[Haskell-cafe] Re: trouble compiling "import GHC.Prim(MutableByteArray#, ....." (building regex-tdfa from darcs) -- what's that # sign doing?

[Stefan O'Rear]

On Sun, Aug 19, 2007 at 11:25:49PM +0100, ChrisK wrote:
> > #ifdef __GLASGOW_HASKELL__
> > foreign import ccall unsafe "memcpy"
> >     memcpy :: MutableByteArray# RealWorld -> MutableByteArray# RealWorld -> Int# -> IO ()
> > 
> > {-# INLINE copySTU #-}
> > copySTU :: (Show i,Ix i,MArray (STUArray s) e (ST s)) => STUArray s i e -> STUArray s i e -> ST s ()
> > copySTU (STUArray _ _ msource) (STUArray _ _ mdest) =
> > -- do b1 <- getBounds s1
> > --  b2 <- getBounds s2
> > --  when (b1/=b2) (error ("\n\nWTF copySTU: "++show (b1,b2)))
> >   ST $ \s1# ->
> >     case sizeofMutableByteArray# msource        of { n# ->
> >     case unsafeCoerce# memcpy mdest msource n# s1# of { (# s2#, () #) ->
> >     (# s2#, () #) }}
> > 
> > #else /* !__GLASGOW_HASKELL__ */
> > 
> > copySTU :: (MArray (STUArray s) e (ST s))=> STUArray s Tag e -> STUArray s Tag e -> ST s ()
> > copySTU source destination = do
> >   b@(start,stop) <- getBounds source
> >   b' <- getBounds destination
> >   -- traceCopy ("> copySTArray "++show b) $ do
> >   when (b/=b') (fail $ "Text.Regex.TDFA.RunMutState copySTUArray bounds mismatch"++show (b,b'))
> >   forM_ (range b) $ \index ->
> >     unsafeRead source index >>= unsafeWrite destination index
> > #endif /* !__GLASGOW_HASKELL__ */
> 
> The entire point of using the ST monad is manage memory more efficiently than
> with (U)Array.  The copySTU simply uses a "memcpy" to copy the whole source
> array into the destination efficiently.  This lets me re-use the already
> allocated destination array.  If there had been a high level "copyMArray" then
> this would not have been needed.  The CPP is used to let non-GHC compilers copy
> element by element.  The *right* solution is to patch the STUArray and/or MArray
> code to do this behind the scenes.
> 
> So how does one get the array pointer without GHC.Prim in 6.7 ?

Import GHC.Exts, which exports everything GHC.Prim does, and according
to the docs is "GHC Extensions: this is the Approved Way to get at
GHC-specific extensions.".

(Can't help you with the CPP issue though.)

Stefan
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