haskell4a.md

4A. Conditions in do Blocks I

Using when

The Control.Monad module exposes a function that help us conditionally execute do blocks. This is called when. When we run the code below:

import Control.Monad (when)

main = do
 when False (do putStrLn "A")
 when True (do putStrLn "B")

The first putStrLn is not executed because we pass a false condition, and the second putStrLn is executed because the condition is true, and we'd see B in standard out.

Output

B

To summarise the behaviour of when:

when - when <condition> <do-block>, if <condition> is true, then <do-block> is executed, otherwise the do-block is not executed.

We can for example, read a name from standard-in and print something if the name matches our codename 007:

import Control.Monad (when)

main = do
 name <- getLine
 when (name == "007") (do putStrLn "Hello, Mr. Bond")

Output

runhaskell foo.js
> 001
runhaskell foo.js
> 007
Hello, Mr. Bond

Using unless

In general, if you have when (not C) b, you can remove that annoying not in the condition by using unless. unless is the boolean opposite of when. If runs the block when the condition is false. It the condition is true, then it does not execute the do block.

Using if-then-else for handling both cases

What if you want to do something in both the true case or in the false case. We could use a when followed by an unless like so:

But this is duplicating the condition somewhat. Luckily Haskell has a contruct called if-then-else.

import Control.Monad (when)

main = do
 putStrLn "Codename?"
 name <- getLine
 if (name == "007") then do
   putStrLn "Hello, Mr. Bond"
   putStrLn "What cocktail would you like sir?"
   cocktail <- getLine
   putStrLn $ "Here it your " ++ cocktail
 else do
   putStrLn "Go away!"

Output

In the case where you are 007:

Codename?
> 007
Hello, Mr. Bond
What cocktail would you like sir?
> Vodka Martini, shaken not stirred
Here it your Vodka Martini, shaken not stirred

And in the case where you are not 007:

Codename?
> 001
Go away!

Note that if-then-else is to be used when, and only when you want to do something in both the true case and the false case. Unlike other programming langauges, in Haskell, you can't write if without the else - they come as a pair! So the following would not compile:

---

Example

What is the output from the following:

main = do
  let foo x = do
      if (x == 3)
      then putStrLn "magic"
      else putStrLn "boring"
  foo 1
  foo 2
  foo 3

Solution:

boring
boring
magic!

Explanation: