TIP #295 Version 1.2: Enhance Arguments to lrange

This is not necessarily the current version of this TIP.

Abstract

This TIP proposes an enhancement to lrange that lets it take more than one start-end index pair.

Rationale

Sometimes you need to extract a non-continuous selection of elements from some list and build a new list from them. This requires the use of multiple calls to lrange and a surrounding concat to build the final list. However, since the following fails with a syntax error:

    lrange {a b c d e f}  1 3  5 5

It would seem a reasonable extension for such a usage of lrange to return "b c d f". This would also make following pattern of usage feasible:

 lassign [lrange $someList 0 2  10 12]  var0 var1 var2 var10 var11 var12

The index-pairs should not be required to be in ascending order (this might be difficult to determine when one index is end-relative), or even disjoint. Even:

 lrange {a b c d}  3 3  0 3  0 0

should be made legal and return "d a b c d a".

Proposed Change

Change the implementation of lrange to accept any odd number of arguments, with semantics (upon supply of a correct number of arguments) equivalent to:

 proc lrange-new {list args} {
    set result [list]
    foreach {start end} $args {
       lappend result {*}[lrange $list $start $end]
    }
    return $result
 }

Draft Implementation

Just the above mentioned procedure, at the moment.

Further Thoughts

Uwe Klein asks: "Should this support reverse ranges?"

I, the TIP author: "I don't want to touch this with this TIP"

To avoid compatibility issues, it would at least require some extra flags. I'd welcome a new TIP with a suggestion on how it can be done compatibly. As of this TIP, anything below s-1 would be equivalent to s-1 and denote an empty range.

Copyright

This document has been placed in the public domain.

This is not necessarily the current version of this TIP.