Require case prefix for patterns in for-comprehension generators

[propensive]

A common beginners' mistake when using for-comprehensions is to write a pattern on the left-side of a generator, and to not realize that it desugars to filter/withFilter.

I propose that this behavior should be supported only if the patter is prefixed with the keyword case, for example,

def collect(xs: List[Option[Int]]) = for {
  case Some(i) <- xs
} yield i

This would be a compile error:

def collect(xs: List[Option[Int]]) = for { Some(i) <- xs } yield i

[dwijnand]

Using case makes it look like it'll match error.

What about if? Then it's obviously like other if filters. (Also it's similar to Rust's if let.)

[He-Pin]

def collect(xs: List[Option[Int]]) = for {
  case Some(i) <- xs
} yield i

and with no error

def collect(xs: List[Option[Int]]) = for {
  ifcase Some(i) <- xs
} yield i

[lihaoyi]

duplicate of #2578

[DavidGregory084]

See also https://github.com/oleg-py/better-monadic-for/blob/master/README.md which changes the desugaring to use map and match

[propensive]

@dwijnand No strong feelings either way in using case or if. The former does certainly cause a potential match error everywhere else, so that might be worth keeping as a universal truth.

[smarter]

So there's been a lot of proposals related to improving for-comprehensions, but it seems like there's just no one working on Dotty currently who's motivated enough to lead the redesign. To move forward, I suggest people who are interested in the topic build consensus and collaboratively write a SIP. Once we have an accepted SIP, the implementation should be easy. Therefore, I'm going to close this and redirect people to https://contributors.scala-lang.org/t/making-for-simpler-and-more-regular/2160 where the discussion has already started.