        map.idx = internalMap(true);

Adds a map-like function for objects.

TODO: consider mapObj.key in parallel with mapObj.idx. Also consider folding together with map implementation.

        R.mapObj = _(function(fn, obj) {
            return foldl(function(acc, key) {acc[key] = fn(obj[key]); return acc;}, {}, keys(obj))
        });

Reports the number of elements in the list

        R.size = function(arr) {return arr.length;};

(Internal use only) The basic implementation of filter.

        var internalFilter = _(function(useIdx, fn, list) {
            if (hasMethod('filter', list)) {
                return list.filter(fn); // TODO: figure out useIdx
            }
            var idx = -1, len = list.length, result = [];
            while (++idx < len) {
                if (!useIdx && fn(list[idx]) || fn(list[idx], idx, list)) {
                    result.push(list[idx]);
                }
            }
            return result;
        });

¶

Returns a new list containing only those items that match a given predicate function.
```
        var filter = R.filter = internalFilter(false);
```

Like filter, but passes additional parameters to the predicate function. Parameters are list item, index of item in list, entire list.

Example:

var lastTwo = function(val, idx, list) {
    return list.length - idx <= 2;
};
filter.idx(lastTwo, [8, 6, 7, 5, 3, 0 ,9]); //=> [0, 9]

        filter.idx = internalFilter(true);

Similar to filter, except that it keeps only those that don't match the given predicate functions.

        var reject = R.reject = _(function(fn, list) {
            return filter(not(fn), list);
        });

Like reject, but passes additional parameters to the predicate function. Parameters are list item, index of item in list, entire list.

Example:

var lastTwo = function(val, idx, list) {
    return list.length - idx <= 2;
};
reject.idx(lastTwo, [8, 6, 7, 5, 3, 0 ,9]);
//=> [8, 6, 7, 5, 3]

        reject.idx = _(function(fn, list) {
            return filter.idx(not(fn), list);
        });

Returns a new list containing the elements of the given list up until the first one where the function supplied returns false when passed the element.

        R.takeWhile = _(function(fn, list) {
            if (hasMethod('takeWhile', list)) {
                return list.takeWhile(fn);
            }
            var idx = -1, len = list.length, taking = true, result = [];
            while (taking) {
                ++idx;
                if (idx < len && fn(list[idx])) {
                    result.push(list[idx]);
                } else {
                    taking = false;
                }
            }
            return result;
        });

Returns a new list containing the first n elements of the given list.

        var take = R.take = _(function(n, list) {
            if (hasMethod('take', list)) {
                return list.take(n);
            }
            var ls = clone(list);
            ls.length = n;
            return ls;
        });

Returns a new list containing the elements of the given list starting with the first one where the function supplied returns false when passed the element.

        R.skipUntil = _(function(fn, list) {
            var idx = -1, len = list.length, taking = false, result = [];
            while (!taking) {
                ++idx;
                if (idx >= len || fn(list[idx])) {
                    taking = true;
                }
            }
            while (idx < len) {
                result.push(list[idx++]);
            }
            return result;
        });

Returns a new list containing all but the first n elements of the given list.

        R.skip = _(function(n, list) {
            if (hasMethod('skip', list)) {
                return list.skip(n);
            }
            return _slice(list, n);
        });
        aliasFor('skip').is('drop');

Returns the first element of the list which matches the predicate, or undefined if no element matches.

        R.find = _(function(fn, list) {
            var idx = -1, len = list.length;
            while (++idx < len) {
                if (fn(list[idx])) {
                    return list[idx];
                }
            }
            return undef;
        });

Returns the index of first element of the list which matches the predicate, or undefined if no element matches.

        R.findIndex = _(function(fn, list) {
            var idx = -1, len = list.length;
            while (++idx < len) {
                if (fn(list[idx])) {
                    return idx;
                }
            }
            return undef;
        });

Returns the last element of the list which matches the predicate, or undefined if no element matches.

        R.findLast = _(function(fn, list) {
            var idx = list.length;
            while (--idx) {
                if (fn(list[idx])) {
                    return list[idx];
                }
            }
            return undef;
        });

Returns the index of last element of the list which matches the predicate, or undefined if no element matches.

        R.findLastIndex = _(function(fn, list) {
            var idx = list.length;
            while (--idx) {
                if (fn(list[idx])) {
                    return idx;
                }
            }
            return undef;
        });

Returns true if all elements of the list match the predicate, false if there are any that don't.

        var all = R.all = _(function (fn, list) {
            var i = -1;
            while (++i < list.length) {
                if (!fn(list[i])) {
                    return false;
                }
            }
            return true;
        });
        aliasFor("all").is("every");

Returns true if any elements of the list match the predicate, false if none do.

        var any = R.any = _(function(fn, list) {
            var i = -1;
            while (++i < list.length) {
                if (fn(list[i])) {
                    return true;
                }
            }
            return false;
        });
        aliasFor("any").is("some");

¶

Returns true if the list contains the sought element, false if it does not. Equality is strict here, meaning reference equality for objects and non-coercing equality for primitives.
```
        var contains = R.contains = _(function(a, list) {
            return list.indexOf(a) > -1;
        });
```

Returns true if the list contains the sought element, false if it does not, based upon the value returned by applying the supplied predicated to two list elements. Equality is strict here, meaning reference equality for objects and non-coercing equality for primitives. Probably inefficient.

        var containsWith = _(function(pred, x, list) {
            var idx = -1, len = list.length;
            while (++idx < len) {
                if (pred(x, list[idx])) {return true;}
            }
            return false;
        });

Returns a new list containing only one copy of each element in the original list. Equality is strict here, meaning reference equality for objects and non-coercing equality for primitives.

        var uniq = R.uniq = function(list) {
            return foldr(function(acc, x) { return (contains(x, acc)) ? acc : prepend(x, acc); }, EMPTY, list);
        };

returns true if all of the elements in the list are unique.

        R.isSet = function(list) {
            return uniq(list).length === list.length;
        };

¶

Returns a new list containing only one copy of each element in the original list, based upon the value returned by applying the supplied predicate to two list elements. Equality is strict here, meaning reference equality for objects and non-coercing equality for primitives.
```
        var uniqWith = _(function(pred, list) {
            return foldr(function(acc, x) {return (containsWith(pred, x, acc)) ? acc : prepend(x, acc); }, EMPTY, list);
        });
```
¶

Returns a new list by plucking the same named property off all objects in the list supplied.
```
        var pluck = R.pluck = _(function(p, list) {return map(prop(p), list);});
```

Returns a list that contains a flattened version of the supplied list. For example:

flatten([1, 2, [3, 4], 5, [6, [7, 8, [9, [10, 11], 12]]]]);
// => [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12];

        var flatten = R.flatten = function(list) {
            var idx = -1, len = list ? list.length : 0, result = [], push = result.push, val;
            while (++idx < len) {
                val = list[idx];
                push.apply(result, isArray(val) ? flatten(val) : [val]);
            }
            return result;
        };

Creates a new list out of the two supplied by applying the function to each equally-positioned pair in the lists. For example,

zipWith(f, [1, 2, 3], ['a', 'b', 'c'])
//    => [f(1, 'a'), f(2, 'b'), f(3, 'c')];

Note that the output list will only be as long as the length os the first list passed in.

        R.zipWith = _(function(fn, a, b) {
            var rv = [], i = -1, len = a.length;
            while(++i < len) {
                rv[i] = fn(a[i], b[i]);
            }
            return rv;
        });

Creates a new list out of the two supplied by yielding the pair of each equally-positioned pair in the lists. For example,

zip([1, 2, 3], ['a', 'b', 'c'])
//    => [[1, 'a'], [2, 'b'], [3, 'c']];

        R.zip =  _(function(a, b) { // = zipWith(prepend);
            var rv = [], i = -1, len = a.length;
            while (++i < len) {
                rv[i] = [a[i], b[i]];
            }
            return rv;
        });

Creates a new list out of the two supplied by applying the function to each possible pair in the lists. For example,

xProdWith(f, [1, 2], ['a', 'b'])
//    => [f(1, 'a'), f(1, 'b'), f(2, 'a'), f(2, 'b')];

        R.xprodWith = _(function(fn, a, b) {
            if (isEmpty(a) || isEmpty(b)) {return EMPTY;}
            var i = -1, ilen = a.length, j, jlen = b.length, result = []; // better to push them all or to do `new Array(ilen * jlen)` and calculate indices?
            while (++i < ilen) {
                j = -1;
                while (++j < jlen) {
                    result.push(fn(a[i], b[j]));
                }
            }
            return result;
        });

