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

[Derek Elkins]

Pete Kazmier wrote:
> Bryan O'Sullivan <bos at serpentine.com> writes:
> 
>> After this switch, I found that spellchecking one word still took 4x
>> as long in Haskell as Norvig's Python program.  Since I was checking
>> only one word in each case, essentially all of the runtime was taken
>> up by building the word frequency map.
>>
>> train = foldl' updateMap M.empty . map lower . mkWords
>>     where updateMap model word = M.insertWith' (+) word 1 model
>>           mkWords = filter (not . B.null) . X.splitWith isNotAlpha
>>           lower !s = X.map toLower s
>>           isNotAlpha !c = c < 0x41 || (c > 0x5a && c < 0x61) || c > 0x7a
>>           toLower !c | c >= 0x41 && c <= 0x5a = c + 0x20
>>                      | otherwise = c
> 
> 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?

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