PROPOSAL: Language Escape Operator

[brucechapman at paradise.net.nz]

Title Language Escape Operator

latest html version at http://docs.google.com/Doc?docid=dcvp3mkv_9gdg3jfhg

AUTHOR(S): Bruce Chapman

OVERVIEW


FEATURE SUMMARY: 

There is great potential for syntactic sugars to be implemented in tools rather
than the Java language. One requirement of such a mechanism is that the sugared
form is NOT valid java source code (otherwise the meaning is ambiguous - is a
piece of code intended as is, or intended to be desugared). In order to future
proof such mechanisms it is also desirable that such a syntactic sugar form will
never become a valid java form due to some unanticipated future language change.


A simple way to ensure this is to reserve a character as a language escape,
effectively it indicates that source code is at that point escaping from the
java language.


Such a character should be a printable ascii character to enable it to be typed
and read, and it should have no existing defined meaning in the java language
within the contexts in which it is to be used.


Inside commments, String literals and character literals all printable ascii
characters have meaning and therefore there is no way we can escape from these.
This is not thought to be an overly burdensome restriction.


The candidate characters for the escape operator are therefore # (hash), \
(backslash) and ` (back quote). Neither # or ` have any currently defined
meaning in the Java language specification. \  does have defined meanings but
only as a unicode escape and inside String and character literals and so is also
suitable.


The intention of this proposal is to reserve one of these three characters for a
range of uses that are outside the language. Although the syntactic sugar
example is mentioned above it is but one example. A crystal ball would be
advantageous here - alas we are limited to our ability to dream.


Several proposals and ideas are circulating which suggest uses for each of these
few remaining characters which are becoming a precious resource for language
developers due to their scarcity. It is the author's belief that use of any one
of these characters as a language escape operator offers far more long term
value than use for a single new syntactic form. Let us reserve one of the three
characters as a language escape and let all other language proposals squabble
over the remaining two.


For the rest of this proposal it is assumed the back-quote character is the
language escape operator. Either of the three characteres could be used equally
from a technical point of view however aesthetic considerations would probably
place # as least desirable.


MAJOR ADVANTAGE:
Shuts the gate before the horse has bolted. Once all these characters are
assigned meaning by the language, then there is no single character that can be
used as a language escape.


MAJOR DISADVANTAGE: 

Prevents the back-quote (GRAVE-ACCENT) character from being used in other
language features.

ALTERNATIVES: 

Alternative characters have been discussed above. Another alternative is to do
nothing and let language tools develop further (using any of the three available
characters) in order to get a slightly better idea of where this might be
heading before committing to a language change. The risk with this approach is
that all three characters might get defined uses by other language changes
before one can be reserved as a language escape. At that point it would be too
late and would restrict language tools.

EXAMPLES
SIMPLE EXAMPLE:
`Property String name;

is not valid Java source code, and can be automatically desugared by an IDE. See
references section.


ADVANCED EXAMPLE:


N/A



DETAILS
SPECIFICATION:


Add new section 3.13 after 3.12 operators.


3.12 Language Escape

The Ascii back quote ` character (unicode GRAVE ACCENT) is reserved for use by
tools that process mixed language content to indicate to their parsers while
parsing java source code, the start of something that is not Java source code.
It is the responsibility of such tools to pass only java source to the java
compiler.


It is a compile time error for a ` character to appear in other than a comment,
a String literal or a Character literal.

COMPILATION:
Generate the error if the ` character is encountered in other than a comment,
String or Character literal.

While the protoype does this, a more specific error message might be useful.

TESTING: 

Test for the compiler error.

LIBRARY SUPPORT: 

No library support required.

REFLECTIVE APIS:

N/A

OTHER CHANGES:

N/A

MIGRATION:

N/A

COMPATIBILITY
BREAKING CHANGES:

N/A

EXISTING PROGRAMS: 

N/A

REFERENCES


http://docs.google.com/Doc?docid=dcvp3mkv_8sn3ccbkk describes a working proof of
concept tool which uses ` as a language escape.


EXISTING BUGS:

?

URL FOR PROTOTYPE (optional):


JDK6's javac is the protoype implementation for this language change.

http://java.sun.com/javase/downloads