TIP #176 Version 1.2: Add String Index Values

This is not necessarily the current version of this TIP.

Abstract

This TIP proposes to add small functionality to the standard index (end-1) values acceptable by most Tcl commands.

Rationale

Most of Tcl's commands that accept an integer index of some kind also accept a string index in the format of end-N. This is an extremely useful feature which I hope to extend just slightly by allowing simple addition to the standard index forms to be done during index processing. The change is mostly just syntactic sugar, but it extends a functionality that is already there and does not change any other syntax.

Proposal

Under this TIP, the following would all be valid indexes (N and M are integers):

N

Standard numeric indexing style.

end-N

Standard end-relative indexing style.

M+N

New offset-to-known-point style.

end+N

New past-end style.

This allows for shorthand list and string operations like so:

set x [lsearch $list $elem]
set new [lrange $list $x-1 $x+1]

set x [string first $string "foo"]
set new [string range $string $x $x+5]

And so on...

Implementation

The code itself is just some simple changes to TclGetIntForIndex() in generic/tclUtil.c. Because the core commands in Tcl and Tk alredy use this private function for index values, the commands which already accept the standard end-N index will now accept the new indexes.

Tcl Patch 1165695 is a draft implementation.

Copyright

This document has been placed in the public domain.

This is not necessarily the current version of this TIP.