base.rst

Ajax下日志框架

.. title:: MochiKit.Base - functional programming and useful comparisons

Name
====

MochiKit.Base - functional programming and useful comparisons


Synopsis
========

::

    myObjectRepr = function () {
        // gives a nice, stable string representation for objects,
        // ignoring any methods
        var keyValuePairs = [];
        for (var k in this) {
            var v = this[k];
            if (typeof(v) != 'function') {
                keyValuePairs.push([k, v]);
            }
        };
        keyValuePairs.sort(compare);
        return "{" + map(
            function (pair) {
                return map(repr, pair).join(":");
            }, 
            keyValuePairs
        ).join(", ") + "}";
    };
            
    // repr() will look for objects that have a repr method
    myObjectArray = [
        {"a": 3, "b": 2, "repr": myObjectRepr},
        {"a": 1, "b": 2, "repr": myObjectRepr}
    ];

    // sort it by the "a" property, check to see if it matches
    myObjectArray.sort(keyComparator("a"));
    expectedRepr = '[{"a": 1, "b": 2}, {"a": 3, "b": 2}]';
    assert( repr(myObjectArray) == expectedRepr );

    // get just the "a" values out into an array
    sortedAValues = map(itemgetter("a"), myObjectArray);
    assert( compare(sortedAValues, [1, 3]) == 0 );

    // serialize an array as JSON, unserialize it, expect something equivalent
    myArray = [1, 2, "3", null, undefined];
    assert( objEqual(evalJSON(serializeJSON(myArray)), myArray) );


Description
===========

:mochiref:`MochiKit.Base` is the foundation for the MochiKit suite.
It provides:

-   An extensible comparison facility
    (:mochiref:`compare`, :mochiref:`registerComparator`)
-   An extensible programmer representation facility
    (:mochiref:`repr`, :mochiref:`registerRepr`)
-   An extensible JSON [1]_ serialization and evaluation facility 
    (:mochiref:`serializeJSON`, :mochiref:`evalJSON`,
    :mochiref:`registerJSON`)
-   A simple adaptation facility (:mochiref:`AdapterRegistry`)
-   Convenience functions for manipulating objects and Arrays
    (:mochiref:`update`, :mochiref:`setdefault`, :mochiref:`extend`, etc.)
-   Array-based functional programming
    (:mochiref:`map`, :mochiref:`filter`, etc.)
-   Bound and partially applied functions
    (:mochiref:`bind`, :mochiref:`method`, :mochiref:`partial`)

Python users will feel at home with :mochiref:`MochiKit.Base`, as the
facilities are quite similar to those available as part of Python and the 
Python standard library.


Dependencies
============

None.


Overview
========

Comparison
----------

The comparators (operators for comparison) in JavaScript are deeply broken,
and it is not possible to teach them new tricks.

MochiKit exposes an extensible comparison facility as a simple
:mochiref:`compare(a, b)` function, which should be used in lieu of
JavaScript's operators whenever comparing objects other than numbers
or strings (though you can certainly use :mochiref:`compare` for those, too!).

The :mochiref:`compare` function has the same signature and return value as a
sort function for ``Array.prototype.sort``, and is often used in that context.

Defining new comparators for the :mochiref:`compare` function to use is done
by adding an entry to its :mochiref:`AdapterRegistry` with the
:mochiref:`registerComparator` function.


Programmer Representation
-------------------------

JavaScript's default representation mechanism, ``toString``, is notorious
for having terrible default behavior. It's also very unwise to change that
default, as other JavaScript code you may be using may depend on it.

It's also useful to separate the concept of a "string representation" and a
"string representation for programmers", much like Python does with its str
and repr protocols.

:mochiref:`repr` provides this programmer representation for JavaScript,
in a way that doesn't require object prototype hacking: using an
:mochiref:`AdapterRegistry`.

Objects that implement the repr protocol can either implement a ``.repr()``
or ``.__repr__()`` method, or they can simply have an adapter setup to
generate programmer representations. By default, the registry provides
nice representations for ``null``, ``undefined``, ``Array``, and objects or
functions with a ``NAME`` attribute that use the default ``toString``. For
objects that ``repr`` doesn't already understand, it simply defaults to
``toString``, so it will integrate seamlessly with code that implements
the idiomatic JavaScript ``toString`` method!

