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 pstruct[[2, 2, 1]][[0:-1]]

temp = pstruct(2, 2, 1);
result = range(temp, 0, length(temp) - 1);

Allowing the lindex command to accept multiple arguments would allow this more-natural style of coding to be written in Tcl.

Specification

1. Allow lindex to accept an arbitrary number of arguments.

   1. The first argument (the list argument) must be a proper Tcl list. No change is required from current behaviour.

   2. The remaining arguments (the index arguments) must be proper list indices (either integer, end, or end-integer).

2. When only one index argument is given, the behaviour is unchanged from the current lindex command.

3. When multiple index arguments are given, the behaviour is defined recursively as:

   lindex alist i0 i1 i2 ... === lindex [lindex alist i0] i1 i2 ...
   

   Note that this does not define any restrictions on the implementation, which may be recursive or iterative.

4. When an invalid index is given, an error of the form, bad index "invalid_index": must be integer or end?-integer?, where invalid_index is the first invalid index encountered, must be returned.

5. If the list argument is malformed, the error resulting from an attempt to convert the list argument to a list must be returned. This behaviour is unchanged from the current implementation.

Side Effects

1. Whether the result of the lindex operation is successful, the underlying Tcl_Obj that represents the list argument may have its internal representation invalidated or changed to that of a list.

Copyright

This document has been placed in the public domain.

This is not necessarily the current version of this TIP.