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From: Georg Martius <mai99dgf@studserv.uni-leipzig.de>
To: glasgow-haskell-users@haskell.org
Subject: Annotation for unfolding wanted
Date: Sat, 28 Jul 2007 20:50:36 +0200
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Hi,

I was wondering why we don't have an annotation or pragma for function to t=
ell=20
the compiler that we need this particular recursive function to be unfolded=
=2E=20
If the compiler cannot do this for some reason it should produce an error=20
message to help you modifying your code. I have regularly problems that my=
=20
code is either not strict enough or my functions are not unfolded. I find i=
t=20
annoying that this is a regular show stopper and consumes much time to fix.=
=20
Here is an example: a search function for strings, which should return the=
=20
index and the rest of the string after the first occurrence: search0 will n=
ot=20
be unfolded by ghc -O. (I don't know why, it looks tail-recursive to me)
whereas search1 is just fine.=20

search0 :: Int -> String -> String -> (String, Int)
search0 i [] _ =3D ([],i)
search0 i haystack needle =3D=20
    let len =3D length needle
    in if isPrefixOf needle haystack then (drop len haystack, i+len)
       else search0 (seq i (i+1)) (drop 1 haystack) needle

search1 :: Int -> String -> String -> (String, Int)
search1 i [] _ =3D ([],i)
search1 i haystack needle =3D=20
    let i =3D elemIndex True $ map (isPrefixOf needle) $ tails haystack
        len =3D length needle
    in case i of
         Just  index -> (drop (index+len) haystack, index + len)
         Nothing     -> ([],0)

Regards!
        Georg

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