parser error: multiple if-guards on the same line in for-comprehensions

[unkarjedy]

using Scala 2.13.4

According to scala syntax (https://www.scala-lang.org/files/archive/spec/2.13/13-syntax-summary.html)

Generator ::=  Pattern1 ‘<-’ Expr {[semi] Guard | semi Pattern1 ‘=’ Expr}

several guards on the same line is a valid code.
But unfortunately, if you place several guards on the same line, starting from a new line, a compilation error occurs.

for {
  x <- 1 to 5 if true if true
} yield x

for {
  x <- 1 to 5 if true
  if true
} yield x

for {
  x <- 1 to 5
  if true if true
} yield x

for {
  x <- 1 to 5
  if true
  if true
} yield x

related issue: https://youtrack.jetbrains.com/issue/SCL-13220

[unkarjedy]

Note that it works fine in Scala 3.0.0-M3.
(though it's not reflected in the syntax reference scala/scala3#11215)

[dwijnand]

My assumption is we need to fix the spec, not the other way round. But let's see what other say (including in the Dotty PR).

[som-snytt]

Quick PR.

Full disclosure, I once advocated https://github.com/scala/scala/pull/3463/files

[SethTisue]

I think I lean towards changing the Scala 2 implementation to match Scala 3's, and changing the Scala 3 spec to match Scala 2's.

It is well established that a guard doesn't need a semicolon before it; and it's well established that you can have multiple guards. So I don't see a good grounds for forbidding any of the four examples above. if a if b on the same line is a bit silly since it could be if a and b, but it's not really the spec's job to forbid things that are a bit silly.

Regardless, I'd like to see what other people think, and in particular, how the Dotty team reacts to scala/scala3#11215 .

There is one piece of information I feel I'm missing here in order to really feel confident in any decision, namely: how is it that the Scala 3 spec ended up being different than the Scala 2 one on this point? Who did that when and why? I'll ask on the Dotty PR.

[SethTisue]

I think I lean towards changing the Scala 2 implementation to match Scala 3's, and changing the Scala 3 spec to match Scala 2's.

confirmed by Martin on the Dotty PR