[Haskell] Using newArray with hGetArray

[Andre Pang]

Hi all, I have the following snippet of code:

    mutableMemoryArray <- newArray_ (0, bufferSize - 1)
    bytesRead <- hGetArray handle mutableMemoryArray bufferSize

The problem is that hGetArray (like hGetBuf) may read less than the  
amount that you specify (since you may hit EOF before filling up the  
whole array).  If this happens, the mutableMemoryArray array has a  
bounds which is greater than it should be.  This is quite bad if,  
say, you hPutArray the array to another writable Handle.

e.g. if bufferSize is 4096, and the input file is 4100 bytes long,  
the code will properly read 4096 bytes on the first pass, but only 4  
bytes on the second pass; yet the bounds of the array will still be  
4096 bytes in both cases, rather than 4096 and 4.  You can't simply  
swap the two expressions since you don't know how many bytes you've  
read until you execute hGetArray, and you can't execute hGetArray  
until you allocate the array, which requires that you know the array  
bounds.  Chicken-and-egg.

Am I missing something obvious here?  I'd even be happy if there was  
some way to adjust the bounds on a mutable array after it's been  
created, or even if I could use hGetBuf and somehow efficiently  
create an array from a (Ptr Word8).  I know I could simply re- 
allocate another array and garbage collect the first one if  
bytesRead /= bufferSize, but that seems pretty inelegant.

Thanks!


-- 
% Andre Pang : trust.in.love.to.save  <http://www.algorithm.com.au/>