Hi all, I am having space issues with some decompression code; I've attached a much simplified version as Test1.hs. At the bottom (foo/bar) is the equivalent of deflate. This should be a standalone module which doesn't know about the rest. In the middle (readChunks) is the equivalent of gunzip. It repeatedly calls foo until there is no more input left. At the top is a simple main function that calls them. If I do dd if=/dev/zero of=data bs=1000 count=3000" # making data around 3MB ghc --make Test1 -o Test1 -O -Wall ./Test1 then in top I see Test1 increasing memory usage to around 150MB. I think this is because the "let (ys, zs) = foo xs" means zs holds on to xs (it's hard to be sure as compiling for profiling is too happy to change the behaviour). I tried (Test2) changing foo to be a monad transformer over the calling monad, so the caller's remaining input was updated as we went along, but (as well as memory usage not obviously being fixed) this is giving me a stack overflow. Has anyone got any suggestions for making a constant space, constant stack version? Thanks Ian