Using functional tools

[Gerhard Häring]

Andrae Muys wrote in comp.lang.python:
> def xmap(func, *args):
>     iterlist = [iter(arg) for arg in args]
>     while 1:
>         yield func(*[itr.next() for itr in iterlist])

Incidentally, yesterday I tried to come up with a little functional
utility library for myself. My xmap looks quite a lot the same ;-) But
in order to be compatible with map, I had to add a special case for
func is None, which I never used in my own code. This is the module:

def xmap(func, *seqs):
    """A lazy map."""
    # map defaults to the identity function if func is None:
    if func is None:
        func = lambda x: x
    iters = map(iter, seqs)
    while 1:
        yield func(*map(lambda x: x.next(), iters))

def xzip(*seqs):
    """A lazy zip."""
    iters = map(iter, seqs)
    while 1:
        # The .next() method on the curiter raises a StopIteration exception
        # when it runs out of items. We don't catch it, so it automagically
        # stops our generator.
        yield tuple([curiter.next() for curiter in iters])

def permutations(items):
    """Yields all permutations of the items."""
    if items == []:
        yield []
    else:
        for i in range(len(items)):
            for j in permutations(items[:i] + items[i+1:]):
                yield [items[i]] + j

def combinations(items, k=None):
    """Yields all k-combinations of items."""
    if k is None:
        k = len(items)
    if k == 0:
        yield []
    else:
        for i in items:
            for j in combinations(items, k-1):
                yield [i] + j

Gerhard
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reduce(lambda x,y:x+y,map(lambda x:chr(ord(x)^42),tuple('zS^BED\nX_FOY\x0b')))