TIP 296: Enhanced Syntax for Pair-Wise Indices

Author:         Andreas Leitgeb <[email protected]>
State:          Draft
Type:           Project
Vote:           Pending
Created:        06-Nov-2006
Keywords:       Tcl,lrange,lreplace
Tcl-Version:    9.1


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


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, 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 (from the range-start argument), and use that in case the new anchor s occurs in the range-end index.

No change at all is proposed for each range's start index.

Draft Implementation

None, at the moment.

For a sketch see previous section. In bytecode, it could be done such, that if an s-prefix is seen for an (eventually bytecoded range-related command), then the current stack-top (the corresponding start-index) is dup'ed and then incremented or decremented according to any offset (s+#, s-#).

Further Thoughts

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 written such as to use the proposed TclGetIntForIndex2 function for parsing end-indices.


This document has been placed in the public domain.