[haskell-llvm] [Haskell-cafe] LLVM, type-level?

[Henning Thielemann]

On Sat, 11 Dec 2010, Lally Singh wrote:

> I had tried that as well, but figured it would be best to post about
> exactly what you had told me :-)
>
> Heres' what I have:
> buildReaderFun :: String -> CodeGenModule (MainFunction)
> buildReaderFun nm  = do
>  puts <- newNamedFunction ExternalLinkage "puts" ::
>    TFunction (Ptr Word8 -> IO Word32)
>  let callPuts format = (
>        createNamedFunction ExternalLinkage "main" $
>        \ argc argv -> do
>          tmp <- getElementPtr (argv ::Value (Ptr (Ptr Word8)))
>                 (0 :: Int32,  (1 :: Int32, ()))
>          _ <- call puts tmp
>          ret (0 :: Int32)) :: CodeGenModule (MainFunction)
>
>  withStringNul nm callPuts
>
> And the result:
> /research/phd/libmet/Listener.hs:51:17:
>    No instance for (GetElementPtr (Ptr Word8) (Int32, ()) Word8)
>      arising from a use of `getElementPtr'
>                   at /research/phd/libmet/Listener.hs:(51,17)-(52,47)
>
> Is there another instance I should add?

No, these GetElementPtr type errors really point you to a serioues 
problem. GHC proposes to add instances because in the general case this 
might be a solution. But here you try to do something that is simply not 
supported by LLVM, and thus 'llvm' forbids it.

I look into
   http://hackage.haskell.org/packages/archive/llvm/0.9.0.1/doc/html/LLVM-Core.html#t:GetElementPtr
  and see that there is no instance for 'optr = Ptr a'. That means in order 
to follow two pointers successively, you have to call getElementPtr twice. 
I assume that with

>          tmp <- getElementPtr (argv ::Value (Ptr (Ptr Word8)))
>                 (0 :: Int32,  (1 :: Int32, ()))

you want to have a pointer to the first (not zeroth) character of the 
zeroth of argv. Is that what you want? I think you have to write

>   tmp0 <- getElementPtr (argv ::Value (Ptr (Ptr Word8))) (0 :: Int32, ())
>   tmp1 <- load tmp0
>   tmp  <- getElementPtr tmp1 (1 :: Int32, ())

http://www.llvm.org/docs/GetElementPtr.html#firstptr
"Why is it okay to index through the first pointer, but subsequent pointers won't be dereferenced?"