Creates a new list out of the two supplied by yielding the pair of each possible pair in the lists. For example,

xProd([1, 2], ['a', 'b'])
//    => [[1, 'a'], [1, 'b')], [2, 'a'], [2, 'b']];

        R.xprod = _(function(a, b) { // = xprodWith(prepend); (takes about 3 times as long...)
            if (isEmpty(a) || isEmpty(b)) {return EMPTY;}
            var i = -1, ilen = a.length, j, jlen = b.length, result = []; // better to push them all or to do `new Array(ilen * jlen)` and calculate indices?
            while (++i < ilen) {
                j = -1;
                while (++j < jlen) {
                    result.push([a[i], b[j]]);
                }
            }
            return result;
        });

Returns a new list with the same elements as the original list, just in the reverse order.

        R.reverse = function(list) {
            return clone(list || []).reverse();
        };

// Returns a list of numbers from from (inclusive) to to (exclusive). For example,

range(1, 5) // => [1, 2, 3, 4]
range(50, 53) // => [50, 51, 52]

        R.range = _(function(from, to) {
            if (from >= to) {return EMPTY;}
            var idx, result = new Array(Math.floor(to) - Math.ceil(from));
            for (idx = 0; from < to; idx++, from++) {
                result[idx] = from;
            }
            return result;
        });

¶

Returns the first zero-indexed position of an object in a flat list
```
        R.indexOf = invoker("indexOf", Array.prototype, 1);
```

Returns the last zero-indexed position of an object in a flat list

        R.lastIndexOf = invoker("lastIndexOf", Array.prototype, 1);

¶

Returns the elements of the list as a string joined by a separator.
```
        R.join = invoker("join", Array.prototype);
```

Returns the sublist of a list starting with the first index and ending before the second one.

        R.slice = invoker("slice", Array.prototype);
        R.slice.from = flip(R.slice)(undef);

¶

ramda.splice has a different contract than Array.splice. Array.splice mutates its array and returns the removed elements. ramda.splice does not mutate the passed in list (well, it makes a shallow copy), and returns a new list with the specified elements removed.
```
        R.splice = _(function(start, len, list) {
            var ls = _slice(list, 0);
            ls.splice(start, len);
            return ls;
        });
```

Returns the nth element of a list (zero-indexed)

        R.nth = _(function(n, list) {
          return list[n];
        });

Makes a comparator function out of a function that reports whether the first element is less than the second.

var cmp = comparator(function(a, b) {
    return a.age < b.age;
};
sort(cmp, people);

        var comparator = R.comparator = function(pred) {
            return function(a, b) {
                return pred(a, b) ? -1 : pred(b, a) ? 1 : 0;
            };
        };

¶

Returns a copy of the list, sorted according to the comparator function, which should accept two values at a time and return a negative number if the first value is smaller, a positive number if it's larger, and zero if they are equal. Please note that this is a copy of the list. It does not modify the original.
```
        var sort = R.sort = _(function(comparator, list) {
            return clone(list).sort(comparator);
        });
```

Splits a list into sublists stored in an object, based on the result of calling a String-returning function on each element, and grouping the results according to values returned.

var byGrade = partition(function(student) {
    var score = student.score
    return (score < 65) ? 'F' : (score < 70) ? 'D' :
           (score < 80) ? 'C' : (score < 90) ? 'B' : 'A';
};
var students = [{name: "Abby", score: 84} /*, ... */,
                {name: 'Jack', score: 69}];
byGrade(students);
//=> {
//   "A": [{name: 'Dianne', score: 99} /*, ... */],
//   "B": [{name: "Abby", score: 84} /*, ... */]
//   /*, ... */
//   "F": [{name: 'Eddy', score: 58}]
// }

        R.partition = _(function(fn, list) {
            return foldl(function(acc, elt) {
                var key = fn(elt);
                acc[key] = append(elt, acc[key] || (acc[key] = []));
                return acc;
            }, {}, list);
        });

¶

Object Functions
¶

These functions operate on plain Javascript object, adding simple functions to test properties on these objects. Many of these are of most use in conjunction with the list functions, operating on lists of objects.

