11 Dec
2004
11 Dec
'04
12:56 p.m.
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}
Ah! Got it. Thanks Stefan, J.A.