# If else in for comprehension when calculating monadic expression

I can’t find this anywhere. In Haskell we can write a monadic expression like this:

``````fun  = do
if 1 > 0 then do
return 1
else do
return 0
``````

I am not sure if I can do sth that concise in Scala with for comprehension. Or maybe I can write it the other way without explicit if-else? Everything seems fine when I am calculating things in a single line like this:

``````for {
l1 <- calc1()
l2 <- calc2()
l3 <- calc3()
} yield (l1 + l2  + l3)
``````

but what to do when I want to use if-else inside? Is it a sign that I am doing sth wrong?

Do you just mean

``````...
yield (if (l1 > l2) l1 + l3 else l2 + l3)
``````

?

(You can make it a block, too: `yield { ... }`. You can always turn an expression into a block whose last statement is an expression. But if-statements are expressions already.)

Should I keep my monadic expressions short? I thought about sth like this when if and else are between calculated value (with <-)

``````fun  = do
if 1 > 0 then do
if ... then do
x1 <- f1()
return x1
else
x2 <- f2()
return x2
else do
x3 <- f3()
return x3
``````

is Scala for comprehension as expressive as Haskell do?

In general they are both syntax sugar for monadic bind/flatMap and fmap/map functions, so they should be equivalent in expressiveness.

The last example you posted wouldn’t work in either language. Keep in mind that `return` in Haskell has nothing to do with `return` in imperative languages like Python/Java, etc… In particular you cannot use it as a statement to return early from a computation. `return` is an alias for `pure`, which takes a value and lifts it into the monad you’re using. So if the do-notation is used with `Maybe`, `return` will lift `1` into `Maybe`, so it evaluates to `Just 1`.

The very first example you posted has redundant usage of `do` and `return`.

You’d normally write it like this (and probably replace `return` with `pure`, which is the same but a clearer name):

``````fun :: Maybe Int
fun  =
if 1 > 0 then
return 1
else
return 0
``````

Using a conditional in for-comprehensions isn’t any different from using one anywhere else in Scala. You just have to follow the types and understand that if-else is an expression, not a statement.

``````def calc1(): Option[Int]
def calc2(): Option[Int]
def calc3(): Option[Int]

for {
intA <- calc1()

intB <- if (intA > 0) calc2() else calc3()
} yield intA + intB
``````

Note that the if-else-expression has type Option[Int], so we unwrap it the same way we did with the result of calc1().

edit: The Haskell do-notation is confusing for beginners because the last expression needs to have the same type as the Monad it operates on. But the often needed lifting into the Monad isn’t automatic, so people use `return` to do this. But `return` is just an alias for `pure` and confuses people coming from imperative languages, where `return` has special meaning.

Learn You A Haskell introduces the do-notation without `return` or `pure` and uses the specific constructor of the Monad (for `Maybe` this is `Just`). Imo that’s the best way to go about it:

``````foo :: Maybe String
foo = do
x <- Just 3
y <- Just "!"
Just (show x ++ y)``````
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Just to amend the other replies… A straightforward translation to Scala, assuming `Option` as the monadic type, might be

``````def fun: Option[Int] =
for {
res <-
if(1 > 0)
for { o <- Some(1) } yield o
else
for { z <- Some(0) } yield z
} yield res
``````

However, this feels excessively complicated in both languages. The original Haskell expression could be condensed to

``````pure \$ if 1 > 0 then 1 else 0
``````

…translating to Scala…

``````Some(if(1 > 0) 1 else 0)
``````

Note that Scala doesn’t have any builtin notion of a monad, in particular of `pure`/`return` - it just has some “duck typing” that allows to use for expressions on types providing `map`/`flatMap` methods with the proper signature. If you want explicit abstractions for functor, monad, etc., you’ll have to pull in libraries like cats or scalaz.

3 Likes