Runs the given function with the supplied object, then returns the object.

        R.tap = _(function(x, fn) {
            if (typeof fn === "function") {
                fn(x);
            }
            return x;
        });
        aliasFor("tap").is("K");

¶

Tests if two items are equal. Equality is strict here, meaning reference equality for objects and non-coercing equality for primitives.
```
        R.eq = _(function(a, b) {
            return a === b;
        });
```

Returns a function that when supplied an object returns the indicated property of that object, if it exists.

        var prop = R.prop = _(function(p, obj) {return obj[p];});
        aliasFor("prop").is("get"); // TODO: are we sure?  Matches some other libs, but might want to reserve for other use.

¶

Returns a function that when supplied an object returns the result of running the indicated function on that object, if it has such a function.
```
        R.func = _(function(fn, obj) {return obj[fn].apply(obj, _slice(arguments, 2));});
```
¶

Returns a function that when supplied a property name returns that property on the indicated object, if it exists.
```
        R.props = _(function(obj, prop) {return obj && obj[prop];});
```

Returns a function that always returns the given value.

        var always = R.always = function(val) {
            return function() {return val;};
        };

        var anyBlanks = any(function(val) {return val === null || val === undef;});

Returns a list containing the names of all the enumerable own properties of the supplied object.

        var keys = R.keys = function (obj) {
            var prop, ks = [];
            for (prop in obj) {
                if (obj.hasOwnProperty(prop)) {
                    ks.push(prop);
                }
            }
            return ks;
        };

Returns a list of all the enumerable own properties of the supplied object.

        R.values = function (obj) {
            var prop, vs = [];
            for (prop in obj) {
                if (obj.hasOwnProperty(prop)) {
                    vs.push(obj[prop]);
                }
            }
            return vs;
        };

        var partialCopy = function(test, obj) {
            var copy = {};
            each(function(key) {if (test(key, obj)) {copy[key] = obj[key];}}, keys(obj));
            return copy;
        };

Returns a partial copy of an object containing only the keys specified. If the key does not exist, the property is ignored

        R.pick = _(function(names, obj) {
            return partialCopy(function(key) {return contains(key, names);}, obj);
        });

Similar to pick except that this one includes a key: undefined pair for properties that don't exist.

        var pickAll = R.pickAll = _(function(names, obj) {
            var copy = {};
            each(function(name) { copy[name] = obj[name]; }, names);
            return copy;
        });

Returns a partial copy of an object omitting the keys specified.

        R.omit = _(function(names, obj) {
            return partialCopy(function(key) {return !contains(key, names);}, obj);
        });

Returns a new object that mixes in the own properties of two objects.

        R.mixin = _(function(a, b) {
          var mixed = pickAll(R.keys(a), a);
          each(function(key) { mixed[key] = b[key]; }, R.keys(b));
          return mixed;
        });

¶

Reports whether two functions have the same value for the specified property. Useful as a curried predicate.
```
        R.eqProps = _(function(prop, obj1, obj2) {return obj1[prop] === obj2[prop];});
```

where takes a spec object and a test object and returns true if the test satisfies the spec. Any property on the spec that is not a function is interpreted as an equality relation. For example:

var spec = {x: 2};
where(spec, {w: 10, x: 2, y: 300}); // => true, x === 2
where(spec, {x: 1, y: 'moo', z: true}); // => false, x !== 2

If the spec has a property mapped to a function, then where evaluates the function, passing in the test object's value for the property in question, as well as the whole test object. For example:

var spec = {x: function(val, obj) { return  val + obj.y > 10; };
where(spec, {x: 2, y: 7}); // => false
where(spec, {x: 3, y: 8}); // => true

where is well suited to declarativley expressing constraints for other functions, e.g., filter:

var xs = [{x: 2, y: 1}, {x: 10, y: 2}, 
          {x: 8, y: 3}, {x: 10, y: 4}];
var fxs = filter(where({x: 10}), xs); 
// fxs ==> [{x: 10, y: 2}, {x: 10, y: 4}]

        R.where = _(function(spec, test) {
            return all(function(key) {
                var val = spec[key];
                return (typeof val === 'function') ? val(test[key], test) : (test[key] === val);
            }, keys(spec));
        });

