TIP #212 Version 1.1: Temporarily Opening out a Dictionary

This is not necessarily the current version of this TIP.

Abstract

This TIP proposes a new subcommand of dict that associates variables with dictionary entries while a Tcl script (the "body" of the command) runs.

Rationale

The standard set of operations provided by the dict command (see TIP #111 and TIP #163) give a fairly rich set of operations, but there are always further things that script authors want to do. Instead of ever-expanding numbers of subcommands offering tiny variants on existing operations, this TIP provides a generalized mechanism for associating variables and dictionary entries for the duration of a script. This makes writing arbitrary complex manipulations of dictionaries much simpler.

Proposed Change

This TIP proposes a new subcommand to dict with the following syntax:

dict update dictVarName dictKey varName ?dictKey varName ...? bodyScript

In prose, the first argument is the name of a variable containing a dictionary that will be updated, the last argument is a script to be executed with the variables set up and whose effects are expected to be to update the associated variables, and the other arguments between those two (of which there must be an even number, with a minimum of two) are a list of dictionary keys and the names of variables with which to associate those keys.

The semantics of the command are this:

1. For each dictionary key (dictKey), the value from the dictionary contained in the variable named by dictVarName which that key maps to will be written to the variable associated with that key (varName). If the key is not present in the dictionary, the variable will be unset.

2. The script (bodyScript) is executed.

3. The dictionary is read out of dictVarName again and, for each dictKey, the varName variable is consulted and the contained value placed in the dictionary (if varName is unset, any existing mapping for dictKey will be removed.) Finally, the resulting dictionary value is written back to dictVarName.

Note that dictVarName is read twice (each time, the value read must be a valid dictionary or the variable must be unset, which is interpreted as an empty dictionary) and written once, and that no traces are set up; dictVarName is completely independent of the varNames during the execution of bodyScript.

Examples

A somewhat-less-efficient version of dict set:

 dict update someVar $key local {
     set local $value
 }

Sorting a list inside a nested dictionary:

 dict update someVar OuterKey temp {
     dict update temp InnerKey temp2 {
         set temp2 [lsort $temp2]
     }
 }

"Renaming" a key:

 dict update someVar oldKey foo newKey bar {
     set bar $foo
     unset foo
 }

Reference Implementation

Not yet available.

Copyright

This document has been placed in the public domain.

This is not necessarily the current version of this TIP.