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# Name

MochiKit.Iter - itertools for JavaScript; iteration made HARD, and then easy

# Synopsis

    theSum = sum(takewhile(
            partial(operator.gt, 10),
            imap(
                partial(operator.mul, 2),
                count()
            )
        )
    ));

    assert( theSum == (0 + 0 + 2 + 4 + 6 + 8) );

# Description

All of the functional programming missing from MochiKit.Base lives here. The functionality in this module is largely inspired by Python's iteration protocol [1], and the itertools module [2].

MochiKit.Iter defines a standard way to iterate over anything, that you can extend with registerIteratorFactory, or by implementing the `.iter()` or `.__iterator__()` (in MochiKit 1.4+) protocol. Iterators are lazy, so it can potentially be cheaper to build a filter chain of iterators than to build lots of intermediate arrays. Especially when the data set is very large, but the result is not.

# Dependencies

- MochiKit.Base

# Overview

## Iteration for JavaScript

The best overview right now is in my Iteration for JavaScript [3] blog entry. This information will migrate here eventually.

# API Reference

## Errors

StopIteration:

> The singleton MochiKit.Base.NamedError that signifies the end of an iterator

> Availability:
>   Available in MochiKit 1.3.1+

## Functions

applymap(fun, seq[, self]):

> `applymap(fun, seq)` --> fun.apply(self, seq0), fun.apply(self, seq1), ...

> Availability:
>   Available in MochiKit 1.3.1+

chain(p, q[, ...]):

> `chain(p, q, ...)` --> p0, p1, ... plast, q0, q1, ...

> Availability:
>   Available in MochiKit 1.3.1+

count(n=0):

> `count(n=0)` --> n, n + 1, n + 2, ...

> Availability:
>   Available in MochiKit 1.3.1+

cycle(p):

> `cycle(p)` --> p0, p1, ... plast, p0, p1, ...

> Availability:
>   Available in MochiKit 1.3.1+

dropwhile(pred, seq):

> `dropwhile(pred, seq)` --> seq[n], seq[n + 1], starting when pred(seq[n]) fails

> Availability:
>   Available in MochiKit 1.3.1+

every(iterable, func):

> Return `true` if `func(item)` is `true` for every item in `iterable`.

> Availability:
>   Available in MochiKit 1.3.1+

exhaust(iterable):

> Exhausts an iterable without saving the results anywhere, like list(iterable) when you don't care what the output is.

> Availability:
>   Available in MochiKit 1.3.1+

forEach(iterable, func[, self]):

> Call `func` for each item in `iterable`, and don't save the results.

> Availability:
>   Available in MochiKit 1.3.1+

groupby(iterable[, keyfunc]):

> Make an iterator that returns consecutive keys and groups from the iterable. The key is a function computing a key value for each element. If not specified or is None, key defaults to an identity function and returns the element unchanged. Generally, the iterable needs to already be sorted on the same key function.

> The returned group is itself an iterator that shares the underlying iterable with groupby(). Because the source is shared, when the groupby object is advanced, the previous group is no longer visible. So, if that data is needed later, it should be stored as an array:

>     var groups = [];
>     var uniquekeys = [];
>     forEach(groupby(data, keyfunc), function (key_group) {
>         groups.push(list(key_group[1]));
>         uniquekeys.push(key_group[0]);
>     });

> As a convenience, groupby_as_array() is provided to suit the above use case.

> Availability:
>   Available in MochiKit 1.3.1+

groupby_as_array(iterable[, keyfunc]):

> Perform the same task as groupby(), except return an array of arrays instead of an iterator of iterators.

> Availability:
>   Available in MochiKit 1.3.1+

iextend(lst, iterable):

> Just like list(iterable), except it pushes results on `lst` rather than creating a new one.

> Availability:
>   Available in MochiKit 1.3.1+

ifilter(pred, seq):

> `ifilter(pred, seq)` --> elements of seq where `pred(elem)` is `true`

> Availability:
>   Available in MochiKit 1.3.1+

ifilterfalse(pred, seq):

> `ifilterfalse(pred, seq)` --> elements of seq where `pred(elem)` is `false`

> Availability:
>   Available in MochiKit 1.3.1+

imap(fun, p, q[, ...]):

> `imap(fun, p, q, ...)` --> fun(p0, q0, ...), fun(p1, q1, ...), ...

> Availability:
>   Available in MochiKit 1.3.1+

islice(seq, [start,] stop[, step]):

> `islice(seq, [start,] stop[, step])` --> elements from seq[start:stop:step] (in Python slice syntax)

> Availability:
>   Available in MochiKit 1.3.1+

iter(iterable[, sentinel]):

> Convert the given argument to an iterator (object implementing `.next()`).

