Declare in Haskell

In Haskell, we declare a class using the class keyword followed by the name of the class and its constraints. Here is the general syntax for declaring a class:

class ClassName constraints where
    function1 :: Type1
    function2 :: Type2
    ...
    functionN :: TypeN

ClassName is the name of the class, and constraints are optional constraints on the type variables used in the class.

Here is an example of declaring a class Printable that defines a single function printMe. This function takes an argument of type a and returns nothing.

class Printable a where
    printMe :: a -> IO ()
instance Printable Int where

    printMe = print
instance Printable Char where

    printMe c = putChar c >> putChar '\n'
main :: IO ()

main = do

    printMe (5 :: Int)

    printMe 'c'

In this example, we declare the class Printable with one member function printMe that takes an argument of type a and returns IO(). We then provide instances for the Printable class for types Int and Char.

The instance Printable Int declaration uses the print function which is pre-defined in Haskell to print the value of the integer. The instance Printable Char declaration creates a custom implementation of printMe that takes a Char argument and uses putChar to print the character and then a newline character.

Finally, in the main function, we use printMe to print an integer and a character using their corresponding instances.