[GHC] #9273: TypeNats and record syntax don't compile

[GHC]

#9273: TypeNats and record syntax don't compile
----------------------------------------------+----------------------------
        Reporter:  br1                        |            Owner:
            Type:  bug                        |           Status:  closed
        Priority:  normal                     |        Milestone:
       Component:  Compiler (Type checker)    |          Version:  7.8.2
      Resolution:  invalid                    |         Keywords:
Operating System:  Unknown/Multiple           |     Architecture:
 Type of failure:  GHC rejects valid program  |  Unknown/Multiple
       Test Case:                             |       Difficulty:  Unknown
        Blocking:                             |       Blocked By:
                                              |  Related Tickets:
----------------------------------------------+----------------------------
Changes (by rwbarton):

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


Comment:

 That's because record syntax additionally defines an accessor function
 {{{
 sNatPred :: SNat (n+1) -> SNat n
 sNatPred (SNatS x) = x
 }}}
 which you cannot define manually using the non-record syntax `SNat`
 either. As the error message says, GHC does not (yet) know that `n+1`
 determines `n`, so it cannot type check the above definition because as
 far as it is concerned, `x` might have type `SNat n1` for some other `n1`
 with `n1+1 = n+1`. Once the Nat solver is improved, your record syntax
 `SNat` should compile.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9273#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler