What is the difference between
data T0 f a = MkT0 a
instance Eq (T0 f a) where ...
and
data T0 f a = MkT0 a
instance Eq a => Eq (T0 f a) where ...
The second one says that "TO f a" is only an instance of "Eq" if "a"
is, while the first says that "TO f a" is an instance regardless of
the type of its arguments.
More explanation please. :-)
Much obliged, Paul
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