A side-effect free Haskell program?

What are side-effects?

Functional programming is all about controlling side-effects in your program.

According to Wikipedia:

[…] a function or expression is said to have a side-effect if it modifies some state outside its scope or has an observable interaction with its calling functions or the outside world besides returning a value. For example, a particular function might modify a global variable or static variable, modify one of its arguments, raise an exception, write data to a display or file, read data, or call other side-effecting functions.

Haskell is particularly explicit about these side-effects.
Here are a few examples of functions from the Haskell standard library:

-- Write a string to the standard output device
putStrLn :: String -> IO ()

-- Read a file and returns the contents of the file
readFile :: FilePath -> IO String

Both of these functions are returning a special type; the IO monad. It represents the result of a computation involving I/O (writing to the console or reading a file on disk).

A program

A particularity of Haskell is that it even consider your program as an IO monad.
The following example is a simple hello world:

main :: IO ()
main = do
    putStrLn "Hello, World!"

You can see that in the type definition the main function returns an IO monad of any type (unit here).

Now imagine that you want to create a program that doesn't invoke any side-effect functions. From the Haskell point of view this program would do absolutely nothing, you even cannot express it since the main needs an IO monad.


Is it possible a write a side-effect free program in Haskell?

The answer is no. A program is by definition (in Haskell at least) a side-effect.

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