#13218: <$ is bad in derived functor instances -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: dfeuer Type: bug | Status: new Priority: normal | Milestone: 8.4.1 Component: Compiler | Version: 8.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Runtime Unknown/Multiple | performance bug Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- `Functor` deriving derives the definition of `fmap`, leaving the definition of `<$` to the default. This is quite bad for recursive types: {{{#!hs data Tree a = Bin !(Tree a) a !(Tree a) | Tip deriving Functor }}} produces {{{ Replace.$fFunctorTree_$c<$ = \ (@ a_aGl) (@ b_aGm) (eta_aGn :: a_aGl) (eta1_B1 :: Tree b_aGm) -> Replace.$fFunctorTree_$cfmap @ b_aGm @ a_aGl (\ _ [Occ=Dead] -> eta_aGn) eta1_B1 }}} Why is this bad? It fills the tree with thunks keeping the original values (which we never use again) alive. What we want to generate is {{{#!hs x <$ Bin l _ r = Bin (x <$ l) x (x <$ r) }}} When there are other functor types in the constructor, like {{{#!hs | Whatever (Tree (Tree a)) }}} we will need to insert `fmap (x <$) t`. The overall shape should be basically the same as `fmap` deriving. Note: there are some types for which we will not realistically be able to derive optimal definitions. In particular, fixed-shape, undecorated types that appear in nested types allow special treatment: {{{#!hs data Pair a = Pair a a deriving Functor data Tree2 a = Z a | S (Tree2 (Pair a)) deriving Functor }}} The ideal definition for this type is {{{#!hs x <$ Z _ = Z x x <$ S t = S (Pair x x <$ t) }}} but that requires cleverness. We should probably settle for {{{#!hs x <$ Z _ = Z x x <$ S t = S (fmap (x <$) t) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13218 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler