Declare a function with no return value?

Question

Can we create a function with void (i.e with no return value) in functional languages? Like Haskell or Scheme

Answer 1

In Haskell, you can write a whole family of functions that return () (unit), which is equivalent to void in C languages:

foo :: a -> ()
foo _ = ()

bar :: a -> b -> ()
bar _ _ = ()

You'll notice, however, that my implementations ignore their input, and simply return (), so they don't do anything.

You can call them like this:

Prelude> foo 42
()
Prelude> bar 42 "foo"
()

but this still doesn't accomplish anything.

You can, on the other hand, write functions that return IO (), like this:

main :: IO ()
main = putStrLn "Hello, world!"

but this is now impure. While this produces a side-effect, you could argue whether or not it's functional. At the very least, in Haskell, you can't call impure code from pure code (this is by design), so it doesn't compose.