> class A a
> type T = (forall x.Num x=>x)
> instance A T
"type" declares a synonym, like #define in C - but working only on types. So,
essentially, you wrote
instance A (forall x.Num x => x)
which is not very Haskelly.
> I am simply trying to state that all members of typeclass Num are of
> typeclass A....
You can't do that. But, if there would not be any other instances of A, then
you don't need it at all, you can just use Num class. And if there are some,
for example
data D = D
instance A D
then it can happen (well, it's unlikely, but possible) that you or some other
developer working on your code would declare
instance Num D where ...
After that, you would have two instances of A for the type D, one defined
explicitly and one derived from the Num instance. That's not a problem for this
empty class, but if class A is not empty, say
class A x where a :: x
then the compiler would be unable to decide which instance (and which "a") to
choose. So allowing that leads to non-obvious bugs.
However, if you trust yourself enough, you can do what you want to in this way:
{-# OPTIONS_GHC -fglasgow-exts -fallow-undecidable-instances #-}
class A a
instance Num a => A a
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