Haskell Type and TypeClass

Types#

:t value

You can check the type of data. Haskell is a strongly typed language.

The same applies to functions.

For example:

fun :: [Char] -> [Char]

For the function "fun", the parameter must be a character array, which is equivalent to a String.

Type Classes#

Type classes are used to define a common interface.

They provide a set of common characteristics for different types.

Similar to interfaces:

class typeclassname a where
    funtionname :: a -> a -> returntype

This can be compared to Java:

interface<T,R> name{
    R funtion(T t,T t);
}

Then implement instances of the type class:

instance typeclassname type where
    funtionname arg arg = return

For example:

class EqualClass a where
    equalFun :: a->a->Bool


instance EqualClass Bool where
    equalFun True True = True
    equalFun True False = False
    equalFun False False = True
    equalFun _ _ = False
    --XNOR

Comparable to Java:

class interfaceImpl<Bool,Bool> implement interface{
    Bool funtion(Bool l, Bool r){
        retutn !(l^r);
        //XNOR
    }
}

Therefore, type classes are more like interfaces that can implement different behaviors for different classes.