PrelBase contains the appended code. Am I correct in assuming that the stuff is structured as it is, because the "map" rule first `breaks' the map `open', which exposes it to the various foldr/build rules, and if no foldr/build rule matches, the "mapList" rule `closes' it again in a later phase of optimisation - after build was inlined? If so, it seems like the whole thing depends a bit on the timing of the various optimsations (the map might be closed again before any of the foldr/build rules fires). Is this maybe the reason that build has an INLINE 2 pragma, which delays its inlining, and thus, the closing of the map? All very cunning, it seems ;-) Cheers, Manuel -=- map :: (a -> b) -> [a] -> [b] map = mapList -- Note eta expanded mapFB :: (elt -> lst -> lst) -> (a -> elt) -> a -> lst -> lst mapFB c f x ys = c (f x) ys mapList :: (a -> b) -> [a] -> [b] mapList _ [] = [] mapList f (x:xs) = f x : mapList f xs {-# RULES "map" forall f xs. map f xs = build (\c n -> foldr (mapFB c f) n xs) "mapFB" forall c f g. mapFB (mapFB c f) g = mapFB c (f.g) "mapList" forall f. foldr (mapFB (:) f) [] = mapList f #-}