Scala equivalent of Haskell's do-notation (yet again)

Question

I know that Haskell\'s

do
  x <- [1, 2, 3]
  y <- [7, 8, 9]
  let z = (x + y)
  return z

can be expressed in Scala as

for         


        

Answer 1

Yes, that translation is valid.

do { x <- m; n } is equivalent to m >>= \x -> n, and do { m; n } is equivalent to m >> n. Since m >> n is defined as m >>= \_ -> n (where _ means "don't bind this value to anything"), that is indeed a valid translation; do { m; n } is the same as do { _ <- m; n }, or do { unusedVariable <- m; n }.

A statement without a variable binding in a do block simply disregards the result, usually because there's no meaningful result to speak of. For instance, there's nothing interesting to do with the result of putStrLn "Hello, world!", so you wouldn't bind its result to a variable.

(As for monads being black magic, the best realisation you can have is that they're not really complicated at all; trying to find deeper meaning in them is not generally a productive way of learning how they work. They're simply an interface to compose computations that happen to be particularly common. I recommend reading the Typeclassopedia to get a solid grasp on Haskell's abstract typeclasses, though you'll need to have read a general Haskell introduction to get much out of it.)