Check-in [11ef41277a]

# TIP 508: New subcommand [array default]
	Author:         Frédéric Bonnet <[email protected]>
	State:          Draft
	Type:           Project
	Vote:           
	Created:        13-May-2018
	Post-History:   
	Keywords:       Tcl,array
	Tcl-Version:	8.7
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# Abstract

This TIP proposes a new `[array default]` subcommand that allows default values
for arrays, causing that value to be returned any time an attempt is made to
access an element of the array that isn't present.

# Rationale

For its most basic usages, a Tcl array variable can be seen as a collection of
scalar variables. However, an array also has its own existence as a variable and
provides specficic actions on itself and on its elements. For exampe, the
regular way to check for a variable existence is to use the `[info exists]`
command, and this is true for both arrays and scalars (including array
elements); likewise, the `[set]` command is used to read and write the value of
a scalar vars and array elements, however array variables have no value in
themselves but provide a way to get and set several of its elements at once.

For more advanced usages, one must get past the simple 'collection of scalars'
viewpoint. A typical example is sparse arrays: at present, an implementation
requires a combination of `[array exists]` (or `[info exists]`) and `[set]`: the
former to test the presence of a key in an array, the latter to get or set the
actual value, and in this order. Failure to do so is likely to cause runtime
errors.

This document therefore proposes to add a new `[array default]` subcommand in
order to support such advanced usages. Sparse arrays built on this new feature
will be simplerw, more robust, faster, all the while being memory efficient.

# Specification

A new `array default` subcommand will be created with the following syntax:

> `array default ` _`option arrayName ?arg?`_ : Default value management. 
   
The following _option_ values are supported:

* <code>array default get <em>arrayName</em></code>: Get _arrayName_
  default value. Raise an error if _arrayName_ is not an array variable or if it
  has no default value.
* <code>array default set <em>arrayName</em> <em>value</em></code>: Set
  _arrayName_ default value. If the variable _arrayName_ does not already exist,
  it is created with an empty array value. Raise an error if _arrayName_ is an
  existing scalar variable or an invalid array name.
* <code>array default exists <em>arrayName</em></code>: Test whether
  _arrayName_ has a default value. Return false if _arrayName_ does not exist.
  Raise an error if _arrayName_ is not an array variable.
* <code>array default unset <em>arrayName</em></code>: Unset _arrayName_ default

  value. Does nothing if _arrayName_ does not exist. Raise an error if
  _arrayName_ is not an array variable.

The default value is only used as a last resort where an error would normally
occur when accessing a non-existing array element. Read traces are processed
before that point and are free to set the element value or modify the default
value.

# Examples

Initial state:

	% array exists var

	% set var(a)
	1
	% set var(b)
	0 2
	% set var(c)
	9999

Default values don't show in entry lists or iterations:

	% array names var
	a b
	% array get var
	a 1 b {0 2}
	% array for {key value} var {puts "$key => $value"}
	a => 1