#15205: Unnecessary equality superclass -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Consider {{{ {-# LANGUAGE MultiParamTypeClasses, GADTs, TypeOperators #-} module Foo where class (a ~ b) => C a b where op :: a -> a -> b f :: C a b => a -> b f x = op x x }}} If you compile this you'll end up with {{{ f = \ (@ a_a1dk) (@ b_a1dl) ($dC_a1dn :: C a_a1dk b_a1dl) (eta_B1 :: a_a1dk) -> case GHC.Types.heq_sel @ * @ * @ a_a1dk @ b_a1dl ((Foo.$p1C @ a_a1dk @ b_a1dl $dC_a1dn) `cast` (Data.Type.Equality.N:~[0] <*>_N <a_a1dk>_N <b_a1dl>_N :: (a_a1dk ~ b_a1dl) ~R# (a_a1dk ~~ b_a1dl))) of co_a1dw { __DEFAULT -> op @ a_a1dk @ b_a1dl $dC_a1dn eta_B1 eta_B1 } }}} What is that unused `heq_sel` doing? It happens during solving * We have `[G] a ~ b` * And, by superclasses we have `[G] a ~# b` * We use this to rewrite `a` to `b` in both givens and wanteds which is all fine, but we end up with {{{ f = \ (@ a_a1dk) (@ b_a1dl) ($dC_a1dn :: C a_a1dk b_a1dl) -> case GHC.Types.heq_sel @ * @ * @ a_a1dk @ b_a1dl (Data.Type.Equality.$p1~ @ * @ a_a1dk @ b_a1dl (Foo.$p1C @ a_a1dk @ b_a1dl $dC_a1dn)) of co_a1dw { __DEFAULT -> \ (x_axX :: a_a1dk) -> op @ a_a1dk @ b_a1dl (($dC_a1dn `cast` ((C co_a1dw <b_a1dl>_N)_R :: C a_a1dk b_a1dl ~R# C b_a1dl b_a1dl)) `cast` ((C (Sym co_a1dw) <b_a1dl>_N)_R :: C b_a1dl b_a1dl ~R# C a_a1dk b_a1dl)) x_axX x_axX } }}} Notice that `co_a1dw` is used. But we are just casting and casting back, so it ends up as Refl and `co_a1dw` is unused. Nothing is wrong here, but it seems inelegant * Can we discard that `heq_sel`? Perhaps we can declare that dictionary- valued terms are always treated strictly (#2439), so that `(heq_sel ...)` is guaranteed non-bottom, and we can discard the case. * Or maybe we can expand the given superclasses less aggressively, so that the equality isn't exposed until necessary. But see `Note [Eagerly expand given superclasses]` in `TcCanonical`. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15205> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler