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List:       haskell-c2hs
Subject:    [C2hs] bug in C2HSMarsh.extractBitMasks function
From:       Duncan Coutts <duncan.coutts () worc ! ox ! ac ! uk>
Date:       2005-05-09 11:31:57
Message-ID: 1115638317.8291.280.camel () localhost
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Hi,

> extractBitMasks bits =
>   [bm | bm <- [minBound..maxBound], bits `containsBitMask` bm]

Sadly this does not work since [minBound..maxBound] uses succ to
enmerate the flags and the Enum instance that c2hs generates does not
include a definition of succ and the default assumes that the enum is
packed whereas of course for flags it is sparse (only has values for
2^n).

We had this bug in gtk2hs too:

> toFlags :: Flags a => Int -> [a]
> toFlags n = andNum n minBound
>     where
>       andNum n m = (if (n .|. fromEnum m) == n then (m:) else id)
>         (if fromEnum m==fromEnum (maxBound `asTypeOf` m) then [] else andNum n (succ m))

We changed this to:

> toFlags :: Flags a => Int -> [a]
> toFlags f = testBits f 1
>   where testBits f n
>           | f == 0        = []
>           | f `testBit` 0 = toEnum n : testBits (f `shiftR` 1) (n `shiftL` 1)
>           | otherwise     =            testBits (f `shiftR` 1) (n `shiftL` 1)

So, extractBitMasks needs fixing, probably in a similar way.

Duncan


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