¶

Miscellaneous Functions
¶

A few functions in need of a good home.
¶

Expose the functions from ramda as properties on another object. If the passed-in object is the global object, or the passed-in object is "falsy", then the ramda functions become global functions.
```
        R.installTo = function(obj) {
            each(function(key) {
                (obj || global)[key] = R[key];
            })(keys(R));
        };
```
¶

A function that always returns 0.
```
        R.alwaysZero = always(0);
```
¶

A function that always returns false.
```
        R.alwaysFalse = always(false);
```
¶

A function that always returns true.
```
        R.alwaysTrue = always(true);
```
¶

Logic Functions
¶

These functions are very simple wrappers around the built-in logical operators, useful in building up more complex functional forms.
¶

A function wrapping calls to the two functions in an && operation, returning true or false. Note that this is short-circuited, meaning that the second function will not be invoked if the first returns a false-y value.
```
        R.and = _(function(f, g) { // TODO: arity?
           return function() {return !!(f.apply(this, arguments) && g.apply(this, arguments));};
        });
```
¶

A function wrapping calls to the two functions in an || operation, returning true or false. Note that this is short-circuited, meaning that the second function will not be invoked if the first returns a truth-y value. (Note also that at least Oliver Twist can pronounce this one...)
```
        R.or = _(function(f, g) { // TODO: arity?
           return function() {return !!(f.apply(this, arguments) || g.apply(this, arguments));};
        });
```
¶

A function wrapping a call to the given function in a ! operation. It will return true when the underlying function would return a false-y value, and false when it would return a truth-y one.
```
        var not = R.not = function (f) {
            return function() {return !f.apply(this, arguments);};
        };
```

TODO: is there a way to unify allPredicates and anyPredicates? they are sooooo similar

Given a list of predicates returns a new predicate that will be true exactly when all of them are.

        R.allPredicates = function(preds /*, val1, val12, ... */) {
            var args = _slice(arguments, 1);
            var maxArity = max(pluck("length", preds));

            var andPreds = arity(maxArity, function() {
                var idx = -1;
                while (++idx < preds.length) {
                    if (!preds[idx].apply(null, arguments)) { return false; }
                }
                return true;
            });
            return (isEmpty(args)) ? andPreds : andPreds.apply(null, args);
        };

Given a list of predicates returns a new predicate that will be true exactly when any one of them is.

        R.anyPredicates = function(preds /*, val1, val12, ... */) {
            var args = _slice(arguments, 1);
            var maxArity = max(pluck("length", preds));

            var orPreds = arity(maxArity, function() {
                var idx = -1;
                while (++idx < preds.length) {
                    if (preds[idx].apply(null, arguments)) { return true; }
                }
                return false;
            });
            return (isEmpty(args)) ? orPreds : orPreds.apply(null, args);
        };

¶

Arithmetic Functions
¶

These functions wrap up the certain core arithmetic operators

Adds two numbers. Automatic curried:

var add7 = add(7);
add7(10); // => 17

        var add = R.add = _(function(a, b) {return a + b;});

Multiplies two numbers. Automatically curried:

var mult3 = multiply(3);
mult3(7); // => 21

        var multiply = R.multiply = _(function(a, b) {return a * b;});

¶

Subtracts the second parameter from the first. This is automatically curried, and while at times the curried version might be useful, often the curried version of subtractN might be what's wanted.
```
var complementaryAngle = subtract(90);
complementaryAngle(30) ; // => 60
```
```
        var subtract = R.subtract = _(function(a, b) {return a - b;});
```
¶

Reversed version of subtract, where first parameter is subtracted from the second. The curried version of this one might me more useful than that of subtract. For instance:
```
var decrement = subtractN(1);
decrement(10); // => 9;
```
```
        R.subtractN = flip(subtract);
```
¶

Divides the first parameter by the second. This is automatically curried, and while at times the curried version might be useful, often the curried version of divideBy might be what's wanted.
```
        var divide = R.divide = _(function(a, b) {return a / b;});
```
¶

Reversed version of divide, where the second parameter is divided by the first. The curried version of this one might be more useful than that of divide. For instance:
```
var half = divideBy(2);
half(42); // => 21
```
```
        R.divideBy = flip(divide);
```

Divides the second parameter by the first and returns the remainder.

