Constants in Haskell

Constants in Haskell

Haskell supports constants of character, string, boolean, and numeric values.

Defining Constants#

In Haskell, constants can be defined using the let or where clauses, but for simplicity, constants are usually defined at the top level using pattern matching.

-- Defining a constant value
s :: String
s = "constant"

main :: IO ()
main = do
    putStrLn s

Defining Numeric Constants#

A constant statement can appear anywhere a variable assignment can.

-- Defining a numeric constant
n :: Integer
n = 500000000

Arithmetic with Constants#

Constant expressions in Haskell perform arithmetic with arbitrary precision.

-- Defining a constant with an arithmetic expression
d :: Double
d = 3e20 / fromIntegral n

main :: IO ()
main = do
    putStrLn s
    print d

Type Conversion#

A numeric constant has no type until it’s given one, such as by an explicit conversion.

main :: IO ()
main = do
    putStrLn s
    print d
    print (round d :: Int)

Context-Based Typing#

A number can be given a type by using it in a context that requires one, such as a variable assignment or function call. For example, here sin expects a Floating type.

main :: IO ()
main = do
    putStrLn s
    print d
    print (round d :: Int)
    print (sin (fromIntegral n :: Double))

To run the program, save the code in a file named constants.hs and use runhaskell to execute it.

$ runhaskell constants.hs
constant
6.0e11
600000000000
-0.28470407323754404

Now that we can run and build basic Haskell programs, let’s learn more about the language.