Proposal: Make intersect(By) lazier and faster

With the current implementation of intersectBy, the calculation of intersectBy _ xs [] takes O(length xs) time, hence doesn't finish on infinite lists and evaluates to _|_ for partial lists xs (... : _|_). The proposed new implementation, intersectBy :: (a -> a -> Bool) -> [a] -> [a] -> [a] intersectBy _ [] _ = [] intersectBy _ _ [] = [] intersectBy eq xs ys = [x | x <- xs, any (eq x) ys] makes this an O(1) operation, returning [] also for infinite or partial xs. The first equation retains the property intersectBy _ [] _|_ = [] Trac ticket: http://hackage.haskell.org/trac/ghc/ticket/4323 Period of discussion: Two weeks, until 30 Sep. 2010. Cheers, Daniel