In GHC/FFI, is there some way (is it possible ?) to access DIRECTLY an array of UArray type (immutable) in a sequential contiguous memory buffer (in C side, for example) without to need to copy the array elements one by one ? GHC hackers and implementors are wellcome... :-) My interest is to transmit the array over a network using MPI, but avoiding marshalling overheads.

It is easy to do this with StorableArray's in the IO monad, of course, but I need to use UArray.

I've been pondering this issue for a while. In fact, I have a replacement for StorableArray which addresses this issue, amongst others. I provide the following function: unsafeStorableArrayToIOUArray :: StorableArray i e -> IOUArray i e This function works in O(1) time in GHC's implementation, because it uses the same representation type for StorableArray and IOUArray (well, in my hacked version of StorableArray this is true). The function has an 'unsafe' prefix, because it only works when the representations of the two arrays are the same (currently true for all 'e' except Bool).

From an IOUArray, you can get a UArray by using unsafeFreeze.

A better way might be to have an immutable StorableArray type, that you can get by freezing a StorableArray, and that also supports passing to C. This involves a lot of extra code, though. Cheers, Simon