Handling nested if-else-then in do blocks in Haskell

Question

I've found that I don't know how to handle nested if-else-then in do-blocks in Haskell.

I know already that I can use a case, but that would require all my conditions (a, b and c) to return the same type (Bool, so there are only two cases, but I need three distinct ones) and is therefore not as general (correct me if I'm wrong). I have also tried considering using guards here, but I don't know how to make this work in a do statement, especially if the -- something expressions are meant to be of type IO ().

Suppose I have the following code that is inside a do:

if a then
     -- something
else 
    if b then
        -- something
    else
        if c then
            -- something
        else
            -- something

How do I create the equivalent logic but without all the indenting?

What you have written already requires all of your -- something placeholders to return the same type, so your objection to case doesn't really apply. — amalloy
– amalloy, Commented Apr 23, 2019 at 4:12
(1) "that would require all my cases to return the same type and is therefore not as general (correct me if I'm wrong)" -- you also have to return the same type in both branches of an if-expression. Other than syntax, there is essentially no difference between an if-expression and a case-expression on Bool. (2) "I have also tried considering using guards here, but I don't know how to make this work in a do statement" -- You can't use guards within a do-block; if-expressions are indeed the right thing to reach for. — duplode
– duplode, Commented Apr 23, 2019 at 4:16
Sorry, I misspoke in my original question, it is not the -- something placeholders returning the same type that I am trying to avoid, it is the conditions (a, b and c). I've edited my answer. — MonkeySeeMonkeyCode
– MonkeySeeMonkeyCode, Commented Apr 23, 2019 at 4:43

duplode · Accepted Answer · 2019-04-23 04:44:04Z

9

To begin with, it is worth noting that if-expressions do not actually require extra indentation (if-within-do used to be an exception, but Haskell 2010 eliminated that). That means you might collapse all the extra indentation:

test = do
    len <- length <$> getLine
    if len < 4
    then putStrLn "Short"
    else if len > 6
    then putStrLn "Long"
    else putStrLn "Mid"

Personally, though, I don't find that too pleasing aesthetically, as I feel some indentation makes if-expressions easier to follow. A nice alternative is using the MultiWayIf extension:

{-# LANGUAGE MultiWayIf #-}

test = do
    len <- length <$> getLine
    if | len < 4 -> putStrLn "Short"
       | len > 6 -> putStrLn "Long"
       | otherwise -> putStrLn "Mid"

On a final note, nested if-expressions getting unwieldy might reveal a good occasion for breaking things down in separate definitions, or otherwise reorganising your code. See also: How do I deal with many levels of indentation?

answered Apr 23, 2019 at 4:44

duplode

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4 Comments

MonkeySeeMonkeyCode Over a year ago

Thank you! This is exactly what I was looking for as an answer. Sorry for my typo in my original question. Haskell is great fun so far!

duplode Over a year ago

@Utagai You're welcome, and worry not -- if you only knew how many typos I make in my posts... :)

dfeuer Over a year ago

@Utagai, if you don't want to use MultiWayIf, you can use case () of _ | .... Uglier, but standard.

MonkeySeeMonkeyCode Over a year ago

Thanks @dfeuer, didn't realize I could use a case like that but reading it, I see why it works. Thank you!

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