TIP #296: Enhanced Syntax for Pair-Wise Indices
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TIP:	296
Title:	Enhanced Syntax for Pair-Wise Indices
Version:	$Revision: 1.7 $
Authors:	Andreas Leitgeb <avl@logic.at>
State:	Draft
Type:	Project
Tcl-Version:	8.7
Vote:	Pending
Created:	Mon Nov 06 00:00:00 UTC 2006
Keywords:	Tcl, lrange, lreplace
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, 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.
Copyright
This document has been placed in the public domain.
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