[GHC] #12888: ‘Identity instance’: Outputable SDoc

[GHC]

#12888: ‘Identity instance’: Outputable SDoc
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  GHC API        |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Is there reason why there isn't an ‘identity instance’¹ for `Outputable`?

 {{{#!hs
 class Outputable a where
   ppr :: a -> SDoc

 instance Outputabe SDoc where
   ppr :: SDoc -> SDoc
   ppr = id
 }}}

 A benefit: lets me always use `pprTraceIt :: Outputable a => String -> a
 -> a`, I know I can use `pprTrace` but I like not having to think about
 the type of what I'm tracing and neither functions work to debug something
 like `Either SDoc Name`.

 ----

 ¹ For lack of a better term, something like

 {{{#!hs
 class Foo a where
   foo :: a -> A

 instance Foo A where
   foo :: A -> A
   foo = id @A
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12888>
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