TIP #342: Dict Get With Default
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TIP:	342
Title:	Dict Get With Default
Version:	$Revision: 1.3 $
Authors:	Lars Hellström <Lars.Hellstrom@residenset.net>
State:	Draft
Type:	Project
Tcl-Version:	8.7
Vote:	Pending
Created:	Thu Nov 27 00:00:00 UTC 2008
Keywords:	dictionary, default value
Abstract
A new subcommand of dict is proposed, which returns a dictionary value if it exists and returns a per-call default otherwise.
Specification
The dict command will get a new subcommand
dict getwithdefault dictionary key ?key ...? value
(I consider the name of this subcommand very much open for discussion) which modulo error messages behaves like
 proc dict_getwithdefault D args 
     if [dict exists $D *[lrange $args 0 end-1]] then 
         dict get $D *[lrange $args 0 end-1]
      else 
         lindex $args end

i.e., it returns the value from the dictionary corresponding to the sequence of keys if it exists, or the default value otherwise. As with dict exists, it is OK (and will cause the default value to be returned) if one of the keys is missing from its dictionary, but an error is thrown if this path of keys cannot be traversed because the value associated with the previous key is not a dictionary.
Rationale
It is clear that getting a value from a dictionary if it exists and using a default otherwise is a common operation, but it is also clear that this can be carried out with a combination of existing Tcl commands. Hence the issue is whether a new subcommand for this improves efficiency and convenience of this operation enough to justify the possible bloat it brings.
Alternative Methods
One approach that has been suggested for providing default values is to combine dict get with dict merge, like so:
  dict get [dict merge -apa bar $D] -apa
This approach is however appropriate mainly in situations where several keys are given fixed defaults simultaneously. Compared to dict getwithdefault, it has the following disadvantages:
It cannot be used for keys in nested dictionaries.
It takes time proportional to the size of the dictionary, even when only one value is inspected. Since dict filter key has an optimisation for this kind of situation, there are apparently maintainers which consider such differences relevant.
The "one dict merge early providing defaults for all keys" approach cannot deal with keys that have dynamic defaults, e.g. that the default for option -foo is the effective value of option -bar.
Hence although dict merge is sometimes appropriate for providing defaults, it is not a universal solution.
The basic approach is instead to, as in the dict_getwithdefault proc above, first use dict exists and then dict get if the value existed. Problems with this approach are:
* It is redundant	already dict exists retrieves the value, but doesn't return it, so dict get has to look it up all over again.
* It is bulky	if the value in dictionary D of option -apa (or its default bar) is to be passed as an argument to the command foo, then the complete command is
     foo [if [dict exists $D -apa] then dict get $D -apa\par       else return -level 0 bar]
or 
     foo [expr [dict exists $D -apa] ? [dict get $D -apa] : "bar"]
which many programmers would find objectionable. The dict getwithdefault counterpart is merely
     foo [dict getwithdefault $D -apa bar]
The only way to avoid the redundance of an extra look-up seems to be to combine dict get with catch, like so:
  if [catch dict get $D -apa value] then set value bar else set value
but this has the disadvantage of hiding other sources of error, such as D not being a dictionary in the first place. This kind of error in a normal processing path is also considered poor style by some.
Implementation Choices
Even if it is deemed appropriate to have a dedicated subcommand of dict for this, it could be argued that it needn't be part of the compiled Tcl core; since dict is an ensemble, anyone can extend it at the script level and "the core can do without this bloat". However, it turns out than an in-core implementation is very easy whereas the alternatives are not so easy.
Concretely, the necessary DictGetWithDefaultCmd is a trivial modification of DictExistsCmd, to take one extra argument after the keys and change the final
  Tcl_SetObjResult(interp, Tcl_NewBooleanObj(valuePtr != NULL));
to
  Tcl_SetObjResult(interp, valuePtr != NULL ? valuePtr : objv[objc-1]);
It is nowhere near as easy to do this in a well-behaved extension, since DictExistsCmd relies on TclTraceDictPath to do most of the work, and the latter is AFAICT at best available in the internal stubs table.
A script-level implementation is certainly possible, but the minute details of producing core-looking error messages in this case appears considerable both compared to the functional parts of the command and compared to the amount of code needed to do it in the core.
Reference Implementation
An implementation is provided on SF, in patch #2370575. [https://sourceforge.net/support/tracker.php?aid=2370575]
Copyright
This document has been placed in the public domain. 
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