[Openmcl-devel] Compiler warnings

Daniel Weinreb dlw at itasoftware.com
Tue Oct 20 18:55:03 UTC 2009



Ron Garret wrote:
>
> Yes, but it is defensible behavior in light of what the spec says, 
> namely (section 3.1.2.1.1):
>
> "If a form is a symbol, then it is either a symbol macro or a variable."
>
> and
>
> "If a form is a symbol that is not a symbol macro, then it is the name 
> of a variable, .... There are three kinds of variables: lexical 
> variables, dynamic variables, and constant variables."
>
> Since a "naked" symbol that is not bound and has not been DEF...ed is 
> clearly not a lexical variable nor a constant variable, one could 
> reasonably conclude that it must be a dynamic variable.
I agree with all of this.  The only thing that (setq a 4)
at top level can possibly mean, if it's not considered
to be illegal, is to treat the symbol a as a dynamic variable.

But for it to automatically create a declaration, which will
cause future uses of that symbol to be treated as
dynamic variables even when inside a lexical scope
that defines a variable named by that symbol, is
a different thing, and isn't necessarily implied by
the spec.  So I don't agree that the CMUCL behavior
is the only reasonable thing to do.
>
> In fact, if one does not conclude this and simply does what CCL does 
> (which I agree is reasonable behavior) then one is outside the spec.  
> You have a thing that acts like a variable, but does not act like any 
> of the three kinds of variables that the spec defines.  (Hence my 
> claim earlier in this thread that a variable created this way is 
> neither lexical nor dynamic).
I'm not sure I'd interpret it that way.  The thing is a symbol,
and it acts like a dynamic variable when used in this way.

But the bottom line is that there's no entirely satisfactory
answer.
>
>
>> Note: Talking about Lisp "creating a variable" can be
>> confusing.  Symbols are created by the reader, at
>> read time.
>
> But symbols are not variables.
I know.  (I probably should have put a paragraph break
between those two sentences.)
>
>> Symbols can be used for many things.
>> Sometimes, symbols are interpreted as variables,
>> depending on the context in which they are seen.
>> There isn't quite anything that is best called "creating
>> a variable".
>
> No, that's not true.  When you create a binding or use DEFVAR or 
> DEFCONSTANT you are creating a variable.
Well, when you do a let on a symbol that isn't declared
special, then I'd say you're creating a lexical variable,
but if you do one on a symbol that is declared special,
I would not say that you're creating a variable.
OK, I guess you could say that defvar "creates
a variable" in a sense, even though it does not actually
create any Lisp object per se.  That is, the symbol foo
in (defvar foo 1) wasn't created by the defvar.

I guess it's like the phrase "he was created Earl of Gurnsey." :):)

The vocabulary just isn't nice and clear and straightforward,
which it really ought to be.

-- Dan


>   The problem is that there is no way to create a global variable 
> whose name is a symbol that is not pervasively declared special 
> without going beyond the behavior specific in the spec. But users want 
> that behavior.
>
>>
>> I hope this helps.
>
> Here is my attempt at cutting through this Gordian knot:
>
> http://rondam.blogspot.com/2009/08/global-variables-done-right.html
>
> rg
>



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