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

9 Feb 2017

      #13249: Default signature check can be quite onerous
-------------------------------------+-------------------------------------
           Reporter:  bgamari        |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  8.1
  (Type checker)                     |
           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:
-------------------------------------+-------------------------------------
 The `bytes` library currently doesn't compile with GHC `master` due to the
 new check of default signatures (7363d5380e600e2ef868a069d5df6857d9e5c17e,
 #12918). Consider this example,
 {{{#!hs
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE DefaultSignatures #-}
 module Hi where
 import Control.Monad.Trans.Class

 class MonadGet m where
   type Remaining m :: *

   remaining :: m (Remaining m)
   default remaining :: (MonadTrans t, MonadGet n, m ~ t n) => m (Remaining
 (t m))
   remaining = lift remaining
 }}}

 Patching this up requires a fair amount of hand-holding,
 {{{#!hs
   default remaining :: (MonadTrans t, MonadGet n, m ~ t n, Remaining m ~
 Remaining n, Monad n) => m (Remaining m)
 }}}
 Yuck.

 I suppose this is just how the world works, but I thought I'd leave this
 here in case anyone had a clever idea for improvement.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13249
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