Hi,
Take a look at the following program, making use of
derivable type classes.
>>>
module Bug where
import Data.Generics
class Foo a where
foo :: a -> Int
foo{| Unit |} _ = 1
foo{| a :*: b |} _ = 2
foo{| a :+: b |} _ = 3
instance Foo [a]
<<<
GHC 6.2.2 produces the following error message:
>>>
Bug.hs:12:
Could not deduce (Foo a) from the context (Foo [a])
arising from use of `foo' at Bug.hs:12
<<<
Why is the context needed? 'foo' is not a recursive
function?
I guess it is because the default declaration is split up
into several instances:
>>>
instance Foo Unit where
foo _ = 1
instance (Foo a, Foo b) => Foo (a :*: b) where
foo _ = 2
instance (Foo a, Foo b) => Foo (a :+: b) where
foo _ = 3
<<<
Why not generating:
>>>
instance Foo Unit where
foo _ = 1
instance Foo (a :*: b) where
foo _ = 2
instance Foo (a :+: b) where
foo _ = 3
<<<
when the context is not needed?
(My motivation is: I have a class like this:
class Arbitrary a => Shrink a where
shrinkSub :: a -> [a]
shrinkSub{| ... |} = ... shrink ...
The definition of shrinkSub is not recursive, it calls a
function 'shrink' from the Arbitrary class instead.)
Regards,
/Koen
PS. Has the implementation of Generics changed from some
earlier compiler version (GHC 5.xx)? I have code lying
around that I am almost certain of used to compile with an
earlier version of GHC (that I do not have access to
anymore).
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