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Re: A solution to fix hidden references in reference chains

> +#define PM_UNSET       (1<<24) /* If PM_DECLARED is also present, parameter
> +                                 * has null value. Otherwise, parameter was
> +                                 * unset.
> +                                 */
>  #define PM_DEFAULTED (PM_DECLARED|PM_UNSET)

It's inaccurate to say the parameter has a null value.  There's no
such thing as a null value in shell.  That's why the following #define
calls it PM_DEFAULTED.  It means the parameter exists in a scope
(which, at least previous to this patch, also implied it has a known
type) but does not have a value (which is distinct from languages
where null/undef IS a value). This goes back to the discussion in
which PM_NULL was rejected, which as I previously said I don't want to
rehash.

The flag PM_UNSET has two meanings:

- When PM_DECLARED is absent, it indicates that the parameter was unset, typically by a call to the builtin "unset".
  - The parameter is no longer considered to exist but it's not fully deleted and still hides any parameter of the same in an enclosing scope.
  - Calls to "typeset -p" don't list it.
  - Expansions consider that the parameter isn't set.
  - Assignments revive the parameter (i.e., clear the PM_UNSET flag) but ignore all its previous attributes.

- When PM_DECLARED is present, it indicates that the parameter was declared, typically by a call to the builtin "typeset", but without any value to initialize it while the option TYPESET_TO_UNSET was enabled.
  - The parameter exists but it has no value.
  - Calls to "typeset -p" list it but without any value.
  - Expansions consider that the parameter isn't set. In this regard, the parameter can be seen as being unset.
  - Assignments initialize the parameter (and clear the PM_UNSET flag).

The distinction is important because there are cases where either only the first or only the second kind of PM_UNSET parameters are to be considered (see for example my patch workers/54261). I think that it's important to document that these two kinds exist. I initially assumed that PM_UNSET only covered the first kind.

I agree that "has a null value" is a bad way of describing the second kind. As you point out there is no null value in Zsh and even worse the term "non-null" is already used in the Zsh documentation to designate parameters whose value isn't the empty string or the empty array.

What about the following description?

#define PM_UNSET (1<<24) /* If PM_DECLARED is present, the parameter
                          * was declared but with no value. Otherwise,
                          * the parameter was undeclared/unset and no
                          * longer exists. In both cases, expansions
                          * consider the parameter as unset.
                          */

Philippe

diff --git a/Src/zsh.h b/Src/zsh.h
index dd58c0816..b017fdc09 100644
--- a/Src/zsh.h
+++ b/Src/zsh.h
@@ -1927,9 +1927,11 @@ struct tieddata {
 #define PM_DONTIMPORT	(1<<22)	/* do not import this variable              */
 #define PM_DECLARED	(1<<22) /* explicitly named with typeset            */
 #define PM_RESTRICTED	(1<<23) /* cannot be changed in restricted mode     */
-#define PM_UNSET	(1<<24)	/* If PM_DECLARED is also present, parameter
-				 * has null value. Otherwise, parameter was
-				 * unset.
+#define PM_UNSET	(1<<24)	/* If PM_DECLARED is present, the parameter
+				 * was declared but with no value. Otherwise,
+				 * the parameter was undeclared/unset and no
+				 * longer exists. In both cases, expansions
+				 * consider the parameter as unset.
 				 */
 #define PM_DEFAULTED	(PM_DECLARED|PM_UNSET)
 #define PM_REMOVABLE	(1<<25)	/* special can be removed from paramtab     */
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