Re: [Haskell-cafe] Re: Haskell version of Norvig's Python Spelling Corrector

Pete Kazmier wrote:

Derek Elkins writes:

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After reading Bryan's post, I switched my right fold to a strict left fold and almost tripled my original speed. Could someone provide some guidelines on when to use each? I thought foldr should be used when building some large data structure such as the map I build. Bryan, out of curiosity, is a non bytestring version of your code faster? http://www.haskell.org/hawiki/StackOverflow

I read the article and understand it, but I am having a hard time applying that specifically to my use of foldr. Here is how I was using foldr:

type WordFreq = M.Map B.ByteString Int

train:: [B.ByteString] -> WordFreq train words = frequencyMap where frequencyMap = foldr incWordCount M.empty words incWordCount w m = M.insertWith (+) w 1 m

So is 'incWordCount' strict in its second argument? I'm still not sure exactly what that means. According to the wiki page, if it is strict in the second argument, I should have used foldl' instead of foldr.

insertWith will definitely need to examine 'm' to perform it's action, therefore it is strict.