Type system compiler flags

[Carsten Schultz]

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