[GHC] #10719: Malformed data type quotation accepted

31 Jul 2015

      #10719: Malformed data type quotation accepted
-------------------------------------+-------------------------------------
              Reporter:  heisenbug   |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.8.3
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
-------------------------------------+-------------------------------------
 This is accepted:

 Prelude Language.Haskell.TH> {{{$(stringE . show =<< [d| data A where C ::
 C |])}}}

 "[DataD [] A_1627402878 [] [ForallC [] [] (NormalC C_1627402879 [])] []]"

 In contrast this is rejected:
 Prelude Language.Haskell.TH> {{{$(stringE . show =<< [d| data A p where C
 :: C |])}}}

 <interactive>:29:22: Malformed constructor result type: C

 However it would make sense to form an equality constraint (for later
 kind/type checking) in these cases, something along the lines of:
 {{{#!hs
 data A p where
   C :: (A p ~ C) => C
 }}}
 as there could be type synonym (or family) `C`.

 I have tested various versions >= 7.8.3 and all seem to behave the same.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10719
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler