Re: [Haskell-cafe] Efficient functional idiom for histogram

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