[Haskell-cafe] Class Synonyms - example 2

Stefan Holdermans sholderm at students.cs.uu.nl
Sat Dec 11 04:00:55 EST 2004


> 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) [...]

Again, with -fallow-undecidable-instances:

class (Foo a b, Foo b a) => DoubleFoo a b
instance (Foo a b, Foo b a) => DoubleFoo a b



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