#1825: standalone deriving for typeable fails
------------------------+---------------------------------------------------
Reporter: jpbernardy | Owner: simonpj
Type: bug | Status: new
Priority: normal | Milestone: 6.8.2
Component: Compiler | Version: 6.8.1
Severity: normal | Resolution:
Keywords: | Difficulty: Unknown
Testcase: | Architecture: Unknown
Os: Unknown |
------------------------+---------------------------------------------------
Changes (by simonpj):
* owner: => simonpj
* milestone: => 6.8.2
Comment:
Oh bother! The deriving for `Typeable` is unique. If you use the
`deriving` clause on the `data` decl, you'll get this instance:
{{{
instance Typeable1 D where
typeOf1 _ = mkTyConApp (mkTyCon "D") []
}}}
So that's what the user should write in a standalone deriving clause too.
thus
{{{
deriving instance Typeable1 D
}}}
(There's a uniform instance for type applications.)
Sadly, GHC 6.8.1 rejects the above because "`Typeable1` is not a derivable
class".
Ok so this is broken and there's no workaround. I will milestone it for
6.8.1. What I propose is to require you to write "`deriving instance
Typeable1 D`". That's not as obvious as the declaration you started with,
but it fits with the current impl, and with the (current) Typeable design;
see Section 7.4 of [http://www.cs.vu.nl/boilerplate/gmap2.pdf the second
SYB paper].
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1825#comment:1>
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
