[Haskell-cafe] Efficient functional idiom for histogram

[Malcolm Wallace]

> I'm expecting the answer to be that I've got unnecessary laziness -  
> which
> is fine, but ultimately my interest is in ease of expression and
> performance combined, so I'm looking for beginner-level improvements
> rather than subtle advanced techniques like unboxing.

You're right, it is too lazy.  Here are a couple of strictifications  
that should help:

> histogram :: Ord a => [a] -> [(a,Int)]
> histogram = Map.assocs . foldl f Map.empty
>  where
>   f m k = Map.insertWith (+) k 1 m

Turn foldl into foldl' (from Data.List) and Map.insertWith into  
Map.insertWith'.
The strict versions simply force the intermediate structures to be  
evaluated, rather than hanging around as large accumulations of  
closures.

Regards,
     Malcolm