To define a programmer representation for your own objects, simply add
a ``.repr()`` or ``.__repr__()`` method that returns a string. For
objects that you didn't create (e.g., from a script you didn't write, or a 
built-in object), it is instead recommended that you create an adapter
with :mochiref:`registerRepr`.


JSON Serialization
------------------

JSON [1]_, JavaScript Object Notation, is a widely used serialization format
in the context of web development. It's extremely simple, lightweight, and
fast. In its essence, JSON is a restricted subset of JavaScript syntax
suitable for sending over the wire that can be unserialized with a simple
eval. It's often used as an alternative to XML in "AJAX" contexts because it
is compact, fast, and much simpler to use for most purposes.

To create a JSON serialization of any object, simply call
:mochiref:`serializeJSON()` with that object. To unserialize a JSON string,
simply call :mochiref:`evalJSON()`
with the serialization.

In order of precedence, :mochiref:`serializeJSON` coerces the given argument
into a JSON serialization:

1.  Primitive types are returned as their JSON representation: 
    ``undefined``, ``string``, ``number``, ``boolean``, ``null``.
2.  If the object has a ``__json__`` or ``json`` method, then it is called
    with no arguments. If the result of this method is not the object itself,
    then the new object goes through rule processing again (e.g. it may return
    a string, which is then serialized in JSON format).
3.  If the object is ``Array``-like (has a ``length`` property that is a
    number, and is not a function), then it is serialized as a JSON array.
    Each element will be processed according to these rules in order.
    Elements that can not be serialized (e.g. functions) will be replaced with
    ``undefined``.
4.  The ``jsonRegistry`` :mochiref:`AdapterRegistry` is consulted for an
    adapter for this object. JSON adapters take one argument (the object),
    and are expected to behave like a ``__json__`` or ``json`` method
    (return another object to be serialized, or itself).
5.  If no adapter is available, the object is enumerated and serialized as a
    JSON object (name:value pairs). All names are expected to be strings.
    Each value is serialized according to these rules, and if it can not be 
    serialized (e.g. methods), then that name:value pair will be skipped.


Adapter Registries
------------------

MochiKit makes extensive use of adapter registries, which enable you to
implement object-specific behaviors for objects that you do not necessarily
want to modify, such as built-in objects. This is especially useful because
JavaScript does not provide a method for hiding user-defined properties from
``for propName in obj`` enumeration.

:mochiref:`AdapterRegistry` is simply an encapsulation for an ordered list of
"check" and "wrap" function pairs. Each :mochiref:`AdapterRegistry` instance
should perform one function, but may have multiple ways to achieve that
function based upon the arguments. One way to think of it is as a poor man's
generic function, or multiple dispatch (on arbitrary functions, not just type!).

Check functions take one or more arguments, and return ``true`` if the
argument list is suitable for the wrap function. Check functions should
perform "cheap" checks of an object's type or contents, before the
"expensive" wrap function is called.

Wrap functions take the same arguments as check functions and do some
operation, such as creating a programmer representation or comparing
both arguments.


Convenience Functions
---------------------

Much of :mochiref:`MochiKit.Base` is there to simply remove the grunt work of
doing generic JavaScript programming.

Need to take every property from one object and set them on another? No
problem, just call :mochiref:`update(dest, src)`! What if you just wanted to
update keys that weren't already set? Look no further than
:mochiref:`setdefault(dest, src[, ...])`.

Want to return a mutable object, but don't want to suffer the consequences
if the user mutates it? Just :mochiref:`clone(it)` and you'll get a
copy-on-write clone. Cheaper than a copy!

Need to extend an ``Array`` with another array? Or even an ``Array``-like
object such as a ``NodeList`` or the special ``arguments`` object? Even if you
need to skip the first few elements of the source ``Array``-like object, it's
no problem with :mochiref:`extend(dstArray, srcArrayLike[, skip])`!

Wouldn't it be convenient to have all of the JavaScript operators were
available as functions somewhere? That's what the :mochiref:`operators` table
is for, and it even comes with additional operators based on the
:mochiref:`compare` function.

Need to walk some tree of objects and manipulate or find something in it?
A DOM element tree perhaps? Use :mochiref:`nodeWalk(node, visitor)`!

There's plenty more, so check out the `API Reference`_ below.


Functional Programming
----------------------

Functional programming constructs such as :mochiref:`map` and
:mochiref:`filter` can save you a lot of time, because JavaScript iteration is
error-prone and arduous. Writing less code is the best way to prevent bugs,
and functional programming can help you do that.

