syntax.md: update syntax of for-expressions

[unkarjedy]

Comments to ForExpr ::=

1. new line before yield or do is supported in braceless syntax as well
2. do can also go in for-expressions with braces/parenthesis
3. we could save some space and replace 3 lines with 2 by merging (‘(’ Enumerators ‘)’ | ‘{’ Enumerators ‘}’)
   but I find my version more readable

Comments to Enumerators0 ::=

1. enumerators can be preceded by an arbitrary amount of newline character
2. ; is possible after the last enumerator

All-in-one example with valid for-expressions which are accepted by Scala 3 compiler:

for x <- 0 to 1 do println(42)
for x <- 0 to 1 yield 42

for { x <- 0 to 1 } do println(42)
for { x <- 0 to 1 } yield 42

for (x <- 0 to 1) do println(42)
for (x <- 0 to 1) yield 42

////////////////////////////////////////

for x <- 0 to 1
do println(42)
for x <- 0 to 1
yield 42

for { x <- 0 to 1 }
do println(42)
for { x <- 0 to 1 }
yield 42

for (x <- 0 to 1)
do println(42)
for (x <- 0 to 1)
yield 42


for (x <- 0 to 1) println(42)
for (x <- 0 to 1)
  println(42)
// DO or YIELD is obligatory when no FOR is braceless
//for x <- 0 to 1 println(42)
//for x <- 0 to 1
// println(42)

for

x <- 0 to 1;

do println(42)

[odersky]

No {nl} needs to be inserted here.

[unkarjedy]

Enumerators themselves do not accept new line before them.

No {nl} needs to be inserted here.

Is it because it's kinda "obvious" that here can go an arbitrary amount of new lines?
For me, it's as obvious as {nl} after ) in for / if / while, still they contain {nl} in rules.

[odersky]

No, nl means an newline token inserted by the lexer. But there is never a newline inserted in front of a do or yield.

[odersky]

Same comment about nl. Also, there's a divergence between parser and syntax.md regarding the trailing [semi] in Enumerators0. The syntax is also not what is supported by the parser. The parser supports trailing semicolons in braces and in braceless syntax but not in parens. That is a question we should clear up. When should enumerators suppport a trailing ;. I believe all other proposed changes to the syntax are unnecessary.

[odersky]

Finally, any PR for syntax.md needs to change both versions: in internals and in reference.

[unkarjedy]

The parser supports trailing semicolons in braces and in braceless syntax but not in parens. That is a question we should clear up

Yeah, that I have missed...

I believe all other proposed changes to the syntax are UNnecessary.

Do you mean that's ok there is a divergence between parser and syntax reference?
(syntax doesn't support do after for with brackets and new line before do/yield in braceless version

Finally, any PR for syntax.md needs to change both versions: in internals and in reference.

The PR contains the change in both reference/syntax.md and internals/syntax.md (you've commented in both files)
What did I miss?

[odersky]

No, nl means an newline token inserted by the lexer. But there is never a newline inserted in front of a do or yield.

[dwijnand]

@unkarjedy so to me this looks like Martin approved applying some changes, but then it failed due to failures we had in master. Do you want to rebase (and squash) this branch so we can merge the PR?

[unkarjedy]

@dwijnand done