        var modulo = R.modulo = _(function(a, b) { return a % b; });

¶

Reversed version of modulo, where the second parameter is divided by the first. The curried version of this one might be more useful than that of modulo. For instance:
```
var isOdd = moduloBy(2);
isOdd(42); // => 0
isOdd(21); // => 1
```
```
        R.moduloBy = flip(modulo);
```
¶

Adds together all the elements of a list.
```
        R.sum = foldl(add, 0);
```
¶

Multiplies together all the elements of a list.
```
        R.product = foldl(multiply, 1);
```
¶

Returns true if the first parameter is less than the second.
```
        R.lt = _(function(a, b) {return a < b;});
```
¶

Returns true if the first parameter is less than or equal to the second.
```
        R.lte = _(function(a, b) {return a <= b;});
```
¶

Returns true if the first parameter is greater than the second.
```
        R.gt = _(function(a, b) {return a > b;});
```
¶

Returns true if the first parameter is greater than or equal to the second.
```
        R.gte = _(function(a, b) {return a >= b;});
```

Determines the largest of a list of numbers (or elements that can be cast to numbers)

        var max = R.max = function(list) {return Math.max.apply(null, list);};

Determines the largest of a list of items as determined by pairwise comparisons from the supplied comparator

        R.maxWith = _(function(comparator, list) {
            if (!isArray(list) || !list.length) {
                return null;
            }
            var idx = 0, max = list[idx];
            while (++idx < list.length) {
                if (comparator(max, list[idx]) < 0) {
                    max = list[idx];
                }
            }
            return max;
        });

TODO: combine this with maxWith?

Determines the smallest of a list of items as determined by pairwise comparisons from the supplied comparator

        R.minWith = _(function(comparator, list) {
            if (!isArray(list) || !list.length) {
                return null;
            }
            var idx = 0, max = list[idx];
            while (++idx < list.length) {
                if (comparator(max, list[idx]) > 0) {
                    max = list[idx];
                }
            }
            return max;
        });

¶

Determines the smallest of a list of numbers (or elements that can be cast to numbers)
```
        R.min = function(list) {return Math.min.apply(null, list);};
```
¶

String Functions
¶

Much of the String.prototype API exposed as simple functions.

A substring of a String:

substring(2, 5, "abcdefghijklm"); //=> "cde"

        var substring = R.substring = invoker("substring", String.prototype);

The trailing substring of a String starting with the nth character:

substringFrom(8, "abcdefghijklm"); //=> "ijklm"

        R.substringFrom = flip(substring)(undef);

The leading substring of a String ending before the nth character:

substringTo(8, "abcdefghijklm"); //=> "abcdefgh"

        R.substringTo = substring(0);

The character at the nth position in a String:

charAt(8, "abcdefghijklm"); //=> "i"

        R.charAt = invoker("charAt", String.prototype);

The ascii code of the character at the nth position in a String:

charCodeAt(8, "abcdefghijklm"); //=> 105
// (... 'a' ~ 97, 'b' ~ 98, ... 'i' ~ 105)

        R.charCodeAt = invoker("charCodeAt", String.prototype);

Tests a regular expression agains a String

match(/([a-z]a)/g, "bananas"); //=> ["ba", "na", "na"]

        R.match = invoker("match", String.prototype);

Finds the index of a substring in a string, returning -1 if it's not present

strIndexOf('c', 'abcdefg) //=> 2

        R.strIndexOf = invoker("indexOf", String.prototype);

Finds the last index of a substring in a string, returning -1 if it's not present

strLastIndexOf('a', 'banana split') //=> 5

        R.strLastIndexOf = invoker("lastIndexOf", String.prototype);

The uppercase version of a string.

toUpperCase('abc') //=> 'ABC'

        R.toUpperCase = invoker("toUpperCase", String.prototype);

The lowercase version of a string.

toLowerCase('XYZ') //=> 'xyz'

        R.toLowerCase = invoker("toLowerCase", String.prototype);

The string split into substring at the specified token

split('.', 'a.b.c.xyz.d') //=>
    ['a', 'b', 'c', 'xyz', 'd']

        R.split = invoker("split", String.prototype, 1);

¶

Data Analysis and Grouping Functions
¶

Functions performing SQL-like actions on lists of objects. These do not have any SQL-like optimizations performed on them, however.