:mochiref:`MochiKit.Base` ships with a few simple Array-based functional
programming constructs, namely :mochiref:`map` and :mochiref:`filter`, and
their "extended" brethren, :mochiref:`xmap` and :mochiref:`xfilter`.

:mochiref:`map(func, arrayLike[, ...])` takes a function and an ``Array``-like
object, and creates a new ``Array``. The new ``Array`` is the result of
``func(element)`` for every element of ``arrayLike``, much
like the ``Array.prototype.map`` extension in Mozilla. However,
:mochiref:`MochiKit.Base` takes that a step further and gives you the full
blown Python version of :mochiref:`map`, which will take several
``Array``-like objects, and calls the function with one argument per given
``Array``-like, e.g.::

   var arrayOne = [1, 2, 3, 4, 5];
   var arrayTwo = [1, 5, 2, 4, 3];
   var arrayThree = [5, 2, 1, 3, 4];
   var biggestElements = map(objMax, arrayOne, arrayTwo, arrayThree);
   assert( objEqual(biggestElements, [5, 5, 3, 4, 5]) );

:mochiref:`filter(func, arrayLike[, self])` takes a function and an
``Array``-like object, and returns a new ``Array``.
This is basically identical to the ``Array.prototype.filter``
extension in Mozilla. self, if given, will be
used as ``this`` in the context of func when called.

:mochiref:`xmap` and :mochiref:`xfilter` are just special forms of
:mochiref:`map` and :mochiref:`filter` that accept a function as the first
argument, and use the extra arguments as the ``Array``-like. Not terribly
interesting, but a definite time-saver in some cases.

If you appreciate the functional programming facilities here,
you should definitely check out :mochiref:`MochiKit.Iter`, which provides
full blown iterators, :mochiref:`MochiKit.Iter.range`,
:mochiref:`MochiKit.Iter.reduce`, and a near-complete port of Python's
itertools [2]_ module, with some extra stuff thrown in for good measure!


Bound and Partial Functions
---------------------------

JavaScript's method-calling special form and lack of bound functions (functions
that know what ``this`` should be) are one of the first stumbling blocks that
programmers new to JavaScript face. The :mochiref:`bind(func, self)` method
fixes that right up by returning a new function that calls func with the right
``this``.

In order to take real advantage of all this fancy functional programming stuff,
you're probably going to want partial application. This allows you to create
a new function from an existing function that remembers some of the arguments.
For example, if you wanted to compare a given object to a slew of other 
objects, you could do something like this::

    compareWithOne = partial(compare, 1);
    results = map(compareWithOne, [0, 1, 2, 3]);
    assert( objEqual(results, [-1, 0, 1, 1]) );

One of the better uses of partial functions is in :mochiref:`MochiKit.DOM`,
which is certainly a must-see for those of you creating lots of DOM elements
with JavaScript!


API Reference
=============

Errors
------

:mochidef:`NotFound`:

    A singleton error raised when no suitable adapter is found


Constructors
------------

:mochidef:`AdapterRegistry`:
    
    A registry to facilitate adaptation.

    All ``check``/``wrap`` function pairs in a given registry
    should take the same number of arguments.


:mochidef:`AdapterRegistry.prototype.register(name, check, wrap[, override])`:

    ``name``:
        a unique identifier used to identify this adapter so that it
        may be unregistered.

    ``check``:
        function that should return ``true`` if the given arguments are
        appropriate for the ``wrap`` function.
        
    ``wrap``:
        function that takes the same parameters as ``check`` and does
        the adaptation.  Every ``wrap``/``check`` function pair in the
        registry should have the same number of arguments.

    ``override``:
        if ``true``, the ``check`` function will be
        given highest priority. Otherwise, the lowest.


:mochidef:`AdapterRegistry.prototype.match(obj[, ...])`:

    Find an adapter for the given arguments by calling every
    ``check`` function until one returns ``true``.
    
    If no suitable adapter is found, throws :mochiref:`NotFound`.


:mochidef:`AdapterRegistry.prototype.unregister(name)`:

    Remove a named adapter from the registry


:mochidef:`NamedError`:

    Convenience constructor for creating new errors (e.g. :mochiref:`NotFound`)


Functions
---------

:mochidef:`arrayEqual(self, arr)`:

    Compare the arrays ``self`` and ``arr`` for equality using ``compare``