[Haskell-cafe] Re: POSIX AIO (asych I/O) ...

Brandon S. Allbery KF8NH allbery at ece.cmu.edu
Wed Jul 2 02:07:25 EDT 2008

On 2008 Jul 2, at 1:42, Galchin, Vasili wrote:

>       errno <- throwErrnoIfMinus1 "aioError" (c_aio_error  p_aiocb)
> "ghc" thinks that "Errno" should be an instance of "Num":
> System/Posix/Aio.hsc:117:15:
>     No instance for (Num Errno)

I expect so it can compare it to -1(throwErrnoIfMinusOne).  But if the  
return value is actually an errno and not -1 to indicate error (which  
it is if I read the manpage correctly), you don't want  
throwErrnoIfMinus1 anyway; I suspect you want to wrap the return value  
of c_aio_return (which should be IO CInt) in an Errno constructor,  
then use errnoToIOError if you really want to raise an IOError.

(What were you expecting for "count"?  I see none, just an errno.)

Note that it *never* returns -1; it returns 0 for successful  
completion for the aiocb, EINPROGRESS if it's still working, and the  
appropriate errno if it failed.

You might want to decide if you want to use the aio_return style  
interface or something more Haskell-ish before designing this part of  
the API.  If you want to stick close to the C interface:

aioReturn :: AIOCB -> IO (AIOCB, Errno)
aioReturn aiocb = do
    allocaBytes (#const sizeof(struct aiocb)) $ \ p_aiocb -> do
       poke p_aiocb aiocb
       err <- c_aio_return  p_aiocb
       aiocb <- peek p_aiocb
       return (aiocb, Errno err)

I'd actually consider something more Haskellish, e.g. a variant of  
StateT IO where the state is the aiocb and errno, the errno  
initialized to eINPROGRESS and set by aioReturn and aioError (and once  
aioReturn is called, it can't be called again so return the cached  
value if needed).

brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery at kf8nh.com
system administrator [openafs,heimdal,too many hats] allbery at ece.cmu.edu
electrical and computer engineering, carnegie mellon university    KF8NH

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