#4529: Deriving Data does not work for attached code
--------------------------------------+-------------------------------------
Reporter: mitar | Owner:
Type: bug | Status: closed
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 7.1
Resolution: invalid | Keywords:
Testcase: | Blockedby:
Difficulty: | Os: Unknown/Multiple
Blocking: | Architecture: Unknown/Multiple
Failure: None/Unknown |
--------------------------------------+-------------------------------------
Changes (by simonpj):
* status: new => closed
* resolution: => invalid
Comment:
You have
{{{
data (Real r, Show r, Random r, Data r)
=> RandomFoo r = RandomFoo (RandomBar r)
deriving (Typeable, Data)
}}}
As you'll see if you say `-ddump-deriv` (I'm improving the layout) you get
a `Data` instance with `(Real r, Random r)` in the context? Why? Because
the instance pattern matches on the constructor.
Solution: simply remove the context from the `data` declaration, giving
{{{
data RandomFoo r = RandomFoo (RandomBar r)
deriving (Typeable, Data)
}}}
The context (which is a Haskell mis-feature) does you no good, and as you
have seen, it does you harm.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4529#comment:3>
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
