Add monadic val

[b-studios]

WIP

Adding monadic value bindings to block syntax. That is,

{
  val a <- ma
  val b <- mb
  foo(a, b)
}

desugars to:

{
  ma.flatMap { a => 
  mb.flatMap { b => 
    foo(a, b)
  }}
}

More details can be found in the contributors forum.

[b-studios]

Here is another example of using the monadic val syntax:

trait Monad[+A] {
  def flatMap[B](f: A => Monad[B]): Monad[B]
}
def pure[A](a: A): Monad[A] = ???

val foo = 42
val ma: Monad[Int] = ???

val x: Monad[String] = {
  val a: Int <- ma
  println("real side effect")
  val b <- ma
  if (a == b) {
    pure("foo")
  } else {
    pure("bar")
  }
}

[nafg]

Does the first statement of the block have to be a val ... <-? Because otherwise it means that as you're reading code you don't know what it does until later retroactively, when you encounter the val ... <- in the block.

[b-studios]

No, since the transformation really just desugars to flatMap ignorant of the rest of the block, the first statement does not need to be a monadic val.

[b-studios]

Also, all statements except for monadic binds just preserve their semantics. So, what you read is what you get ;)

[b-studios]

It would be nice to have syntax for monadic binds that ignore their results. That is, those which are only executed for their side effects.

Currently one has to write:

{
...
val _ <- log("foo")
val _ <- log("bar")
.. 
}

But I don't have a good proposal for a syntax there, yet. The best I could come up with is using do:

{
...
do log("foo")
do log("bar")
.. 
}

While it nicely fits as a marker that the rhs is monadic and is reminiscent of Haskell, it might not ideal to overload it as a keyword. It also will clash syntactically with do ... while ... expressions.

Any other ideas?