Just as a quck note, I thought I'd post the cleanest python I could come up with to solve the roman numerals problem I discussed earlier. It tries to use a functional style while actually avoiding recursion. To do so, I wrote an iterative python unfold:
def unfold(f, x): res =  while 1: try: w, x = f(x) res.append(w) except TypeError: return res
And then the answer becomes:
numerals = [("M", 1000), ("CM", 900), ("D", 500), ("CD", 400), ("C", 100), ("XC", 90), ("L", 50), ("XL", 40), ("X", 10), ("IX", 9), ("V", 5), ("IV", 4), ("I", 1)] def next(x): for n in numerals: if n <= x: return (n, x-n) def romanize(n): return "".join(unfold(next, n))
Compare to the haskell I posted before:
romanize = concat . unfoldr next where next 0 = Nothing next x = Just $ second (x-) $ head $ filter ((<=x) . snd) numerals numerals = [("M", 1000), ("CM", 900), ("D", 500), ("CD", 400), ("C", 100), ("XC", 90), ("L", 50), ("XL", 40), ("X", 10), ("IX", 9), ("V", 5), ("IV", 4), ("I", 1)]
The "where" idiom allows you to group helper functions and constants underneath the main function, and the unimportance of function order means you can highlight the high-level logic. The haskell code is much shorter and tighter than the python.
Having powerful iterative functions like unfoldr in the standard library is another clear win for Haskell; it took me more lines to define unfold than it did to define both next() and romanize(). (By the by, some googling failed to turn up a previous python implementation of unfold(); I think this code gets a lot uglier without it. I also don't see any obvious way to do it with itertools.)
On the other hand, I find the simplicity of the python next() appealing, with its constructive instead of declarative approach. It's a less generic solution that's intuitively simpler (for me, still an imperative thinker).
You could match the haskell more exactly:
from itertools import ifilter def second(f, (a, b)): return (a, f(b)) def next(x): return second(lambda a: x-a, ifilter(lambda (y, z): z <= x, numerals).next()
but the result is pretty ugly. The inability to compose functions compactly results in a lot of boilerplate, and in having to pick a lot of meaningless variable names that obscure what's going on. We could move the lambdas out of the call and give them names:
def next(x): subx = lambda a: x-a ltx = lambda (y, z): z <= x return second(subx, ifilter(ltx, numerals).next())
And it's a little better, but I think we're now pretty far from idiomatic python.
The python translation of the haskell isn't bad, but I do think it loses something. The unfold() trick works really well, and translates easily into an imperative, pythonic implementation; I'll look for ways to use it in the future. And, finally, I'm pretty jealous of Haskell's function combination abilities.
In the comments at reddit, nostrademons posts a nicer unfold function:
def unfold(f, x): while True: w, x = f(x) yield w
And Kay Schluehr prefers the iterative solution:
numerals = (("M", 1000), ("CM", 900), ("D", 500), ("CD", 400), ("C", 100),("XC", 90),("L", 50),("XL", 40), ("X", 10), ("IX", 9), ("V", 5), ("IV", 4), ("I", 1)) def romanize(n): roman =  for ltr, num in numerals: (k,n) = divmod(n, num) roman.append(ltr*k) return "".join(roman)
David Pollak contributes a Scala unfold and romanize at his blog.