[type-annos-observers] Constraining type annotations to occur just before type

[Alex Buckley]

On 3/21/2013 10:21 AM, Michael Ernst wrote:
> I agree entirely that it is terrible style to mix type annotations and
> declaration annotations (the declaration annotations should always come
> first) and that it is terrible style to mix type annotations with modifiers
> (the modifiers should come first).  So, Alex's suggestion to change the
> advice is definitely good.  Alex, can you make that change?  It's outside
> the scope of the JSR 308 specification.

Since type annotations are the reason to change the advice about 
modifier order, the 308 spec should have a note giving the preferred 
layout of declaration annotations v. type annotations. I will then 
incorporate it into JLS sections which use modifiers.

> I consider it an infelicity in the Java grammar that (declaration)
> annotations are considered modifiers and are lumped in with them, and even
> worse that type annotations get lumped in with declaration annotations.
> Logically, there are types and modifiers, and a type consists of optional
> type annotations followed by a Java type.  This is how people should write
> them.

Carefully distinguishing declaration annotations from type annotations 
would be the heart of the new note :-)

> Now, the question is whether or not the Java language should strictly
> enforce this practice.
>
> I'm sympathetic to this change -- probably my position is between
> Srikanth's and Alex's.  I see the benefit of preventing confusing code
> rather than just advising programmers not to write it in the first place
> and then giving them rope to hang themselves.

I understand, but there are infinite ways to writing confusing code in 
Java, and none of them get their own compile-time error. Even 
potentially unsafe use of raw types - which can lead to unpredictable 
program termination! - only gets a compile-time warning. I think people 
are over-rotating towards the IDE context, where informative messages 
and warnings abound, rather than focusing on hard-edged rules of the 
language.

> Incidentally, the Annotation File Utilities (AFU) grammar has these
> productions:
...
> Elsewhere, the AFU grammar always uses the decl-annotation or
> type-annotation non-terminal, rather than just using the "annotation"
> non-terminal.  The AFU grammar would accept exactly the same annotation
> files if it did not contain the decl-annotation and type-annotation
> non-terminals, but those clarify the meaning.
>
> We could imagine doing a similar thing with the JLS grammar, though the
> required changes would be more extensive.  Modifiers would include
> declaration annotations but not type annotations, which would be rolled
> into types.  I suspect that the reason I didn't do this in the first place
> was because I was trying to make the changes to the JLS grammar as minimal
> as possible to clarify what was actually changing.  We've since moved away
> from that position (and even if we hadn't, what's good for a diff isn't
> necessarily good for a final version).

As Jon implied in [1], the grammar would have to be relaxed (use only 
the Annotation non-terminal) because parsers cannot in general do the 
semantic analysis required to distinguish decl. from type annotations. 
It would have to be a higher-level complaint - either a standard 
compile-time error or a compiler-specific lint warning - if a decl. 
annotation is used where a type annotation is required, and vice-versa.

However, we cannot say that decl. annotations must come before 
traditional modifiers (private/public/static/final etc) because it has 
been legal since Java SE 5.0 to mix them up. The only rule which could 
possibly be added is "type annotations are not modifiers, and occur 
after all modifiers (which include declaration annotations)". I 
personally remain to be convinced that this rule is necessary.

Alex

[1] 
http://mail.openjdk.java.net/pipermail/type-annotations-spec-observers/2013-March/000103.html