[Haskell-cafe] Class Synonyms - example 2

[Stefan Holdermans]

Jorge,

> 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:

\begin{code}
class (Foo a b, Foo b a) => DoubleFoo a b
instance (Foo a b, Foo b a) => DoubleFoo a b
\end{code}

HTH,

Stefan