An alternative to "restricted keywords"

[Remi Forax]

I want to answer this before we start the meetings because i really think that restricted keyword as i propose solve the issues Stephan raised.


----- Mail original -----
> De: "Stephan Herrmann" <stephan.herrmann at berlin.de>
> À: jigsaw-dev at openjdk.java.net
> Envoyé: Mardi 16 Mai 2017 11:49:45
> Objet: Re: An alternative to "restricted keywords"

> Thanks, Remi, for taking this to the EG list.
> 
> Some collected responses:
> 
> 
> Remi: "from the user point of view, '^' looks like a hack"
> 
> This is, of course, a subjective statement. I don't share this view
> and in years of experience with Xtext-languages (where this concept
> is used by default) I never heard any user complain about this.
> 
> More importantly, I hold that such aesthetic considerations are of
> much lesser significance than the question, whether we can explain
> - unambiguously explain - the concept in a few simple sentences.
> Explaining must be possible at two levels: in a rigorous specification
> and in simple words for users of the language.

I'm not against ^, or ` as it has already asked to escape an identifier, but as you said it's a pervasive change that applies on the whole grammar while i think that with restricted keyword (that really should be called local keywords) the changes only impact the grammar that specifies a module-info.java

> 
> Remi: "a keyword which is activated if you are at a position in the
>  grammar where it can be recognized".
> 
> I don't think 'being at a position in the grammar' is a good way of
> explaining. Parsing doesn't generally have one position in a grammar,
> multiple productions can be active in the same parser state.
> Also speaking of a "loop" for modifiers seems to complicate matters
> more than necessary.
> 
> Under these considerations I still see '^' as the clearest of all
> solutions. Clear as a specification, simple to explain to users.

Eclipse uses a LR parser, for a LR parser, position == dotted production as i have written earlier, so no problem because it corresponds to only one parser state.  Note that even if one do not use an LR or a LL parser, most hand written parser i've seen, javac is one of them, also refers to dotted production in the comments of the corresponding methods.

> 
> 
> 
> Peter spoke about module names vs. package names.
> 
> I think we agree, that module names cannot use "module words",
> whereas package names should be expected to contain them.

yes, that the main issue, package names may contains unqualified name like 'transitive, ''with' or 'to'.
but i think people will also want to use existing package or more exactly prefix of existing package as module name, so we should also support having restricted keyword name as part of a module name.

The grammar is:

  open? module module_name {
    requires (transitive | static)* module_name;
    exports package_name;
    exports package_name to module_name1, module_name2;
    opens package_name;
    opens package_name to module_name1, module_name2;
    uses xxx;
    provides xxx with xxx, yyy;
  }

If we just consider package name, only 'opens' and 'exports' are followed by a package name and a package name can only been followed by ';' or 'to', so once 'opens' is parsed, you know that you can have only an identifier so if it's not an identifier by one of the restricted keywords, it should be considered as an identifier.

As i said earlier, the scanner can see the restricted keyword as keyword and before feeding the token to the parser, you can check the parser state to see if the keyword as to be lowered to an identifier or not.

For module name, there is the supplementary problem of transitive, because if a module starts with transitive, you can have a conflict. As i said earlier, instead of using the next token to know if transitive is the keyword or part of the module name, i think we should consider it as a keyword, as the JLS said a restricted keyword is activated when it can appear, so "requires transitive" is not a valid directive.

> 
> Remi: "you should use reverse DNS naming for package so no problem :)"
> 
> "to" is a "module word" and a TLD.
> I think we should be very careful in judging that a existing conflict
> is not a real problem. Better to clearly and rigorously avoid the
> conflict in the first place.

to as the first part of a package/module and to as in exports ... to can not be present on the same dotted production, because exports as to be followed by a package_name so 'to' here means the start of a package name and then because a package name can not ends with '.' you always know if you are inside the production recognizing the package_name or outside matching the to of the directive exports. 

> 
> 
> 
> Some additional notes from my side:
> 
> In the escape-approach, it may be prudent to technically allow
> escaping even words that are identifiers in Java 9, but could become
> keywords in a future version. This ensures that modules which need
> more escaping in Java 9+X can still be parsed in Java 9.

yes, that's why i think that escaping is not the right mechanism here, because we want to solve a very local problem so we do not need a global grammar-wise way to solve our problem.

> 
> 
> Current focus was on names of modules, packages and types.
> A complete solution must also give an answer for annotations on modules.
> Some possible solutions:
> a. Assume that annotations for modules are designed with modules in mind
>    and thus have to avoid any module words in their names.
> b. Support escaping also in annotations
> c. Refine the scope where "module words" are keywords, let it start only
>    when the word "module" or the group "open module" has been consumed.
>    This would make the words "module" and "open" special, as being
>    switch words, where we switch from one language to another.
>    (For this I previously coined the term "scoped keywords" [1])

For annotation, again, because annotation name are qualified, you now when you see 'module' if you are in the middle of the annotation name or if you are outside.

> 
> 
> I think we all agree that the conflicts we are solving here are rare
> corner cases. Most names do not contain module words. Still, from a
> conceptual and technical p.o.v. the solution must be bullet proof.
> But there's no need to be afraid of module declarations being spammed
> with dozens of '^' characters. Realistically, this will not happen.
> 

I agree, and i strongly believe that scoped keyword, local keywords or restricted keywords, i.e. whatever the name, keywords that are keywords or identifiers depending on the parser state are the general mechanism that solve our problem. 

> Stephan
> 
> [1] http://www.objectteams.org/def/1.3/sA.html#sA.0.1
> 

Rémi