[GHC] #2715: Equality constraint in superclass not supported

[GHC]

#2715: Equality constraint in superclass not supported
-------------------------------------------------+--------------------------
  Reporter:  rodprice                            |          Owner:  chak  
      Type:  task                                |         Status:  closed
  Priority:  low                                 |      Milestone:  7.2.1 
 Component:  Compiler (Type checker)             |        Version:  6.10.1
Resolution:  fixed                               |       Keywords:        
  Testcase:  indexed_types/should_compile/T2715  |      Blockedby:        
Difficulty:  Unknown                             |             Os:  Linux 
  Blocking:                                      |   Architecture:  x86   
   Failure:  None/Unknown                        |  
-------------------------------------------------+--------------------------
Changes (by simonpj):

  * status:  new => closed
  * testcase:  => indexed_types/should_compile/T2715
  * resolution:  => fixed


Comment:

 Hurrah!  We now have superclass equalities. I've added this example as a
 test.
 {{{
 commit 940d1309e58382c889c2665227863fd790bdb21c
 Author: Simon Peyton Jones <simonpj at microsoft.com>
 Date:   Wed Jun 22 17:37:47 2011 +0100

     Add equality superclasses

     Hurrah.  At last we can write

        class (F a ~ b) => C a b where { ... }

     This fruit of the fact that equalities are now values,
     and all evidence is handled uniformly.

     The main tricky point is that when translating to Core
     an evidence variable 'v' is represented either as
       either   Var v
       or       Coercion (CoVar v)
     depending on whether or not v is an equality.  This leads
     to a few annoying calls to 'varToCoreExpr'.

  compiler/basicTypes/MkId.lhs      |    4 +-
  compiler/deSugar/DsExpr.lhs       |   18 +++++++-
  compiler/iface/BuildTyCl.lhs      |   50 ++++++++++------------
  compiler/typecheck/TcInstDcls.lhs |   85
 +++++++++++++++++-------------------
  compiler/typecheck/TcMType.lhs    |    9 +----
  compiler/types/Class.lhs          |   21 ++++------
  6 files changed, 92 insertions(+), 95 deletions(-)
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2715#comment:18>
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