TIP #250 Version 1.1: Efficient Access to Namespace Variables

This is not necessarily the current version of this TIP.

Abstract

This TIP proposes a new namespace subcommand, namespace upvar, to efficiently alias namespace variables into the current scope.

Rationale

A pure-Tcl object system which defines a namespace to contain the variables for each object instance must either duplicate the object's method code in each instance namespace, or define the method code such that it exists in one namespace but accesses data from another. The Snit package [1] does the latter. Instance variables are declared automatically within each method body using code like this, where "selfns" is a variable containing the name of the instance namespace:

 upvar ${selfns}::myvar myvar

The fully-qualified variable name, "${selfns}::myvar", must be recomputed each time the method is called, which is a significant source of method-call overhead. This TIP proposes a mechanism for avoiding many of these costs while also allowing people to write clearer code.

With namespace upvar, the code would look like this:

 namespace upvar $selfns myvar myvar

In this case, both "$selfns" and "myvar" can be resident in the strings table, allowing for better byte-compilation and improved efficiency.

Specification

The syntax of the new subcommand is as follows:

namespace upvar ns otherVar myVar ?otherVar myVar ...?

The semantics are identical to the following upvar call:

upvar ns::otherVar myVar ?ns::otherVar myVar...?

That is, the variable otherVar in namespace ns is aliased to variable myVar in the local scope.

Reference Implementation

A reference implementation of namespace upvar is to be provided by Miguel Sofer.

Copyright

This document has been placed in the public domain.

This is not necessarily the current version of this TIP.