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Type-based dispatch
class Num a where
(+) :: a -> a -> a
negate :: a -> a
fromInteger :: Integer -> a
...
double :: Num a => a -> a
double x = 2*x
means
double :: Num a -> a -> a
double d x = mul d (fromInteger d 2) x
The overloaded value is returned by
fromInteger, not passed to it. It is the
dictionary (and type) that are passed as
argument to fromInteger