# 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.