[Haskell-cafe] Fast number parsing with strict bytestrings [Was: Re: Seemingly subtle change causes large performance variation]

[Conal Elliott]

Lovely!

Perhaps a stylistic shift would encourage writing this sort of elegant,
fusion-friendly code.

    -- Generalized version of "interact".  Encapsulates data getter &
putter.
    genInteract :: IO i -> (o -> IO ()) -> ((i -> o) -> IO ())
    genInteract get put = \ f -> get >>= put . f

The intention here is that i and o are pure value types (no IO).  Solutions
2 and three use the following specialization.

    -- Lazy ByteString in and showable out
    lGetPrint :: Show o => (L.ByteString -> o) -> IO ()
    lGetPrint = genInteract L.getContents print

Then rewrite solution 2 as follows:

    main = lGetPrint f
     where
       f contents = foldl' (test k) 0 . map int . take n $ ls
         where
           (l:ls) = L.lines contents
           [n,k]  = map int (L.split ' ' l)

And solution 3 similarly.

Plug: for additional examples and a more general approach to separating out
IO, my blog post "separating IO from logic -- example
<http://conal-elliott.blogspot.com/2007/02/separating-io-from-logic-example.html>",
which uses the TV <http://www.haskell.org/haskellwiki/TV> library.
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