#7542: GHC doesn't optimize (strict) composition with id ---------------------------------+------------------------------------------ Reporter: shachaf | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.1 Keywords: | Os: Unknown/Multiple Architecture: Unknown/Multiple | Failure: Runtime performance bug Difficulty: Unknown | Testcase: Blockedby: | Blocking: Related: | ---------------------------------+------------------------------------------ Changes (by shachaf):

* status: infoneeded => new Comment: Here's an example of the sort of context this comes up in: {{{ module T7542 where import Unsafe.Coerce newtype Id a = MkId { unId :: a } -- Think of `mapped` as `mapM`, but restricted to Id (we could make it work -- with any Functor, rather than just []). `over` takes the Id wrappers back -- off. The goal is to make it easy to compose mapped with other functions of -- the same form. The wrapper should be "free" because it's just newtype noise. mapped1 :: (a -> Id b) -> [a] -> Id [b] mapped1 f = MkId . map (unId . f) over1 :: ((a -> Id b) -> s -> Id t) -> (a -> b) -> s -> t over1 l f = unId . l (MkId . f) map1 :: (a -> b) -> [a] -> [b] map1 f xs = over1 mapped1 f xs -- Core: map1 = \f xs -> map (\x -> f x) xs -- over1 mapped1 = unId . MkId . map (unId . MkId . f) -- ~ map -- However, if f = ⊥, unId . MkId . f /= f! -- Therefore `over1 mapped1` must turn into \f -> map (\x -> f x) -- We can't expect GHC to compile it to `map` because it has different strictness. -- Let's define strict versions of (MkId .) and (unId .): mkIdDot2 :: (a -> b) -> a -> Id b mkIdDot2 f = f `seq` \x -> MkId (f x) unIdDot2 :: (a -> Id b) -> a -> b unIdDot2 f = f `seq` \x -> unId (f x) mapped2 :: (a -> Id b) -> [a] -> Id [b] mapped2 f = mkIdDot2 (map (unIdDot2 f)) over2 :: ((a -> Id b) -> s -> Id t) -> (a -> b) -> s -> t over2 l f = unIdDot2 (l (mkIdDot2 f)) map2 :: (a -> b) -> [a] -> [b] map2 f xs = over2 mapped2 f xs -- map2 should have the same semantics as map. But the Core isn't the same: -- Without -fpedantic-bottoms: map2 = \f xs -> map (\e -> f e) xs -- With -fpedantic-bottoms: -- map2 = \f xs -> map (case f of g { __DEFAULT -> \x -> g x }) xs -- Ideally, (case f of g { __DEFAULT -> \x -> g x }) would simply be f. -- Let's try manually telling GHC that our newtype compositions are coercions: -- (Ideally, this is what mkIdDot2 and unIdDot2 would compile into.) mkIdDot3 :: (a -> b) -> a -> Id b mkIdDot3 = unsafeCoerce unIdDot3 :: (a -> Id b) -> a -> b unIdDot3 = unsafeCoerce -- (Note: Due to #7398, we couldn't define a strict composition operator and -- rely on RULES to turn (MkId `dot`) into unsafeCoerce -- the `MkId` itself -- gets turned into a coercion before any RULES have a chance to fire.) mapped3 :: (a -> Id b) -> [a] -> Id [b] mapped3 f = mkIdDot3 (map (unIdDot3 f)) over3 :: ((a -> Id b) -> s -> Id t) -> (a -> b) -> s -> t over3 l f = unIdDot3 (l (mkIdDot3 f)) map3 :: (a -> b) -> [a] -> [b] map3 f xs = over3 mapped3 f xs -- Core: map3 = map }}} -- Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7542#comment:2> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler _______________________________________________ Glasgow-haskell-bugs mailing list Glasgow-haskell-bugs@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs