[GHC] #13774: Singletons code fails to typecheck when type signature involving type family is added

[GHC]

#13774: Singletons code fails to typecheck when type signature involving type
family is added
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |             Keywords:  FunDeps,
      Resolution:  invalid           |  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by simonpj):

 * status:  new => closed
 * resolution:   => invalid


Comment:

 Your type signature
 {{{
 sT2 :: Sing (L2r 'False)
 }}}
 means
 {{{
 sT2 :: forall k. Sing k (L2r Bool k 'False)
 }}}
 And, since `k` is universally quantified, we can't unify it with `Nat`. So
 the instance doesn't match.

 You can fix it thus
 {{{
 sT2 :: Sing (L2r 'False :: Nat)
 }}}
 which indeed compiles fine.

 I'm not sure what other error message would be better.   I'll close as
 invalid for now.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13774#comment:3>
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