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