> 1. If `iterable` is an iterator (implements `.next()`), then it will be returned as-is.
> 2. If `iterable` is an iterator factory (implements `.iter()`), then the result of `iterable.iter()` will be returned.
> 3. If `iterable` is a JavaScript 1.7 iterator factory (implements `.__iterable__()`), then the result of `iterable.__iterable__()` will be returned (MochiKit 1.4+).
> 4. Otherwise, the iterator factory MochiKit.Base.AdapterRegistry is used to find a match.
> 5. If no factory is found, it will throw `TypeError`

> Built-in iterator factories are present for Array-like objects, and objects that implement the `iterateNext` protocol (e.g. the result of Mozilla's `document.evaluate`).

> When used directly, using an iterator should look like this:

>     var it = iter(iterable);
>     try {
>         while (var o = it.next()) {
>             // use o
>         }
>     } catch (e) {
>         if (e != StopIteration) {
>             throw e;
>         }
>         // pass
>     }

> This is ugly, so you should use the higher order functions to work with iterators whenever possible.

> Availability:
>   Available in MochiKit 1.3.1+

izip(p, q[, ...]):

> `izip(p, q, ...)` --> [p0, q0, ...], [p1, q1, ...], ...

> Availability:
>   Available in MochiKit 1.3.1+

list(iterable):

> Convert `iterable` to a new `Array`

> Availability:
>   Available in MochiKit 1.3.1+

next(iterator):

> Return `iterator.next()`

> Availability:
>   Available in MochiKit 1.3.1+

range([start,] stop[, step]):

> Return an iterator containing an arithmetic progression of integers.

> `range(i, j)` returns iter([i, i + 1, i + 2, ..., j - 1])

> `start` (!) defaults to `0`. When `step` is given, it specifies the increment (or decrement). The end point is omitted!

> For example, `range(4)` returns iter([0, 1, 2, 3]). This iterates over exactly the valid indexes for an array of 4 elements.

> Availability:
>   Available in MochiKit 1.3.1+

reduce(fn, iterable[, initial]):

> Apply `fn(a, b)` cumulatively to the items of an iterable from left to right, so as to reduce the iterable to a single value.

> For example:

>     reduce(function (a, b) { return a + b; }, [1, 2, 3, 4, 5])

> calculates:

>     ((((1 + 2) + 3) + 4) + 5).

> If initial is given, it is placed before the items of the sequence in the calculation, and serves as a default when the sequence is empty.

> Note that the above example could be written more clearly as:

>     reduce(operator.add, [1, 2, 3, 4, 5])

> Or even simpler:

>     sum([1, 2, 3, 4, 5])

> Availability:
>   Available in MochiKit 1.3.1+

registerIteratorFactory(name, check, iterfactory[, override]):

> Register an iterator factory for use with the iter function.

> `check` is a `function(a)` that returns `true` if `a` can be converted into an iterator with `iterfactory`.

> `iterfactory` is a `function(a)` that returns an object with a `.next()` method that returns the next value in the sequence.

> `iterfactory` is guaranteed to only be called if `check(a)` returns a true value.

> If `override` is `true`, then it will be made the highest precedence iterator factory. Otherwise, the lowest.

> Availability:
>   Available in MochiKit 1.3.1+

repeat(elem[, n]):

> `repeat(elem, [,n])` --> elem, elem, elem, ... endlessly or up to n times

> Availability:
>   Available in MochiKit 1.3.1+

reversed(iterable):

> Return a reversed array from iterable.

> Availability:
>   Available in MochiKit 1.3.1+

some(iterable, func):

> Return `true` if `func(item)` is `true` for at least one item in `iterable`.

> Availability:
>   Available in MochiKit 1.3.1+

sorted(iterable[, cmp]):

> Return a sorted array from iterable.

> Availability:
>   Available in MochiKit 1.3.1+

sum(iterable, start=0):

> Returns the sum of a sequence of numbers plus the value of parameter `start` (with a default of 0). When the sequence is empty, returns start.

> Equivalent to:

>     reduce(operator.add, iterable, start);

> Availability:
>   Available in MochiKit 1.3.1+

takewhile(pred, seq):

> `takewhile(pred, seq)` --> seq[0], seq[1], ... until pred(seq[n]) fails

> Availability:
>   Available in MochiKit 1.3.1+

tee(iterable, n=2):

> `tee(it, n=2)` --> [it1, it2, it3, ... itn] splits one iterator into n

> Availability:
>   Available in MochiKit 1.3.1+

# See Also

[1] | The iteration protocol is described in PEP 234 - Iterators: http://www.python.org/peps/pep-0234.html

[2] | Python's itertools module: http://docs.python.org/lib/module-itertools.html

[3] | Iteration in JavaScript: http://bob.pythonmac.org/archives/2005/07/06/iteration-in-javascript/

# Authors

- Bob Ippolito <bob@redivi.com>

# Copyright

Copyright 2005 Bob Ippolito <bob@redivi.com>. This program is dual-licensed free software; you can redistribute it and/or modify it under the terms of the MIT License or the Academic Free License v2.1.