#5007: "deriving" seems to ignore class context for a type family
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Reporter: jkff | Owner: simonpj
Type: bug | Status: closed
Priority: high | Milestone: 7.2.1
Component: Compiler (Type checker) | Version: 7.0.2
Resolution: invalid | Keywords: type families,
datatype contexts, type classes, deriving
Testcase: | Blockedby:
Difficulty: | Os: Unknown/Multiple
Blocking: | Architecture: Unknown/Multiple
Failure: GHC rejects valid program |
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Changes (by simonpj):
* status: new => closed
* resolution: => invalid
Comment:
The error message is quite right.
Using a context on a data type declaration, as you are doing, is a mis-
feature of Haskell. If you read its specification carefully you'll see
that it is practically useless. Any program using it is suspicious.
Certainly, it has absolutely no effect on 'deriving' declarations.
You can follow the advice in the error message and use a standalone
deriving declaration, thus
{{{
{-# LANGUAGE StandaloneDeriving, FlexibleContexts, UndecidableInstances,
TypeFamilies #-}
module T5007 where
class Foo a where
data Bar a :: *
class (Show (Bar a)) => Qux a
data Xyzzy a = Xyzzy (Bar a)
deriving instance Show (Bar a) => Show (Xyzzy a)
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5007#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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