#810: GHC complains about missing instance in conjunction with GADTs
-------------------------------------+--------------------------------------
Reporter: [EMAIL PROTECTED] | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.4.1
Severity: normal | Keywords:
Os: Linux | Difficulty: Unknown
Architecture: x86 |
-------------------------------------+--------------------------------------
I wanted to do something like that:
{{{
data Pair :: (* -> *) -> * where
Pair :: a b -> b -> Pair a
data Sel :: * -> * where
A :: Sel Bool
B :: Sel Integer
showSnd :: Pair Sel -> String
showSnd (Pair A bool) = show bool
showSnd (Pair B integer) = show integer
}}}
However, GHC 6.4.1 with -fglasgow-exts complains in the second last and
last
line that there is no instance for (Show b). But it is clear that bool ::
Bool and integer :: Integer and that therefore bool and integer are
showable. In addition, Chris Kuklewicz said the following:
I cut and pasted your code into ghci-6.4.2 with -fglasgow-exts and there
was no
error. And I can (print (showSnd (Pair A True)) and (print (showSnd (Pair
B
8))) but when I try to compile it with ghc-6.4.2 it complains about the
Show
instance. I find this deeply wierd, and it seems like a bug.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/810>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler_______________________________________________
Glasgow-haskell-bugs mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
