PR Stanley wrote:
Hi
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.
You can regard an "instance" declaration as an inference rule for the
type checker, with "=>" meaning "implies" (though I don't think its the
answer to your other question about names).
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