[GHC] #16213: Unnecessary error

[GHC]

#16213: Unnecessary error
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        Reporter:  pjljvdlaar        |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.3
      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 RyanGlScott):

 Replying to [ticket:16213 pjljvdlaar]:
 > Yet, both patterns are matched, i.e. '[] and (x ': xs),
 > so I was surprised the compiler could not figure this out!

 This isn't how type class instances work. Having an `Show (TermList '[])`
 instance and a `Show (TermList (x ': xs))` instance doesn't imply an
 instance for `forall (v :: [Bool]). Show (TermList v)`. In the instance
 you're trying to write, GHC needs a `Show` instance for an arbitrary list
 `v`, and GHC can't know //a priori// whether that list is `'[]` or `(_ ':
 _)`.

 That being said, there's not much sense in writing two separate `Show`
 instances like this. Why not just combine them into a single instance,
 like so?

 {{{#!hs
 instance Show (TermList v) where
     show TNil = "Nil"
     show (TCons a b) = "(Cons " ++ show a ++ " " ++ show b ++ ")"
 }}}

 This actually gives you an instance for `forall (v :: [Bool]). Show
 (TermList v)`, so you avoid your earlier issue entirely.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16213#comment:1>
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