| Actually, it's sufficient to do good arity-raising transformations, | and use the definition: | foldl f z xs = foldr (\x k a -> k (f a x)) id xs z | | This yields code which looks a bit like this: | | let loop [] = id | loop (x:xs) = let k = loop xs in (\a -> k (f a x)) | in loop xs z | | In the pH compiler we recognized this idiom (and its generalizations | to multiple accumulating parameters) and arity-raised loop as follows: | | let loop [] a = a | loop (x:xs) a = loop xs (f a x) | in loop xs z Absolutely right. This particular thing has been on my to-do list for at least 10 years (see Andy Gill's thesis). Dana Xu and I worked on it a bit last year, but her focus has shifted to verifying Haskell programs. We identified two analyses to help with arity raising: the simpler one works by looking a function *definitions*, and will deal with this example; the other looks at function *calls*. The good news is that Kirsten Chevalier is coming to Microsoft for an internship Oct-Dec this year, and we plan to work on exactly this. Watch this space. Simon