POV-Ray : Newsgroups : povray.newusers : Ignorance rules! : Re: Ignorance rules! Server Time
10 Jun 2023 07:58:15 EDT (-0400)
  Re: Ignorance rules!  
From: Bald Eagle
Date: 27 Jun 2020 11:10:01
Message: <web.5ef76041ea0d75eafb0b41570@news.povray.org>
Cousin Ricky <ric### [at] yahoocom> wrote:
> One just has to keep in mind POV-Ray's scope leakage

> [2] If you have formal computer science education, you are mortified by
> these restrictions, wondering how people smart enough to create POV-Ray
> could have allowed such boneheaded violations of modular practice.  I am
> assured that the current development team is aware of such problems.
> Yes, I will keep complaining about the scope leakage until it is fixed.

This is an area where I've harbored suspicions and trepidation, due to
subconscious knowledge gathered from reading the newsgroups and docs over the
past years.

Jr and I have recently been working on some code, and while implementing /
rewriting a macro, some recursion was introduced, and a variable identifier was
passed back into the macro as an argument.  Debugging hilarity ensued.

If I understand it all correctly:
When you #declare variable N to have some value, and then use N as the
"temporary" variable name of what gets passed into the macro, and then use
#local to declare N's new in-macro / in-loop value ....  the global / higher
level value of N gets redeclared.
So apparently "POV SDL apparently makes no distinction between passing-by-value
and passing-by-reference, as other languages do."

Or maybe I have that wrong somehow - because reading the docs seems to
contradict that assertion: Identifier Name Collisions

Perhaps we can have some more educated and astute (than I am) commentary on
This goes back to a point I've made many times.   Documentation is fine - but
code examples give you something that is undeniable and that you have far less
likelihood of misunderstanding.

THIS works (as expected intended):

THIS does not: (because...)

THESE are a few tricky examples....:

Now here's the interesting part - which I had NO IDEA was even a thing:
"turns out, there's a (documented *) way to force POV-Ray to always treat a
macro argument as pass-by-value.  Add a '+' in front of the identifier name and
voila, POV-Ray is not "not allowed" to pass-by-reference because we're not using
an identifier, but an expression.

will leave N alone."

* "Consider these examples.

  #declare Value=5.0;
  MyMacro(Value)     //MyMacro can change the value of Value but...
  MyMacro(+Value)    //This version and the rest are not lone
  MyMacro(Value+0.0) // identifiers. They are float expressions
  MyMacro(Value*1.0) // which cannot be changed.
Although all four invocations of MyMacro are passed the value 5.0, only the
first may modify the value of the identifier."

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