[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 sheaf):

 What about the following:


 {{{#!hs
 data Dim
   = D2
   | D3
   | D4

 type family ToDim (n :: Nat) = d | d -> n where
   ToDim 2 = D2
   ToDim 3 = D3
   ToDim 4 = D4
   ToDim n = TypeError
     ( Text "Error: dimension must be 2, 3 or 4 (given: "
      :<>: ShowType n :<>: Text ")" )
 }}}

 This seems useful to me, as it allows one to switch easily between two
 different representations of dimension (which have different uses: with
 Nats I can do arithmetic, but the explicit enum is more convenient with
 singletons for instance).

 I feel like the injectivity annotation should be allowed in this case (but
 I am not aware of any of the theory which backs injective type families).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12119#comment:6>
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