TIP #215 Version 1.1: Make [incr] Auto-Initialize Undefined Variables

This is not necessarily the current version of this TIP.

Abstract

Unlike append and lappend, incr currently does not auto-create yet-undefined variables. This TIP proposes to make incr's behaviour in this regard more like the aforementioned commands.

Rationale

A quite common task is counting the number of occurrences of items in a given list. The usual solution is to iterate the list, and for each item, increment the associated value in a tcl-array. As of now this requires a separate step of determining the not-yet-existence and eventual initialization to 0 or alternatively catch'ing errors from incr and setting the variable, if an error was raised.

If we instead alter incr to treat non-existant variables as if they contained the value 0, this would be more like the auto-initializing behaviour of append and lappend, and would make writing code that does this sort of summing up much easier. It is also very similar to the way that the dict incr subcommand operates.

No Change for Variables that Contain Non-Integers

Just as lappend does complain if passed a variable whose value is an invalid list (e.g. a single open-brace), so it appears reasonable for incr to still throw an error if the variable contains something that is not a number.

The empty string is invalid as an operand for expr's integer operators, so it should remain illegal to incr an existing variable that contains an empty string.

Further Special Cases

If a variable passed to incr is not yet existing, but linked to some other not-yet existing var, or if it is traced, then of course it would add flesh to that existing husk. Care should be taken that any write traces only trigger once (like for lappend), not twice (as in: for initializing and then for incrementing).

Copyright

This document has been placed in the public domain.

This is not necessarily the current version of this TIP.