How to express the following if-else if-else statement in Haskell?

Question

I am trying to replicate the following requirements/code in Haskell

Pseudo-Code (the actual evaluations are much more complicated):

if (x == 1)
   doX()
else if (y == 1)
   doY()
else if (z == 1)
   doZ()
else
   doSomethingElse()

Right now I have this as WHEN statements for each, but that doesn't give me an ELSE, so I have

when (x == 1) $ do doX
when (y == 1) $ do doY
when (z == 1) $ do doZ

But then how do I manage my else?

Although @bheklilr's guards are more idiomatic for this, Haskell does have if ... then ... else ... expressions, so the pseudocode is only missing thens to be legal Haskell. (But watch out for indentation if using if then else together with do.) — Ørjan Johansen
– Ørjan Johansen, Commented Aug 3, 2014 at 3:48
@ØrjanJohansen This is pretty much the reason why I chose guards for my examples, they avoid the nastiness of having to worry about indentation. — bheklilr
– bheklilr, Commented Aug 3, 2014 at 3:52
Not a serious answer: [doX, doY, doZ] !! findIndex (==1) [x, y, z] — Daniel Wagner
– Daniel Wagner, Commented Aug 3, 2014 at 7:27

bheklilr · Accepted Answer · 2014-08-03 03:11:21Z

5

This looks like a good option for guards or a case statement. You could do something like

myAction :: Monad m => Int -> Int -> Int -> m ()
myAction x y z
    | x == 1 = doX
    | y == 1 = doY
    | z == 1 = doZ
    | otherwise = doSomethingElse

Or you can use the MultiWayIf extension:

myAction :: Monad m => m ()
myAction = do
    x <- getX
    y <- getY
    z <- getZ
    if | x == 1 -> doX
       | y == 1 -> doY
       | z == 1 -> doZ
       | otherwise -> doSomethingElse
    -- continue doing more stuff in myAction as needed

answered Aug 3, 2014 at 3:11

bheklilr

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1 Comment

kqr Over a year ago

You can even fake a multiway if with case () of () | p1 -> e1; () | p2 -> e2 ...

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