NOTE: bounded polymorphism

Bounded polymorphism refers to existential quantifiers(\[\exists\]), restricted to range over types of bound type. To understand it only needs a few examples. Let's start! Take a look at the following program:

numSort :: Num a => [a] -> [a]

Num a is how we represent the bounded polymorphism in Haskell, the definition of Num was class Num b where=(Hoogle shows =a, just prevent to confuse reader don't familiar with Haskell) could read as a type b is an instance of class Num.

So numSort takes [a] only if a is an instance of Num. Now we could run down:

numSort [1, 2, 3] :: [Int]
numSort [1.1, 2, 3] :: [Double]

This is really a powerful feature(and you don't need to use Haskell for this, Java also has this feature), consider the old way to do List<A> to List<B>, and unfortunately solution was to copy each element in the list.