Manuel | I have found a way of rephrasing the definition so that it | is properly optimised by GHC. However, I think, it should | be possible to do this automatically and it is maybe not | unlike the optimisation done by simplCore/LiberateCase. I agree with your example. But I think the way to handle it is this. Suppose we have f x y = ....(f ex (C a b))... where C is a constructor, and where y is scrutinised by a case expression somewhere in f's body. Then, in this recursive call to f, we know what y will be, and we could save the case in the recursive call. So make a specialised version of f, thus: fs x a b = let y = C a b in (...original body of f...) Now add a RULE to f, saying f x (C a b) ===> fs x a b This is very much the way type specialisation works. Now every call to f that has a (C a b) argument will benefit. Including the recursive call to f in fs's RHS, so fs will become the self-recursive tight loop you want, leaving f as a sort of 'impedence matcher' for callers elsewhere. I'll try to find time to implement this. Simon