Re: More rabbit-holes with unset variables

[Felipe Contreras <felipe.contreras@gmail.com>]

On Fri, Nov 27, 2020 at 2:54 PM Bart Schaefer <schaefer@brasslantern.com> wrote:
>
> On Thu, Nov 26, 2020 at 5:30 PM Felipe Contreras
> <felipe.contreras@gmail.com> wrote:
> >
> > What zsh refers to as "scalar" internally is a string:
> >
> >   char *str; /* value if declared string  (PM_SCALAR)  */
> >
> > From Src/zsh.h (struct param).
>
> This is exactly the discussion I was trying to avoid when I said "in
> the abstract that doesn't matter".
>
> You can't just pull one field out of a union inside a struct and
> ignore the struct itself and the API for field access that goes with
> it.

I am not. I have been looking at the code and fixing some inconsistencies.

From what I can see PM_SCALAR is considered a string, and the thing
I'm putting more emphasis on is the comment: "value if declared
string".

> > So if you didn't mean string, what did you mean?
>
> I meant a struct param, containing the least specific thing so
> represented, as interpreted through all the layers of code that
> implement a dereference of its value when you write $var or any of its
> variations.  Again this doesn't actually matter, which is why I didn't
> spell it out.

The least specific thing--which happens when you do "static struct
param p"--sets the type to PM_SCALAR (0), which from what I can see
for all intents and purposes is a string.

> > And what did you mean by 'so a the only useful "declared but not set"
> > variable is a simple scalar'?
>
> As the very first message in this thread demonstrated, in both bash
> and ksh (call this "example one", and to be pedantic assume that X is
> not inheriting its name or value from somewhere):
>
> typeset -i X
> echo ${X-nil}
> X="garbage"
> echo ${X-nil}
>
> will output
>
> nil
> 0
>
> However (call this "example two"):
>
> typeset -i X
> unset X
> X="garbage"
> echo ${X-nil}
>
> outputs
>
> garbage
>
> The language you quoted from the posix proposal says "otherwise, the
> variable is initially unset".  Given that proposed language, example
> one is incorrect, because an "unset" variable should not retain its
> (in this example) integer properties when assigned a string.

I agree. I would say "imprecise" rather than "incorrect", since if all
these threads taught us anything is that it's not so clear to define
what we mean by "unset", "nil value", or "no value".

> > What simple scalar other than a string is useful "declared but not set"?
>
> Under this interpretation, there isn't any.  That's what I said.  In
> fact the last paragraph of the very first message in this thread:
>
> "Therefore, this isn't as simple as having zsh create an unset
> variable when typeset is given no assignment, because subsequent
> assignment has to preserve the type of the variable, which normally
> does not apply after unset."

So we can interpret what you said as 'so the only useful "declared but
not set" variable is a string' without losing any meaning.

Cheers.

-- 
Felipe Contreras