After looking more closely at user's manual, I just found that the following works:
{-# OPTIONS_GHC -fglasgow-exts -fallow-undecidable-instances #-} data Foo = Foo deriving Show data Bar c = Bar (c Foo)
deriving instance Show (c Foo) => Show (Bar c)
/ Emil On 2007-12-20 11:18, Emil Axelsson wrote:
Hello all!
How come in GHC 6.6 I could to write
{-# OPTIONS_GHC -fglasgow-exts -fallow-undecidable-instances #-} data Foo = Foo deriving Show data Bar c = Bar (c Foo) deriving Show
but in GHC 6.8.2 I get the error
No instance for (Show (c Foo)) arising from the 'deriving' clause of a data type declaration at Ert.hs:3:0-37 Possible fix: add an instance declaration for (Show (c Foo)) When deriving the instance for (Show (Bar c))
Thanks,
/ Emil
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