# NOTE: bounded polymorphism

Bounded polymorphism refers to existential quantifiers(∃), 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.

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### 林子篆

I am a programming language theory lover; good at system software like Networking, OS.

### Discussion

forgot that MathJax expression xd. Did anyone know how to use LaTeX in dev.to?