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::Numa=>[a]->[a]

Num a is how we represent the bounded polymorphism inHaskell , the definition ofNum wasclass Num b where(Hoogle showsa, just prevent to confuse reader don’t familiar withHaskell ) could read asa typeb is an instance of classNum.

SonumSort takes[a] only ifa is an instance ofNum. 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 useHaskell for this,Java also has this feature), consider the old way to doList<A> toList<B>, and unfortunately solution was to copy each element in the list.