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.
do (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. Thank you, Carsten