Scala Wart: For-comprehensions syntax restrictions

Opening this issue, as suggested by Martin, to provide a place to discuss the individual warts brought up in the blog post Warts of the Scala Programming Language and the possibility of mitigating/fixing them in Dotty (and perhaps later in Scala 2.x). These are based on Scala 2.x behavior, which I understand Dotty follows closely, apologies in advance if it has already been fixed

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As mentioned above, you cannot have defs, classes, or imperative statements in the generators of a for-comprehension:

scala> for{
     |   i <- Seq(1)
     |   println(i)
     |   j <- Seq(2)
<console>:4: error: '<-' expected but ';' found.
  j <- Seq(2)
^

This is a rather arbitrary restriction, and as far as I can tell doesn’t serve any purpose, and forces you to put random _ = prefixes on your statements to make things compile:

scala> for{
     |   i <- Seq(1)
     |   _ = println(i)
     |   j <- Seq(2)
     | } yield j
1
res0: Seq[Int] = List(2)

There really isn’t any reason that this shouldn’t work out-of-the-box, and convert say:

for{
  i <- Seq(1)
  def debug(s: Any) = println("Debug " + s)
  debug(i)
  j <- Seq(2)
  debug(j)
  k <- Seq(3)
} yield i + j + k

Into

Seq(1).flatMap{ i => 
  def debug(s: Any) = println("Debug " + s)
  debug(i)
  Seq(2).flatMap{ j =>
    debug(j)
    Seq(3).map{ k => 
      i + j + k
    }
  }
}

[ritschwumm]

see #1982, too

[odersky]

I don’t see a lot of appetite to tackle this, since it would require major changes to the principle and implementation of parsing. So it would be a high effort project that also comes with high risk of failure.