[Haskell-cafe] [Newbie] What to improve in my code

Ketil Malde ketil at malde.org
Mon Jul 19 03:35:01 EDT 2010

Daniel Fischer <daniel.is.fischer at web.de> writes:

>> First of all: I'm not sure if this question is allowed here. If not, I
>> apologize

You might want to check out the haskell-beginners list, but IMO most
questions are okay to post here.

Just a couple of style issues Daniel didn't mention:

>> process :: [Char] -> [String]
>> process str = words (map toLower (removePunctuation str))

It's a matter of taste, but I think this reads clearer if written:

    process = words . map toLower . removePunctuation

>> unique :: (Eq a) => [a] -> [a]
>> unique [] = []
>> unique (x:xs) = [x] ++ unique (filter (\s -> x /= s) xs)

Also 'filter (\s -> x /= s)' can be written as

     filter (x /=)

> import qualified Data.Map as Map
> import Data.List
> occurrenceCount'' :: Ord a => [a] -> [(a,Int)]
> occurrenceCount'' xs = Map.toList $ 
>       foldl' (\mp x -> Map.insertWith' (+) x 1 mp) Map.empty xs

Note the primes here!  This is perhaps my most common use of Map, and
because of laziness, it is very easy to blow the stack.  Although you
really want to store an Int for each key, the default is to store an
unevaluated computation, in this case a tower of (1+(1+(1+..))).  The
foldl' and insertWith' functions are stricter, and presumably Daniel
gets this right (I'm never comfortable without testing this myself :).

>> 3) The whole process as i'm doing it now feels pretty imperatively (been
>> working for years as a Java / PHP programmer). I've got this feeling
>> that the occurenceCount' function could be implemented using a mapping
>> function. What ways are there to make this more "functional"?

I don't think I agree with this sentiment - you're building a pipeline
of functions, not setting variables or otherwise mixing state or other
imperativeness.  Why do you think it's imperative? 

If I haven't seen further, it is by standing in the footprints of giants

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