Using directive and namespace aliasing with Ksh

Question

Can I create an alias for a korn shell namespace? Is there anything like the using directive in C++ (bring several namespaces together and throw a (compiler) error if a symbol designation is ambiguous)?

Answer 1

Even though ksh has more advanced feature than other shells, it's still at its heart a language to automate simple task and write small scripts, not a language geared towards writing huge applications. You can't expect to have all the features of a typical language designed for large applications, let alone all the features of C++.

I don't think you can create an alias for a namespace. You can create a copy of a namespace of sorts by enumerating its variables (parse the output of typeset +p or typeset -p, or use ${!.foo.*} if you want to include variables inherited from the parent namespace) and copying them one by one. You can make a variable an alias of another with typeset -n, but that only takes effect when used inside the namespace:

$ namespace foo { x=1; }
$ namespace bar { typeset -n x=.foo.x; echo $x; }
1
$ namespace foo { x=2; }
$ namespace bar { echo $x; }
2
$ echo ${.bar.x}
.foo.x

Ksh lets you go further and execute arbitrary code when a variable is read, modified or unset, with discipline functions. For example, if you define a function called x.get, this function is invoked each time the value of x is retrieved.

namespace bar {
  x.get () { _=${.foo.x}; }
  x.set () { .foo.x=${.sh.value}; }
  x.append () { .foo.x+=${.sh.value}; }
  x.unset () { unset ${.sh.name}; }
}

I think you have to define these for every variable, and you can't detect variables that are added to the namespace foo.

This can be extended to look up multiple namespaces.

x.get () {
  if ((${.foo.x+1}+${.bar.x+1}!=1)); then
    echo 1>&2 "Ambiguous variable x, available in more than one of foo and bar"
    exit 125
  fi
  _=${.foo.x-${.bar.x}}
}