Deprecate procedure syntax

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The special procedure syntax has been a constant source of trouble and confusion for people learning Scala, while providing no benefit except a few saved characters over using the standard method syntax.

To not cause pointless work for people, this migration has to happen in multiple steps:

• Update the style guide and the specification
• Update tools (IDEs, Scalariform, ...)
• Deprecate procedure syntax

Mailing list discussion: https://groups.google.com/d/topic/scala-internals/q1YX7NC1geM/discussion

ScalaDays 2013 Keynote: http://www.parleys.com/play/51c1994ae4b0d38b54f4621b/chapter47

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Imported From: https://issues.scala-lang.org/browse/SI-7605?orig=1
Reporter: @soc
See #7523, #7648, #6811

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Tony Sloane (asloane) said:
Just for the record, I'd like to record one vote against removing the procedure syntax.

I may be the only one but I use procedure syntax for situations where I do not intend to return anything, reserving the "normal" syntax for places where a value is returned. I personally find a benefit from having the two different syntaxes to indicate these two situations and I haven't experienced any issues or confusions from having both.

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@Ichoran said (edited on Jun 25, 2013 12:11:12 AM UTC):
I also vote against removing this syntax.

I find it visually useful (more useful than : Unit = especially when specifying/guaranteeing that there is no return type), it is well-established (and thus a change would affect many lines of code), and even if it is a source of confusion for newcomers it should be quickly dispelled.

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@soc said:
As mentioned in the thread, nobody claims that experts who know both procedures and methods can't tell them apart.

The issue is that pretty much everyone learning the language falls into the trap of defining a procedure by forgetting the = when they actually intended to define a method.

I don't consider saving 5 characters enough of a reason to expose people learning the language to such a frustrating, unnecessary language detail.

Just a nitpick: The only type which doesn't return anything is Nothing, not Unit (which returns ()). But = is required for Nothing. Yet another reason why I want to get rid of this inconsistency.)

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Tony Sloane (asloane) said:
I'm certainly not claiming that some beginners don't find this a stumbling point. In my experience of teaching Scala to upper level undergraduates I haven't seen it to be a major problem, certainly not compared to other much trickier things that Scala asks them to learn.

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@soc said:
PR for the compiler part is scala/scala#3076.

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@adriaanm said:
Please open separate issues for separate work items. The compiler part is fixed.

[SethTisue]

new PR to complete the deprecation and pave the way for eventual removal: scala/scala#6325