[GHC] #12119: Can't create injective type family equation with TypeError as the RHS

[GHC]

#12119: Can't create injective type family equation with TypeError as the RHS
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  feature request   |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |             Keywords:
      Resolution:  wontfix           |  CustomTypeErrors, TypeFamilies,
                                     |  InjectiveFamilies
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 AntC):

 The injectivity annotation is a promise that all your equations are
 injective.

 Then consider the return ''type'' of `ToDim 5` compared to `ToDim 6` (if
 you put a `TypeError` equation): they're the same type. Then that equation
 is not injective.

 Specifically, `ShowType n` is not injective:
 [https://downloads.haskell.org/~ghc/latest/docs/html/libraries/base-4.12.0.0/src
 /GHC-TypeLits.html#AppendSymbol it's an existentially-quantified data
 constructor, promoted to a datakind].

 If you omit the `TypeError` equation, there's an implicit `ToDim n = ToDim
 n` added at the end. That is injective.

 Possibly you could put some other error-handling logic, that preserves
 plain `n` on the rhs, then that would be injective.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12119#comment:10>
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