I tend to deal a lot with very very large data files in Haskell and my current approach to dealing with getting data out of one program an into another is writing it to a file using 'show' and then reading it in using 'read'. Unfortunately, this is very slow and produces very large files which are very slow to read and write.
Be careful with your choice of Read and Show instances. Standard reading of lists is ``strict in the closing bracket'', so the first element is only returned after the end of the list has been read. I tend to define my own instances, using a fairly standard set of parser combinators working on the ReadS type --- this speeds up things considerably (and runs in less memory). This can, of course, also be used to get file size down, but in my circumstances this was not a major problem (rarely had files of more than a few tens of megabytes), and human-readability was useful to have. Wolfram