haskell4d.md

3C. Conditions in do blocks II

Using Actions instead of do blocks

How does when "not execute" a do-block?

Haskell if a functional programming language and every function has an output. What exactly do we mean by "not executed".

the when will instead we perform a return (). So if you (for reason) inspected the value the when evaluated to, you'll see it's ().

import Control.Monad (when)

main = do
 result <- when (True) (putStrLn "hi!")
 print $ result

Output

hi!
()

The <do-block> can be either a fully-fledged do-block, or a simple monadic action like putStrLn or getLine.