Hi
Recently I filed an issue [1] requesting to "Allow to run shebang script
with --no-syntax-extensions". This was closed as "Won't Fix" with "Can't
support because # is an extension." While this is true, I meant to say
"Allow to run shebang script without allowing any other syntax
extensions than shebang". Or more generally: allow fine-grained control
over syntax extensions.
Currently, there are only 2 switches that influence syntax extensions:
--no-syntax-extensions and -scripting. However, the nature of the
various extensions is totally disparate: allowing "for each" expressions
has nothing to do with allowing a shebang. Therefore, it should be
possible to enable/disable them individually. Additionally, there could
be a few groups defined: "-scripting" is already one such group, and
"-mozilla" could be another.
So I would like to be able to specify options such as:
--language=es5 --extensions=scripting,foreach
--language=es6 --extensions=shebang,mozilla
where the "--extensions" option implicitly disables all non-specified
extensions
These options should also be validated:
* warnings should be emitted when the specified language provides an
alternative for some enabled extension. For example, in ES6, "for of" is
available as a standard alternative for "for each".
* errors should be emitted when there's an ambiguity. For example, in
ES6, when "Back-quote exec expressions" are enabled, the statement:
var cmd = `format C:`;
is both ambiguous & potentially disastrous if the author meant it as a
template literal, while Nashorn interprets it as a command.
The above also demonstrates why more fine-grained control is required:
certain extensions are obsolete and/or cause ambiguities, while others
are extremely useful. Even the "Shell script style hash comments"
extension is too coarse-grained: it should be split into a "shebang" and
a "hash comment" extension, since there's no good reason to use hash
comments besides for specifying the shebang. In my opinion, it should be
possible to run a shebang script with something like "jjs
--extensions=shebang myscript.js"
Besides more fine-grained control, there should also be a way to specify
options inside the script, no matter how it's invoked. In my opinion,
this is very useful because the script author always knows best how a
script must be interpreted and which extensions were/weren't used.
For example, suppose I wrote the following shell script named "find.js":
//jjs --language=es5 --extensions=execbackquote
load("library.js")
print(`grep abc file`);
where library.js is a library written by someone else, in which template
literals are used (note that "--extensions=" has the same effect as
"--no-syntax-extensions"):
//jjs --language=es6 --extensions=
var cmd = `format C:`;
then by being able to specify the necessary parser directives within the
script, Nashorn would be able to correctly run "find.js" (i.e. without
executing the `format C:` inside library.js). Personally, I think any
parser directives such as language version and extensions should always
be embedded within the script, and I don't really see a need to specify
them on the command-line. As for implementation: this can be done either by:
* always interpreting the shebang. However, this is not
ECMAScript-compliant.
* providing a "pure ECMAScript" alternative. For example, if the script
starts with "//jjs ", then it's interpreted by Nashorn (as in the
example above). Personally, I prefer this approach.
In summary, I'd like 2 things:
* fine-grained control over extensions, and validation of the
combination "language + enabled extensions". This could be done by
introducing a new "--extensions" option, which takes a list of
extensions & disables all others.
* a way to specify options in pure ECMAScript (i.e. by interpreting a
comment in a specific format at the beginning of the script) inside the
script itself, whereby these options are used to load the remainder of
the script, regardless of any "outside-specified" options.
What's your opinion on this?
Kind regards,
Anthony
[1] https://bugs.openjdk.java.net/browse/JDK-8144139
