[GHC] #15421: Refactor (~) to reduce the superclass stack

[GHC]

#15421: Refactor (~) to reduce the superclass stack
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        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:                    |
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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>
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