#15421: Refactor (~) to reduce the superclass stack -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Simon Peyton Jones <simonpj@…>): In [changeset:"f265008fb6f70830e7e92ce563f6d83833cef071/ghc" f265008f/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="f265008fb6f70830e7e92ce563f6d83833cef071" Refactor (~) to reduce the suerpclass stack The constraint (~) used to be (effectively): class a ~~ b => (a :: k) ~ (b :: k) but, with this patch, it is now defined uniformly with (~~) and Coercible like this: class a ~# b => (a :: k) ~ (b :: k) Result: * One less superclass selection when goinng from (~) to (~#) Better for compile time and better for debugging with -ddump-simpl * The code for (~), (~~), and Coercible looks uniform, and appears together, e.g. in TysWiredIn and ClsInst.matchGlobalInst. Previously the code for (~) was different, and unique. Not only is this simpler, but it also makes the compiler a bit faster; T12227: 9% less allocation T12545: 7% less allocation This patch fixes Trac #15421 }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15421#comment:1> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler