51362: Begin documentation for named references.

ChangeLog

@@ -1,5 +1,8 @@
 2023-02-12  Bart Schaefer  <schaefer@zsh.org>
 
+	* 51362: Doc/Zsh/builtins.yo, Doc/Zsh/expn.yo, Doc/Zsh/params.yo,
+	Doc/Zsh/mod_parameter.yo: Begin documentation for named references.
+
 	* 51361: Test/K01nameref.ztst, Test/V10private.ztst: Tests
 	for 51360.
 

Doc/Zsh/builtins.yo

@@ -2037,6 +2037,20 @@ To initialize a parameter var(param) to a command output and mark it readonly,
 use tt(typeset -r )var(param) or tt(readonly )var(param) after the parameter
 assignment statement.
 
+cindex(named reference)
+cindex(reference, named)
+The flag tt(-n) creates a em(named reference) to another parameter.
+The second parameter need not exist at the time the reference is
+created.  No other attribute flags may be used in conjunction with
+tt(-n).  The var(name) assigned-to may not be an array element nor use
+a subscript, but the var(value) assigned may be any valid parameter
+name syntax, even a subscripted array element (incuding an associative
+array element) or an array slice, which is evaluated when the named
+reference is expanded.
+See ifzman(zmanref(zshexpn))ifnzman(noderef(Parameter Expansion)) and
+ifzman(zmanref(zshparam))ifnzman(noderef(Parameters)) for details of the
+behavior of named references.
+
 If no attribute flags are given, and either no var(name) arguments are
 present or the flag tt(+m) is used, then each parameter name printed is
 preceded by a list of the attributes of that parameter (tt(array),
@@ -2242,9 +2256,9 @@ automatically given the tt(-h) attribute to avoid name clashes.
 item(tt(-H))(
 Hide value: specifies that tt(typeset) will not display the value of the
 parameter when listing parameters; the display for such parameters is
-always as if the `tt(PLUS())' flag had been given.  Use of the parameter is
-in other respects normal, and the option does not apply if the parameter is
-specified by name, or by pattern with the tt(-m) option.  This is on by
+always as if the `tt(PLUS())' flag were given, but use of the parameter is
+in other respects normal.  This effect does not apply when the parameter is
+specified by name or by pattern with the tt(-m) option.  This is on by
 default for the parameters in the tt(zsh/parameter) and tt(zsh/mapfile)
 modules.  Note, however, that unlike the tt(-h) flag this is also useful
 for non-special parameters.

Doc/Zsh/expn.yo

@@ -1226,6 +1226,9 @@ for parameters with the `hide' flag (tt(-h))
 item(tt(hideval))(
 for parameters with the `hideval' flag (tt(-H))
 )
+item(tt(nameref))(
+for named references having an empty value (tt(-n))
+)
 item(tt(special))(
 for special parameters defined by the shell
 )
@@ -1523,6 +1526,77 @@ Include the unmatched portion in the result (the em(R)est).
 )
 enditem()
 
+subsect(Named References)
+cindex(named references)
+cindex(namerefs)
+cindex(reference variables)
+cindex(parameters, nameref)
+The command
+ifzman()
+indent(tt(typeset -n )var(pname)tt(=)var(rname))
+
+initializes a parameter var(pname) as a reference to a second
+parameter var(rname).  With the few exceptions described here, when
+var(pname) is used in any of the expansion forms described above, the
+parameter var(rname) is expanded instead.  This is similar to the
+action of the `tt((P))' expansion flag, but when var(rname) has itself
+been declared a named reference, that third parameter referenced by
+var(pname) is also expanded, and so on.  With `tt((P))' this must be
+done explicitly, so for example
+tt(${LPAR()P)tt(RPAR()${LPAR()P)tt(RPAR())var(name)tt(}}).
+
+Unlike `tt((P))', named references in substitutions that perform
+assignment, such as tt(${)var(pname)tt(::=)var(word)tt(}), do not
+create new arrays when var(rname) is in the form of an array element
+or slice and no such array (or associative array) is presently set.
+This includes arrays declared, but not initialized, when the option
+tt(TYPESET_TO_UNSET) is in effect.  The var(word) is substituted but
+no assignment occurs.
+
+Also unlike `tt((P))' named references always expand parameters at
+the scope in which var(rname) existed when `tt(typeset -n)' was
+called.  This can be used to expand or assign parameters from an
+earlier scope even if a local of the same name has been declared at
+a later scope.  Example:
+ifzman()
+example(tt(caller=OUTER)
+tt(func LPAR()RPAR() {)
+tt(  print before local: $caller)
+tt(  typeset -n outer=$1)
+tt(  local caller=INNER)
+tt(  print by reference: $outer)
+tt(  outer=RESULT)
+tt(})
+tt(func caller)
+tt(print after func: $caller))
+
+displays the output
+ifzman()
+example(tt(before local: OUTER)
+tt(by reference: OUTER)
+tt(after func: RESULT))
+
+When var(rname) includes an array subscript, the subscript expression
+is interpreted at the time tt(${)var(pname)tt(}) is expanded.  Any
+form of subscript is allowed, including those that select individual
+elements, substrings of scalar strings, or multiple elements as with
+array slices or the `tt((i))', `tt((I))', `tt((r))', `tt((R))' and
+`tt((w))' subscript flags.
+
+When var(rname) is an array (but not an array element or slice), the
+named reference may also be used in substitutions requiring an
+var(arrayname), so these are equivalent:
+ifzman()
+example(tt(${)var(name)tt(:|)var(rname)tt(})
+tt(${)var(name)tt(:|)var(pname)tt(}))
+
+Expansions of the form `tt(${LPAR()t)tt(RPAR())var(pname)tt(})' expand
+the type information of var(rname), unless var(rname) is empty, in which
+case `tt(nameref)' is expanded, or when no variable var(rname) exists,
+in which case the expansion is empty.
+
+See also ifzman(zmanref(zshparam))ifnzman(noderef(Parameters)).
+
 subsect(Rules)
 cindex(parameter expansion rules)
 cindex(rules, parameter expansion)
@@ -1545,12 +1619,16 @@ substitutions; the nested substitution will return either a scalar or an
 array as determined by the flags, possibly adjusted for quoting.  All the
 following steps take place where applicable at all levels of substitution.
 
-Note that, unless the `tt((P))' flag is present, the flags and any
+Note that, unless the `tt((P))' flag or a named reference is present,
+the flags and any
 subscripts apply directly to the value of the nested substitution; for
 example, the expansion tt(${${foo}}) behaves exactly the same as
-tt(${foo}).  When the `tt((P))' flag is present in a nested substitution,
+tt(${foo}).  When a named reference or the `tt((P))' flag is used in a
+nested substitution,
 the other substitution rules are applied to the value em(before) it is
 interpreted as a name, so tt(${${(P)foo}}) may differ from tt(${(P)foo}).
+When both a named reference and the `tt((P))' flag appear, the named
+reference is resolved before `tt((P))' is applied.
 
 At each nested level of substitution, the substituted words undergo all
 forms of single-word substitution (i.e. not filename generation), including

Doc/Zsh/mod_parameter.yo

@@ -125,7 +125,7 @@ zmanref(zshexpn)
 ifnzman(\
 noderef(Parameter Expansion)
 )\
-.  The value may also be `tt(undefined)' indicating a parameter that
+. The value may also be `tt(undefined)' indicating a parameter that
 may be autoloaded from a module but has not yet been referenced.
 Setting or unsetting keys in this array is not possible.
 )

Doc/Zsh/params.yo

@@ -80,6 +80,7 @@ startmenu()
 menu(Array Parameters)
 menu(Positional Parameters)
 menu(Local Parameters)
+menu(Named References)
 menu(Parameters Set By The Shell)
 menu(Parameters Used By The Shell)
 endmenu()
@@ -592,7 +593,7 @@ array assignment of the form `var(n)tt(=LPAR())var(value) ...tt(RPAR())' is
 allowed, and has the effect of shifting all the values at positions greater
 than var(n) by as many positions as necessary to accommodate the new values.
 
-texinode(Local Parameters)(Parameters Set By The Shell)(Positional Parameters)(Parameters)
+texinode(Local Parameters)(Named References)(Positional Parameters)(Parameters)
 sect(Local Parameters)
 Shell function executions delimit scopes for shell parameters.
 (Parameters are dynamically scoped.)  The tt(typeset) builtin, and its
@@ -626,6 +627,49 @@ find the programs in tt(/new/directory) inside a function.
 Note that the restriction in older versions of zsh that local parameters
 were never exported has been removed.
 
+cindex(named references)
+cindex(references, named)
+texinode(Named References)(Parameters Set By The Shell)(Local Parameters)(Parameters)
+sect(Named References)
+Zsh supports two different mechanisms for indirect parameter referencing:
+ifzman()
+example(tt(typeset )var(name)tt(=)var(rname)
+tt(print -r -- ${LPAR()P)tt(RPAR())var(name)tt(}))
+ifzman()
+example(tt(typeset -n )var(pname)tt(=)var(rname)
+tt(print -r -- ${)var(pname)tt(}))
+
+The `tt((P))' flag method is older and should be used when a script
+needs to be backwards-compatible.  This is described fully in
+ifzman(zmanref(zshexpn))ifnzman(noderef(Parameter Expansion)).
+
+When a em(named reference) is created with `tt(typeset -n)', all uses
+of var(pname) in assignments and expansions instead assign to or
+expand var(rname).  This also applies to `tt(unset )var(pname)' and to
+most subsequent uses of `tt(typeset)' with the exception of
+`tt(typeset +n)', so to remove a named reference it is necessary to
+use:
+ifzman()
+example(tt(typeset +n )var(pname)
+tt(unset )var(pname))
+
+When `tt(typeset -n )var(pname)tt(=)var(rname)' appears in a given
+(global or function) scope, `tt(${)var(pname)tt(})' refers to
+var(rname) in the scope of the tt(typeset) command.  This differs
+from ksh93 where the scope used is that of var(rname) even if the
+current var(rname) would be found in a surrounding scope.
+em(This is a misfeature.)
+
+An empty reference such as one of
+ifzman()
+example(tt(typeset -n )var(pname)
+tt(typeset -n )var(pname)tt(=)
+tt(typeset -n )var(pname)tt(=""))
+
+acts as a placeholder.  The first non-empty assignment to var(pname)
+initializes the reference, and subsequently any expansions of, or
+assignments to, var(pname) act on the referenced parameter.
+
 texinode(Parameters Set By The Shell)(Parameters Used By The Shell)(Local Parameters)(Parameters)
 sect(Parameters Set By The Shell)
 In the parameter lists that follow, the mark `<S>' indicates that the