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Re: literals and keywords

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  Re: literals and keywords markspace <markspace@nospam.nospam> - 2013-12-28 08:59 -0800
    Re: literals and keywords "Chris Uppal" <chris.uppal@metagnostic.REMOVE-THIS.org> - 2013-12-28 17:28 +0000
    Re: literals and keywords markspace <markspace@nospam.nospam> - 2013-12-28 10:56 -0800
      Re: literals and keywords Arne Vajhøj <arne@vajhoej.dk> - 2013-12-28 14:53 -0500
        Re: literals and keywords markspace <markspace@nospam.nospam> - 2013-12-28 12:41 -0800
          Re: literals and keywords Arne Vajhøj <arne@vajhoej.dk> - 2013-12-28 16:36 -0500

#27096 — Re: literals and keywords

On 12/28/2013 8:15 AM, Stefan Ram wrote:
>    Does it matter?
>
>    Can you find any program, where one can observe the difference?
>    E.g., some program that does compile because they are not keywords,
>    but would not compile when they were keywords, or vice versa?
>    Or are there any other observable consequences of this difference?
>

Yes, it matters, or they wouldn't make a distinction.  We talked about 
this already:  literals can be used in expressions, keywords can't.  The 
observable difference is in the implementation of the JLS.

<http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.8>

You can write "int i = 1 + 2;" but you can't write "Object o = for + 
try;" because that section of the JLS says so.  Read it.

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#27098

Stefan Ram wrote:
>  Just as
>  the keyword »this« names a value, the keyword »true« would
>  name a value. So, when they have decided to call »this« a
>  keyword, but not »true«, does it matter? Is there any point
>  where it helps to know that »this« is a keyword, but »true«
>  is not?

I think the distinction is of more use to compiler (and other tool) creators[*] 
than to common-or-garden programmers.

The same can be said of the whole of the spec.

    -- chris

[*] And, of course, to pedant-class language specialists. 

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#27100

On 12/28/2013 9:17 AM, Stefan Ram wrote:
> markspace <markspace@nospam.nospam> writes:
>> this already:  literals can be used in expressions, keywords can't.
>
>    It is not true that keywords can't be used in expressions,
>    for example, the expression »this« contains the keyword »this«.
>

Yes, and there are two other keywords mentioned in that section that get 
used in expressions, if you read it.

>    Keywords can introduce statements, as in the case of »if«,

This is rather untrue.  if takes an expression as an argument because 
the spec says there must be an expression there.  These things aren't 
founded out some inherent principle of ur-computing, they're arbitrarily 
dictated by the the spec.  That's it.

>    or they can name types, as in the case of »int«, or

"int" when it introduces an expression is called a "declaration", not an 
expression.  "(int)" as a cast is a special type of expression.  Both of 
those constructs are dictatated by the spec.

>    references as in the case of »this«. It would have been
>    possible to make »true« a keyword in Java as well. Just as

But it doesn't make sense.  When you look at discreet math, the concept 
of a literal value is already present.  "true" and "false" are the 
literal values that boolean values can have in mathematics.

When the spec says that true and false are literals, it's trying to 
mimic the natural language way of expressing that concept, what already 
exists in the math.  What you seem to be saying to me is contorted and 
unnatural.  Just read that part of the spec the way it's written, and 
understand it naturally.

Why exactly "this" and "new" and "super" aren't literals, I don't know. 
  Possibly because their value isn't fixed, but depends on context, and 
therefore isn't actually a single literal value.  They represent a range 
of values, and the language implementers didn't want to call them 
operators or variables or introduce a new concept like "special 
function," so they just called them a keyword because they had the 
concept already and it worked well enough.

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#27101

On 12/28/2013 1:56 PM, markspace wrote:
> Why exactly "this" and "new" and "super" aren't literals, I don't know.
>   Possibly because their value isn't fixed, but depends on context, and
> therefore isn't actually a single literal value.  They represent a range
> of values, and the language implementers didn't want to call them
> operators or variables or introduce a new concept like "special
> function," so they just called them a keyword because they had the
> concept already and it worked well enough.

this and super are not constant so they could never be called a literal.

And new is an operator. More like instanceof.

Arne

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#27102

On 12/28/2013 11:53 AM, Arne Vajhøj wrote:

> And new is an operator.

Is it?  That section I linked to refers to "new" as a keyword.

<http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.9>

Five paragraphs after that link.

"...the type arguments to the constructor immediately follow the keyword 
_new_."

And a couple of other places too.

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#27103

On 12/28/2013 3:41 PM, markspace wrote:
> On 12/28/2013 11:53 AM, Arne Vajhøj wrote:
>
>> And new is an operator.
>
> Is it?  That section I linked to refers to "new" as a keyword.
>
> <http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.9>
>
> Five paragraphs after that link.
>
> "...the type arguments to the constructor immediately follow the keyword
> _new_."
>
> And a couple of other places too.

Operators that are words and not special characters are also keywords.

But a quick check reveals that you are absolutely right - JLS do not
call new an operator even though it does call instanceof an operator.

That surprises me. In C++ and C# new is considered an operator.

And I do not seem to be the only one surprised. The Java tutorial
actually call it an operator.

Arne

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