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To: internals@lists.php.net
Date: Sun, 17 Jun 2007 08:58:33 -0400
Subject: Proposal: array_get, a more palatable alternative to ifsetor
From: andrew@ashearer.com (Andrew Shearer)

MOTIVATION

There is an unmet need for an accessor that doesn't generate an  
E_NOTICE when the value is missing, as shown by ongoing discussions  
and repeated requests for an ifsetor operator. However, ifsetor had a  
special-case syntax and generally didn't fit very well with the rest  
of the language.

http://devzone.zend.com/node/view/id/1481#Heading2 has a brief  
summary. See the Related Functions and Proposals section for more.

Reading over those ideas (firstset(), coalesce(), :?, ifset(), and a  
workaround using settype()), most of the best uses boil down to  
retrieving values from arrays.


PROPOSAL

As a simpler alternative to constructs such as this common double  
array reference...
	
		$value = isset($_POST['command']) ? $_POST['command'] : '';
		
I propose an array_get function, like this...
	
		$value = array_get($_POST, 'command', '');
		
The third argument provides a default. This function would require no  
special syntax, and makes a very common construct easier to read and  
less error-prone to type. It's a concise way of saying that missing  
values can be handled gracefully.

Though request processing was used as an example, the function has  
wide applicability across many other uses of associative arrays.

	
GREAT, BUT WHY NOT ADD IT TO AN INCLUDE FILE, INSTEAD OF THE CORE?

One of the goals is to make everyday PHP code simpler and clearer.  
Writers of sample code snippets should be able to rely on array_get()  
being available. Otherwise, they will not use it. Clearer sample code  
particularly benefits beginners, who would probably find array_get  
easier to understand, but anyone else who has to read or maintain  
other people's code would benefit from its wide deployment in core as  
well. The function is generally useful enough to be part of the  
language, and the implementation in C is also more efficient than a  
PHP version.

That said, a compatibility function for older versions of PHP is  
given below.


SEMANTICS

	  mixed array_get(array $array, mixed $key[, mixed $default = FALSE]);
	
If $array contains the key $key, $array[$key] is returned. Otherwise  
$default is returned.
	
If $default is not specified, it defaults to FALSE. (NULL would also  
be possible, and would more closely match other languages such as  
Python with its dict.get method, but other PHP functions tend to  
return FALSE to indicate no value.)
	
The semantics match
	
		array_key_exists($key, $array) ? $array[$key] : $default
	
... but for comparison,
	
		isset($array[$key]) ? $array[$key] : $default
	
is subtly different. The preferred array_key_exists version has these  
differences:
	  1. If $array[$key] exists but has been set to null, that null  
value will be returned instead of $default. This is likely to be the  
least surprising thing to do.
	  2. If $array itself is unset, an error is generated. This is good.  
The intention is to gracefully handle a missing $key. But if even  
$array itself doesn't exist, there may be another problem, such as  
misspelling the array variable. isset() ignores all errors, sweeping  
more under the rug than we typically want.


IMPLEMENTATION

A core C implementation of array_get() benchmarked between two and  
three times as fast as the implementation in PHP. I'll attach the  
patch after responding to feedback.

See the last section for the code of the PHP implementation.


RELATED FUNCTIONS AND PROPOSALS

This function is different than the array_get function proposed and  
rejected in http://bugs.php.net/bug.php?id=28185. That function had  
no default value and throws a notice when the key doesn't exist,  
eliminating the major purpose of this function.

The ?: operator doesn't serve the same purpose, because it causes an  
E_NOTICE for missing values. However, ?: and array_get can be used  
together to provide short-circuit evaluation, overcoming the  
limitations of both. See the LIMITATIONS section for an example.

ifsetor: as discussed above, ifsetor wasn't a regular function. It  
required special language syntax support because it attempted to test  
whether a direct parameter itself was set or unset, and was  
ultimately rejected.

ifset: a related proposal to ifsetor, with a simpler syntax, ifset  
was missing a way to control the default value.

See here for more discussion about the 'E_STRICT ternary pain-in-the- 
ass expression' and alternatives:
	http://keithdevens.com/weblog/archive/2005/Nov/24
	http://www.php.net/~derick/meeting-notes.html#id39
	http://devzone.zend.com/node/view/id/1481#Heading2

	
LIMITATIONS

This proposal doesn't address every requested feature. The third  
parameter is always evaluated, so calling a slow function there would  
be undesirable. However, the limitation appears to be unavoidable  
without special language support, and there are workarounds. These  
snippets have approximately equal meanings (though they may differ is  
the handling of array values that convert to false):

     array_get($_GET, 'foo', slowDefaultCalculation())

     $val = array_get($_GET, 'foo'); if (!$val) $val =  
slowDefaultCalculation();

     array_get($_GET, 'foo') ?: slowDefaultCalculation()

The last example uses the new PHP 6 ?: operator.

This function applies only to array elements. Unlike other proposed  
functions, it doesn't also attempt to determine whether variables are  
set. However, the practical uses suggested for the other functions  
generally ended up applying to array elements.


COMPATIBILITY FUNCTION FOR OLDER VERSIONS OF PHP

if (!function_exists('array_get')) {
     function array_get($arr, $key, $default = false) {
         if (array_key_exists($key, $arr)) {
             return $arr[$key];
         }
         else {
             return $default;
         }
     }
}

(This version turned in the fastest times out of several variants.  
Passing $arr by reference or attempting to return the result by  
reference had a huge negative impact, and using the ternary ? :  
operator instead of the if/else was slightly slower.)


Comments?

--
Andrew Shearer
http://ashearer.com/