Type system compiler flags
carsten at codimi.de
Tue Feb 1 11:37:37 CET 2011
Am 01.02.11 11:05, schrieb Daniel Fischer:
> On Tuesday 01 February 2011 10:20:26, Carsten Schultz wrote:
>> Hello everyone,
>> I am trying to compile some code that I have written a long time ago
>> (might have been for ghc 6.3), and I have not done much Haskell in the
>> meantime. I have trouble compiling the code, maybe only because I do
>> not remember the necessary flags (yes, these should be in the source
>> files), maybe because ghc has changed.
> GHC has changed pretty much. I don't know whether there's a way to make
> your code compile with flags, without changing the code itself.
>> I do for example have functions like this:
>> getnArray :: Int -> [Word8] -> Maybe (UArray Int Word8, [Word8])
>> getnArrayST :: Int -> [Word8] ->
>> (forall s . ST s (Maybe (UArray Int Word8, [Word8])))
>> getnArrayST n bs :: ST s (Maybe (UArray Int Word8, [Word8])) =
> Get rid of such signatures, this is where you get a parse error, I don't
> know if there's a way to make GHC parse it at all. I doubt it.
>> (a :: STUArray s Int Word8) <- newArray_ (0,n-1)
> Move the signature to the RHS,
> a <- newArray_ (0,n-1) :: ST s (STUArray s Int Word8)
>> let loop k bs
>> | k == n = do fa <- freeze a
>> return $ Just (fa, bs)
>> | k < n = case bs of
>> (b:bs) -> do
>> writeArray a k b
>> loop (k+1) bs
>>  -> return Nothing
>> loop 0 bs
>> getnArray n bs = runST (getnArrayST n bs)
> With those changes (and ScopedTypeVariables), it compiles.
It indeed does, even though I doubted it at first. As far as I remember
the type in
getnArrayST n bs :: ST s (Maybe (UArray Int Word8, [Word8])) =
used to be necessary to bind the type variable s. Apparently things
have become easier.
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