Reasonable analog to SQL select statement.

var kids = [
    {name: 'Abby', age: 7, hair: 'blond', grade: 2},
    {name: 'Fred', age: 12, hair: 'brown', grade: 7}
];
project(['name', 'grade'], kids);
//=> [{name: 'Abby', grade: 2}, {name: 'Fred', grade: 7}]

        R.project = useWith(map, pickAll, identity); // passing `identity` gives correct arity

Determines whether the given property of an object has a specific value Most likely used to filter a list:

var kids = [
  {name: 'Abby', age: 7, hair: 'blond'},
  {name: 'Fred', age: 12, hair: 'brown'},
  {name: 'Rusty', age: 10, hair: 'brown'},
  {name: 'Alois', age: 15, disposition: 'surly'}
];
filter(propEq("hair", "brown"), kids);
//=> Fred and Rusty

        R.propEq = _(function(name, val, obj) {
            return obj[name] === val;
        });

¶

Combines two lists into a set (i.e. no duplicates) composed of the elements of each list.
```
        R.union = compose(uniq, merge);
```
¶

Combines two lists into a set (i.e. no duplicates) composed of the elements of each list. Duplication is determined according to the value returned by applying the supplied predicate to two list elements.
```
        R.unionWith = _(function(pred, list1, list2) {
            return uniqWith(pred, merge(list1, list2));
        });
```

Finds the set (i.e. no duplicates) of all elements in the first list not contained in the second list.

        R.difference = _(function(first, second) {return uniq(reject(flip(contains)(second))(first));});

¶

Finds the set (i.e. no duplicates) of all elements in the first list not contained in the second list. Duplication is determined according to the value returned by applying the supplied predicate to two list elements.
```
        R.differenceWith = _(function(pred, first, second) {
            return uniqWith(pred)(reject(flip(containsWith(pred))(second), first));
        });
```

Combines two lists into a set (i.e. no duplicates) composed of those elements common to both lists.

        R.intersection = _(function(list1, list2) {
            return uniq(filter(flip(contains)(list1), list2));
        });

Combines two lists into a set (i.e. no duplicates) composed of those elements common to both lists. Duplication is determined according to the value returned by applying the supplied predicate to two list elements.

        R.intersectionWith = _(function(pred, list1, list2) {
            var results = [], idx = -1;
            while (++idx < list1.length) {
                if (containsWith(pred, list1[idx], list2)) {
                    results[results.length] = list1[idx];
                }
            }
            return uniqWith(pred, results);
        });

Creates a new list whose elements each have two properties: val is the value of the corresponding item in the list supplied, and key is the result of applying the supplied function to that item.

        var keyValue = _(function(fn, list) { // TODO: Should this be made public?
            return map(function(item) {return {key: fn(item), val: item};}, list);
        });

Sorts the list according to a key generated by the supplied function.

        R.sortBy = _(function(fn, list) {
            /*
              return sort(comparator(function(a, b) {return fn(a) < fn(b);}), list); // clean, but too time-inefficient
              return pluck("val", sort(comparator(function(a, b) {return a.key < b.key;}), keyValue(fn, list))); // nice, but no need to clone result of keyValue call, so...
            */
            return pluck("val", keyValue(fn, list).sort(comparator(function(a, b) {return a.key < b.key;})));
        });

Counts the elements of a list according to how many match each value of a key generated by the supplied function.

        R.countBy = _(function(fn, list) {
            return foldl(function(counts, obj) {
                counts[obj.key] = (counts[obj.key] || 0) + 1;
                return counts;
            }, {}, keyValue(fn, list));
        });

Groups the elements of a list by a key generated by the supplied function.

        R.groupBy = _(function(fn, list) {
            return foldl(function(groups, obj) {
                (groups[obj.key] || (groups[obj.key] = [])).push(obj.val);
                return groups;
            }, {}, keyValue(fn, list));
        });

¶

All the functional goodness, wrapped in a nice little package, just for you!
```
        return R;
    }());
}));
```

ramda.js

Ramda

Basic Setup

Internal Functions and Properties

Core Functions

Function functions :-)

List Functions

Object Functions

Miscellaneous Functions

Logic Functions

Arithmetic Functions

String Functions

Data Analysis and Grouping Functions