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)