[Haskell-beginners] Another request for code critique...

[Brent Yorgey]

On Thu, Mar 31, 2011 at 02:18:32PM -0400, Mike Meyer wrote:
> Shorter, but thing I've done that wasn't just an exercise from a
> haskell book.
> 
> The problem statement can be found at
> http://kernelbob.wordpress.com/2011/03/20/same-five-digits/.
> 
> My solution can be seen at http://pastebin.com/iW95q2ex.

Looks nice.  A few places things could be made a bit more points-free
(which isn't always a good thing -- but I think in these cases it makes
things more readable, although I suppose that's mostly an issue of
what you're used to)

  (\ s -> length s < 6)   --->   ((<6) . length)  (etc.)

  (\ (_, m) -> (== 5) $ M.size m)  --->  (==5) . M.size . snd   (etc.)

  If you have a recent enough version of base, (\ (t,m) -> (m,t)) is
  available as 'swap' in Data.Tuple.

This one is slightly more advanced:

  map (\ (t,m) -> (t, head . M.keys $ M.filter (== '1') m))

can be replaced by

  (map . second) (head . M.keys . M.filter (== '1'))

which applies the function (head . M.keys . M.filter (== '1')) to the
second component of every element of a list.  The 'second' function is
from Control.Arrow, and can be given the type

  second :: (b -> c) -> (a,b) -> (a,c)

(actually its type is a bit more general than that).

-Brent