1- avoid forming the (iter xs) and (count i+1) closures by passing the function and the arguments instead of the function bound to the argument iter [] next i done = done iter (x:xs) next i done = next i x iter xs
count i x step xs = step xs count (i+1) (i+1)
test xs = iter xs count 0 0
I'm not at all sure what you're aiming for (the above doesn't compile), just some guesses/hints: - your original version allowed both 'next' and 'done' to change at every step, so specializing 'iter' for 'count' would seem to involve something like fixpoint reasoning for the general case or at least unfolding a finite number of recursions for the special case - if 'next' is supposed to be constant, inlining 'iter' would provide enough context to specialize the recursion for that constant parameter, but GHC doesn't inline recursive definitions (see also http://hackage.haskell.org/trac/ghc/ticket/3123 ) and even if it did the equivalent of loop peeling (unfold a few iterations of the recursion at the call site), it might not spot the constant parameter - a common workaround -if there are constant parts to the recursion- is to split the recursive definition into non-recursive wrapper and recursive worker (with some parameters made constant), then to let GHC inline the wrapper; this is particularly interesting if the constant parts are functions So, if you are happy with 'next' being constant throughout the iteration (encoding any variation in the accumulator 'done'), something like this might do: iter next = iterW where iterW done [] = done iterW done (x:xs) = next done x iterW xs count i x step xs = step (i+1) xs test xs = iter count 0 xs GHC should do more wrt recursive definitions, but it can't guess your intentions. There are facilities for library-defined optimizations (RULES), but they wouldn't cover the kind of transformations you seem to be looking for. Work is underway to make library-specified optimizations more expressive (as core2core pass plugins), though I don't know the status of either that (Max?-) or whether #3123 is on anyone's list. Claus