TIP: 342
Title: Dict Get With Default
Version: $Revision: 1.3 $
Author: Lars Hellstr�m <[email protected]>
State: Draft
Type: Project
Vote: Pending
Tcl-Version: 8.7
Created: 27-Nov-2008
Keywords: dictionary, default value
Post-History:
~ 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 ''key''s 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 ''key''s 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}\
| 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 ''key''s 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.