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Re: Operator precedence [message #185028 is a reply to message #185017] Mon, 24 February 2014 21:23 Go to previous messageGo to previous message
Ben Bacarisse is currently offline  Ben Bacarisse
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Registered: November 2013
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Jerry Stuckle <jstucklex(at)attglobal(dot)net> writes:

> On 2/23/2014 11:49 PM, Ben Bacarisse wrote:
>> Jerry Stuckle <jstucklex(at)attglobal(dot)net> writes:
>>
>>> On 2/23/2014 6:36 PM, Richard Damon wrote:
>>>> On 2/23/14, 4:34 PM, Jerry Stuckle wrote:
>>>> > On 2/23/2014 2:42 PM, GS wrote:
>>>> >> This shows one of the major problems with PHP: there is no formal
>>>> >> language
>>>> >> definition, you have to go by examples in the help file to intuit what is
>>>> >> or is not valid. Where is the BNF definition of the language? There is
>>>> >> none
>>>> >> AFAICT.
>>>> >>
>>>> > Incorrect. There is a formal definition, as indicated in the doc at
>>>> > www.php.net.
>>>> >
>>>> Do you have a pointer to where "expression" is precisely defined (other
>>>> than reading code).
>>>
>>> Try any Introduction to Programming book. Documentation (not just
>>> PHP) does not normally define commonly used terms.
>>
>> I don't think it's that clear cut. Expressions are built from
>> operators, but is ',' an operator in PHP? It appears in the operator
>> precedence table but it isn't an operator in an sense that would
>> normally be understood by someone familiar with these terms.
>
> It is an operator, as indicated by its appearance in the table of
> operators. Operators are not limited to +-*/.

I am familiar with languages that have a ',' operator, which is why I
brought it up. Normally, combining expressions with an operator makes a
new expression, but that's not true in PHP for the comma. For example,
f() and g() are expressions, but (f(), g()) is not. That's why,
presumably, there is no description of what it does in the section on
operators -- it isn't one!

<snip>
>>>> The lack of formal rigor says that there are some questions which just
>>>> can't be answered by reading the language documentation.
>>>
>>> It can if you have a basic understanding of programming.
>>
>> Again, I don't think it's that simple. How would you determine if the
>> following program is permitted and, if it is, what it does?
>>
>> if (false) label: else echo "else\n";
>> goto label;
>>
>> You could try it, of course, but that's hardly very formal.
>
> What language gives every possible way for you to screw up your
> programming?

You seemed to be disagreeing with the statement that there are some
questions which just can't be answered by reading the language
documentation. All I was doing was giving an example of a question I
could not answer from the documentation.

I agree it's very much a corner case (and many not worth an answer) but
there are other such questions. For example, from the documentation
alone I can't explain why ($i++, $j++) is not a valid expression, given
that it appears to consist of brackets and operators and variables, all
put together in the usual way.

> If you want to find out how some screwed-up code works in ANY
> language, you try it and find out.

That's a flawed method. If I have an algorithm that shifts bits about,
but sometimes shifts them *all* out, how can I be sure what

1 << PHP_INT_SIZE*8

really does? If I test it, and it does what I want, can I be sure that
on some other hardware it will do the same? Does PHP define the
meaning of this expression, or does it leave it open (and possibly up to
what the hardware does)?

<snip>
>> The assignment operators in PHP have, in effect, different precedences
>> on the left and the right (a feature that some languages flaunt) but
>> that is not common and certainly not part of what I'd call basic
>> knowledge. The documentation gives one example of this peculiarity, but
>> that does not clear the matter up conclusively. For example, in some
>> languages permit assignment to the result of a conditional expression,
>> but I think it's hard to tell from the documentation what these two do:
>>
>> $bool ? $a : $b = 42;
>> ($bool ? $a : $b) = 42;
>
> Show me what language provides you with examples of every possible
> combination of operators and expressions.

I never suggested otherwise. However, in more rigorously defined
languages you can find out what combinations are permitted from the
specification. With PHP, I often end up just trying things because I
can't find the answer in the documentation. I then assume that what
happens now is what will always happen -- i.e. that the reference
implementation is the specification.

>> The operator precedence table suggests that both mean the same thing,
>> but they don't. I don't think the meaning of either is easily
>> determined from the documentation.
>>
>> <snip>
>>
> So you try it out, as you do in any language. But just because you
> can come up with some weird combination of operators which isn't
> listed in the doc does not mean the language is not rigorously
> defined.

I think that's exactly what it means.

> But it does mean you are trolling.

Eh? This whole thread is an interesting technical discussion about the
degree to which PHP programs are well-defined. If you really think my
contributions are trolling, you know the well-worn advice: don't feed
me.

--
Ben.

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