TIP #296 Version 1.2: Enhanced Syntax for Pair-Wise Indices

This is not necessarily the current version of this TIP.

Abstract

This TIP proposes adding another anchor s (for usage exclusively in end-indices) that refers to the respective start-index.

Rationale

Currently, most commands that accept an index also accept the local keyword end as well as simple arithmetics involving this keyword.

Commands that accept a pair of indices for a range, require an explicit start and (inclusive)end index for the requested subrange.

In other languages (C++/stl, java, ...) it appears to be more common to specify ranges by start and length. Since the length has a higher tendency of being constant, this seems actually the better idea, anyway. For compatibility reasons, we just cannot simply change all the commands to take offset/length pairs rather than start/end.

However we could add a new anchor s to be used in the end-index, and allow simple arithmetic on it just like on the other index forms.

Specifying a subrange of length 1 (which makes more sense e.g. with lreplace than with lrange), one would then simply specify s as end-index. For a pair, "s+1" as end index does it.

If the start position of a subrange is obtained as result of a command, currently one typically does in tcl:

 set offs [...]
 ... [lrange $list $offs [expr {$offs+1}]] ...

A previous TIP has added support to do the following instead:

 set offs [...]
 ... [lrange $list $offs $offs+1] ...

This TIP aims at making this the new idiom:

 ... [lrange $list [...] s+1]

Proposed Change

Currently there is a function TclGetIntForIndex, which deals with the current end anchor. It could be accompanied with a second function (e.g., TclGetIntForIndex2) that would accept another argument for the previously calculated index of the range-start argument, and use that in case the new anchow s occurs in the range-end index.

No change at all is proposed for each ranges start index.

Draft Implementation

None, at the moment.

Further Thoughts

Eventually, an empty string could be made equivalent to s.

For lreplace, s-1 as an end-index would actually make it an linsert operation.

This TIP covers all tcl commands that accept a pair of indices to specify a range, including (but not limited to) lrange, lreplace, string range, plus any third-party command that uses Tcl's index parsing functions.

Copyright

This document has been placed in the public domain.

This is not necessarily the current version of this TIP.