TIP #22 Version 1.1: Multiple Index Arguments to lindex

This is not necessarily the current version of this TIP.

Abstract

Obtaining access to elements of sublists in Tcl often requires nested calls to the lindex command. The indices are syntactically listed in most-nested to least-nested order, which is the reverse from other notations. In addition, the nesting of command substitution brackets further decreases readability. This proposal describes an extension to the lindex command that allows it to accept multiple index arguments, in least-nested to most-nested order, to automatically extract elements of sublists.

Rationale

The heterogeneous nature of Tcl lists allows them to be applied to a number of useful data structures. In particular, lists can contain elements that are, themselves, valid lists. In this document, these elements are referred to as sublists.

Extracting elements from sublists often requires nested calls to lindex. Consider, for example, the following Tcl script that prints the center element of a 3-by-3 matrix:

    set A {{1 2 3} {4 5 6} {7 8 9}}
    puts [lindex [lindex $A 2] 2]

When these calls are deeply nested - e.g., embedded in an expr arithmetic expression, having results extracted through lrange, etc. - the results are difficult to read:

# Print the sum of the center indices of two 3x3 matrices
set p [expr {[lindex [lindex $A 2] 2] + [lindex [lindex $A 2] 2]}]

# Get all but the last font in the following parsed structure:
set pstruct {text {ignored-data
                      { ... }
		       }
		       {valid-styles
			   {justifiction {left centered right full}}
			   {font {courier helvetica times}}
		       }
		 }
return [lrange [lindex [lindex [lindex $pstruct 1] 2] 2] 0 end-1]

Note that the list of indices in the latter example is listed in the reverse order of vector indices. In most other languages/domains, the last line might take on one of the following forms:

return list_range(pstruct[2][2][1], 0, end-1);

return