Hi, there is a new ticket that Data.List.intersperse is not as non-strict as possible (http://hackage.haskell.org/trac/ghc/ticket/4282). I have observed some other functions which are unnecessarily strict and it might be advantageous to change their definitions as well. I think it is known that the current implementations of inits and tails are too strict. At least I think I have once read a post to haskell-cafe about this topic. Furthermore intersect is too strict. We have intersect _|_ [] = _|_ and the current implementation is linear in the size of xs if we evaluate intersect xs []. I think simply adding a rule intersect _ [] = [] to the current implementation solves this issue. The implication (<=) :: Bool -> Bool -> Bool is too strict as well. We have False <= _|_ = _|_ as well as _|_ <= True = _|_ while one of these cases could yield True. The problem is that (<=) is defined by means of compare. This effect shows up for all data types with a least element if the default implementation of (<=) by means of compare is used. Furthermore there are a couple of functions which are too strict but whose minimally strict implementations do not provide benefits. For example, reverse is too strict as has already been observed by Olaf Chitil (http://www.cs.kent.ac.uk/people/staff/oc/Talks/ifl2006NonStrictProgramming.p... ). Cheers, Jan