TIP #77 Version 1.3: Support for Nested Paired Item Lists

This is not necessarily the current version of this TIP.

Abstract

Tcl arrays can be transformed to and from lists using the array get and array set commands. This TIP proposes a new command for working directly these paired lists, and extending them to allow nesting in a manner analogous to TIP #22.

Rationale

Tcl lists provide only ordinal access to their items; often it makes more sense to access items by pre-assigned descriptive names. This can be easily accomplished with Tcl arrays. Consider these alternatives:

  set urlList { http tcl.activestate.com 80 /index.html }

  array set urlArray {
       proto   http
       host    tcl.activestate.com
       port    80
       uri     /index.html
  }

Clearly the array approach promotes more readable code ($urlArray(host) versus [lindex $urlList 1]).

However, it's quite unwieldy and sometimes expensive to use arrays to access members of many sets of structured data, particularly when that data contains nested structures.

Consider this structured data:

  set data {
       text    {ignored-data}
       valid-styles {
               justification {left centered right full}
               font          {courier helvetica times}
       }
  }

Extracting items from structures like this can be accomplished by multiple array set commands:

  array set dataArray $data
  array set validStylesArray $dataArray(valid-styles)
  puts "Justification: $validStylesArray(justification)"

To modify an item in struct, we need some pretty ugly code:

  array set dataArray $struct
  array set validStylesArray $dataArray(valid-styles)
  set validStylesArray(justification) {left}
  set dataArray(valid-styles) [array get validStylesArray]
  set data [array get dataArray]

Clearly, all this setting and getting of arrays imposes a rather high overhead; many variables are created and moved around. Also, if this is occurring in a loop, then care must be taken to unset the dataArray and vali