Re: Yet another weakly defined bug report

"Ketil Z. Malde" wrote:

"Simon Marlow" writes:

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-- | add data from a file to the histogram addFile :: FiniteMap String Int -> String -> IO (FiniteMap String Int) addFile fm name = do x <- readFile name return (addHist fm x)

-- | add data from all files in a directory to the histogram addDir :: FiniteMap String Int -> String -> IO (FiniteMap String Int) addDir fm dir = do dc <- getDirectoryContents dir fs <- filterM doesFileExist (map ((dir++"/")++) dc) foldM addFile fm fs

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It's not possible to tell from this code whether the readFiles will be fully evaluated or not: it depends on how much evaluation addHist does, and to what extend the result FiniteMap is demanded, amongst other things.

Of course. Never cut code, I suppose; I thought the parts my understanding would be weakest would be the monadic stuff, not this:

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addHist :: FiniteMap String Int -> String -> FiniteMap String Int addHist fm = foldl add1 fm . words where add1 f w = case lookupFM f w of Just n -> addToFM f w (n+1) Nothing -> addToFM f w 1

I felt pretty sure that this would evaluate the string to the end. Am I wrong?

The string won't be evaluated until the finite map is demanded. None of the code you've shown so far makes that demand. -- Dean