[GHC] #10756: Allow users to indicate inaccessible patterns

[GHC]

#10756: Allow users to indicate inaccessible patterns
-------------------------------------+-------------------------------------
        Reporter:  edsko             |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.10.2
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
-------------------------------------+-------------------------------------

Comment (by bgamari):

 Also, kosmikus has suggested that the syntax proposed above looks a bit
 too similar to a usual RHS. [https://github.com/idris-lang/Idris-dev/wiki
 /Unofficial-FAQ#where-is-agdas--pattern-and-what-is-impossible Idris], for
 instance, has a very clearly different syntax for this,
 {{{
 f : Num a => (n : Nat) -> Vect n a -> Nat
 f Z [] = 0
 f Z (_::_) impossible

 f (S n) (x::xs) = x + f n xs
 f (S _) [] impossible
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10756#comment:3>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler