We started off the PyGTA meeting with brainstorming for fun projects that we could do in a "pure functional" manner. Lots of complex and involved things that we'd whip off in a few minutes. But first, just to be sure we were all on the same page, let's whip off a quicky...

• given a file
• output the file on stdout sorted by the 5th character of each line
• use as pure a functional form as possible, with no variable assignments

It is *amazing* how addicted to variables a Python programmer is. All of us could trivially write the code as something like this:

import sys
print "".join(
    [
        x[1]
        for x in sorted(
            [(line[4:5],line) for line in open(sys.argv[1])]
        )
    ]
)

But sorted is just syntactic sugar for creating and sorting a list, we wanted something that didn't use any variables/state, and we wanted to implement the sort ourselves to see how to do it purely functionally. We eventually came up with something... but OMG it was a mess, iterating over the whole file N times (via itertools.tee) searching for the N+1th item. Worse, the implementation of itertools.tee would mean that it was just creating N copies of the lists in dequeues.

So I looked up the Haskel qsort algorithm when I got home... the Python equivalent looks something like this:

def qsort( seq ):
    try:
        n = iter(seq).next()
    except StopIteration, err:
        pass
    else:
        seqs = itertools.tee(seq)
        for i in qsort( x for x in seqs[0] if x < n ):
            yield i
        yield n
        for i in qsort( x for x in seqs[1] if x > n ):
            yield i

Note the presence of variable assignments... which is one of the things we were considering a disqualification for "pure" functional style. The Haskel version uses "lazy lists" for everything, so they don't have the itertools.tee() call, and the n variable is part of the matching-head "unpack".

Which suggests we were being *way* too rigorous in our functional purity to work within the Python context. Sprinkling a bit of functionalism here and there might be useful, but the utility of eliminating all variable assignments is... suspect. That said, it was a very fun exercise to work on as a brain teaser, you don't realize how addicted you are to variable assignments until you declare that you cannot use them.