Re: [GHC] #13772: Cannot put HasCallStack on instances

12 Jun 2017

      #13772: Cannot put HasCallStack on instances
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        Reporter:  augustss          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      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 Iceland_jack):

 I am not very familiar with `HasCallStack` but it is possible to go from
 `ImplicitParams` to kmett's reflection library, I don't know where you
 would call [https://hackage.haskell.org/package/reflection-2.1.2/docs
 /Data-Reflection.html#v:reify reify] but it may help:

 {{{#!hs
 import Data.Proxy
 import GHC.Stack
 import Data.Reflection
 import Data.Monoid hiding ((<>))
 import Data.Semigroup

 newtype WithClassStack s a = WCS a

 instance Semigroup a => Semigroup (WithClassStack s a) where
   WCS a <> WCS b = WCS (a <> b)

 instance (Semigroup a, Reifies s CallStack) => Monoid (WithClassStack s a)
 where
   mempty = error "No mempty!" where
     ?callStack = reflect (Proxy @s)

   mappend = (<>)
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13772#comment:5
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