Maybe I should have included a more interesting example in the previous mail. So I had this class:
class Foo a b | a -> b where foo_method1 :: ... foo_method2 :: ... ...
Besides the case where 'a' is the same as 'b', there is also another interesting case. That is when you have both, Foo A B and Foo B A. This is a known property (named DoubleFoo), so I'd like to type contexts as,
DoubleFoo a b =>
instead of,
(Foo a b, Foo b a) =>
so I tried:
class (Foo a b, Foo b a) => DoubleFoo a b where
This works fine if I'm going to define functions which need both instances of Foo. Something like:
testDouble :: DoubleFoo a b => a -> b -> c testDouble a b = foo_method1 a b ... foo_method1 b a
but it doesn't help me with:
testDouble2 :: DoubleFoo a b => a -> b -> c testDouble2 a b = foo_method1 a b ... testDouble2 b a
now I need DoubleFoo b a as well. Seems to me like there is no way of saying:
Foo a b , Foo b a <=> DoubleFoo a b
right? J.A.