Check-in [adcf227f66]

	Post-History:	
	Tcl-Ticket:     2370575
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# Abstract

A new subcommand of **dict** is proposed, which returns a dictionary value
if it exists, and a specified-per-call default otherwise.

# Specification

The **dict** command will get a new subcommand

 > **dict getwithdefault** _dictionary_ _key_ ?_key_ ...?  _value_

   * 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 {foo bar} else {foo $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.

An alternate version with **try** fixes that problem... but is ugly:

	try {
	    set value [dict get $D -apa]
	} trap {TCL LOOKUP DICT} {} {
	    set value bar
	}
	foo $value

## 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

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, in [patch #2370575](https://core.tcl-lang.org/tcl/tktview/2370575).


# Copyright

This document has been placed in the public domain.