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
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