structure Constant in Haskell

In Haskell, constants are used to assign fixed values to variables that cannot be changed throughout the program's execution. To declare a constant, you can use the let keyword followed by the name of the constant, an equal sign (which is read as "is defined as"), and the value of the constant.

Here's an example:

module Main where
-- declare constant pi

pi :: Double

pi = 3.14159
-- define a function that calculates the area of a circle

areaOfCircle :: Double -> Double

areaOfCircle r = pi * r * r
main :: IO ()

main = do

    -- calculate the area of a circle with radius 5

    let radius = 5

    let area = areaOfCircle radius

    putStrLn $ "The area of a circle with radius " ++ show radius ++ " is " ++ show area

In this example, we declare a constant pi and assign it the value 3.14159. This constant is then used in the areaOfCircle function to calculate the area of a circle. Finally, in the main function, we declare two more variables radius and area using the let keyword and use them to calculate and display the area of a circle with radius 5.

Note that in Haskell, constant variables are not truly immutable as they can be redefined later in the code. However, it is considered bad practice to do so and should be avoided in most cases.