[GHC] #13249: Default signature check can be quite onerous

[GHC]

#13249: Default signature check can be quite onerous
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        Reporter:  bgamari           |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #12918            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Well it's true, isn't it?  We are simply listing the conditions under
 which you can use the generic default instance.  The more constraints you
 need, the less generally-applicable it is.

 In this case you are saying that you can say
 {{{
 instance MonadGet ty where
   type Remaining ty = blah
    -- no code for 'remaining'
 }}}
 only if:
  * `ty` is of form `(t n)`
  * We have `MonadTrans t, MonadGet n, Monad n`
  * `Remaining (t n)` equals `Remaining n`.

 Fair enough

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13249#comment:2>
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