[GHC] #7862: Could not deduce (A) from the context (A, ...)

[GHC]

#7862: Could not deduce (A) from the context (A, ...)
-------------------------------------+-------------------------------------
        Reporter:  alang9            |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Compiler (Type    |              Version:  7.8.3
  checker)                           |
      Resolution:  fixed             |             Keywords:
Operating System:  Linux             |         Architecture:  x86_64
                                     |  (amd64)
 Type of failure:  GHC rejects       |            Test Case:  indexed-
  valid program                      |  types/should_compile/T7862
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by thomie):

 * status:  new => closed
 * resolution:   => fixed
 * milestone:   => 8.0.1


Comment:

 This was fixed in commit a6f0f5ab45b2643b561e0a0a54a4f14745ab2152:
 {{{
 Author: Simon Peyton Jones <simonpj at microsoft.com>
 Date:   Tue Dec 23 15:39:50 2014 +0000

     Eliminate so-called "silent superclass parameters"

     The purpose of silent superclass parameters was to solve the
     awkward problem of superclass dictinaries being bound to bottom.
     See THE PROBLEM in Note [Recursive superclasses] in TcInstDcls

     Although the silent-superclass idea worked,

       * It had non-local consequences, and had effects even in Haddock,
         where we had to discard silent parameters before displaying
         instance declarations

       * It had unexpected peformance costs, shown up by Trac #3064 and its
         test case.  In monad-transformer code, when constructing a Monad
         dictionary you had to pass an Applicative dictionary; and to
         construct that you neede a Functor dictionary. Yet these extra
         dictionaries were often never used.  (All this got much worse when
         we added Applicative as a superclass of Monad.) Test T3064
         compiled *far* faster after silent superclasses were eliminated.

       * It introduced new bugs.  For example SilentParametersOverlapping,
         T5051, and T7862, all failed to compile because of instance
 overlap
         directly because of the silent-superclass trick.

     So this patch takes a new approach, which I worked out with Dimitrios
     in the closing hours before Christmas.  It is described in detail
     in THE PROBLEM in Note [Recursive superclasses] in TcInstDcls.

     Seems to work great!

     Quite a bit of knock-on effect

      * The main implementation work is in tcSuperClasses in TcInstDcls
        Everything else is fall-out

      * IdInfo.DFunId no longer needs its n-silent argument
        * Ditto IDFunId in IfaceSyn
        * Hence interface file format changes

      * Now that DFunIds do not have silent superclass parameters, printing
        out instance declarations is simpler. There is tiny knock-on effect
        in Haddock, so that submodule is updated

      * I realised that when computing the "size of a dictionary type"
        in TcValidity.sizePred, we should be rather conservative about
        type functions, which can arbitrarily increase the size of a type.
        Hence the new datatype TypeSize, which has a TSBig constructor for
        "arbitrarily big".

      * instDFunType moves from TcSMonad to Inst

      * Interestingly, CmmNode and CmmExpr both now need a non-silent
        (Ord r) in a couple of instance declarations. These were previously
        silent but must now be explicit.

      * Quite a bit of wibbling in error messages
 }}}

 The test output is now:
 {{{
 T7862.hs:23:10: Warning:
     No explicit implementation for
       ‘+’, ‘*’, ‘abs’, ‘signum’, ‘fromInteger’, and (either ‘negate’
                                                             or
                                                             ‘-’)
     In the instance declaration for ‘Num (Tower s a)’
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7862#comment:10>
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