rfc:namespaceresolution

Non fully qualified namespaced identifier resolution RFCs

This RFC discusses the way identifiers inside a namespace are to be resolved that are not fully qualified.

Introduction

Generally in namespaces we support fully qualified names as well as importing namespaces via “use”. However what happens if a non fully qualified name is used that is not defined or imported inside the namespace? Should this cause a fatal error or should an attempt be made to resolve this call to the global namespace?

<?php
namespace foo;
 
$bar = new bar();
bar();
?>

Its important that we make a conscious decision for these questions. Depending on how we approach this, users might unintentionally trigger autoload, call functions in the global namespace they did not expect or they could run into trouble when trying to migrate existing code to namespaces.

One way to avoid this is via an explicit “use” statement or by fully qualifying the name

<?php
namespace foo;
use \dong as bar;
 
$bar = new bar();
\bar();
?>

While there is no way to magically import the right things all namespaces, we do have the option of automatically falling back into the global namespace if the identifier does not resolve in the local namespace. This RFC details some alternative approaches for this as well as how things would be like if such a fallback would not exist.

Possible approaches

Fallback to the global namespace

In this scenario when an identifier does not resolve inside the current namespace and attempt would be made to find the identifier in the global namespace.

When referencing global identifiers in namespaces, it is probably a reasonable assumption that the bulk of that will be function calls. This is because currently most functionality in PHP is provided via functions. Also once an instance of a class has been made, one does not have to reference the identifier again in common usage (there will still be cases, like with instanceof/type hint checks or static calls).

In the past people often created classes for the sole reason of being able to sort of “namespace” their functions. Given that we now have real namespaces, class usage as a namespace replacement is no longer necessary. Still another possible assumption, which is considerably more dangerous to make, is that most code that uses namespaces will mostly use classes for its implementation and considerably less depend on namespaced functions.

One noteworthy aspect here is that for classes we have autoload. If non fully qualified identifiers can be used to reference global identifiers, “lazy” programmers can skip fully qualifying identifiers even if they have the full intention of referencing a global identifier. With autoload this could trigger expensive operations, which are essentially useless.

For functions however we do not have autoload capabilities. This brings the advantage that falling back to the global namespace does not run the performance risk of autoload. So a fallback would be much less expensive, but there would still be overhead for not making intentional references to the global namespace fully qualified.

At the same time the ability to automatically fallback into the global namespace gives the ability to overload global identifiers inside a namespace without having to modify existing code inside that namespace. This however can also be considered dangerously similar to the ambiguity issues we solved by changing the namespace separator (for example static code analysis becomes more difficult).

Furthermore users need to be aware that if they are overloading internal identifiers that they need to make sure that the relevant code is loaded in time. For classes there is the autoload approach which would help ensure that the class to overload is loaded on demand if necessary. However users that do not use autoload or that are overloading function (and constants) run the risk of their code behaving differently in not so obvious ways if they do not always load all files defining relevant functions (and constants) for this namespace.

file1.php

<?php
namespace foo;
function strlen(){}
?>

file2.php

<?php
namespace foo;
// removing the commenting of the following line will change the behavior of file2.php
// include 'file1.php';
strlen();
?>

One approach to make it at least noticeable when a fallback into the global namespace occurs would be to throw an E_NOTICE in this case. This would discourage users from using the fallback for overloading, but it would ensure that people migrating legacy code or people who have not yet fully understood namespaces, would be able to find out about where they are loosing performance.

Another approach to reduce (though it does not remove the issues entirely) some of the issues is by simply removing functions (and constants) from namespaces.

As a result of the above notes, we might decide to go with a few different options based on how one weighs these aspects:

  1. Only for functions/constants
  2. Only for classes
  3. Only for internal identifiers
  4. For everything

Only for functions/constants

Assumption: Most people will use global functions and namespaced classes.

Notes: By throwing an E_NOTICE when a fallback occurs, the performance issues become more manageable, but it would reduce the feasibility of overloading. Also note that if functions (and constants) would be removed from namespaces, then most disadvantages would be removed as functions (and constants) would always directly reference the global namespace.

Advantages
  1. Does not require fully qualified names for functions (and constants)
  2. No performance “bomb” with autoload
  3. Ability to overload global functions (and constants)
Disadvantages
  1. Overloading global identifiers requires ensuring that all relevant files are loaded or unexpected behavior might occur
  2. There is still overhead for the fallback
  3. Classes still need fully qualified names

Only for classes (autoload first)

Assumption: People want to overload global classes

Notes: By throwing an E_NOTICE when a fallback occurs, the performance issues become more manageable, but it would reduce the feasibility of overloading. Also if instead of checking autoload before global, one could first check global before falling back to autoload. This prevents performance issues, but would raise issues with the load order similar to functions/constants.

Advantages
  1. Does not require fully qualified names for classes
  2. Ability to overload global classes
Disadvantages
  1. Functions (and constants) still need fully qualified names
  2. Possible performance bomb with autoload

Only for internal identifiers

Assumption: People will leave the global namespace to PHP and namespace their own code.

Advantages
  1. Does not require fully qualified names for all internal identifiers
  2. Internal identifiers work the same inside and outside of namespaces (though overloading would still be possible)
Disadvantages
  1. Less clear rule as its not possible form just reading the calling code if something is internal or not
  2. Defining a function in userland code to emulate functionality from a newer PHP version will not enable fallbacks
  3. Higher performance overhead (???)

For everything

Assumption: People want to easily migrate their existing code and beginners should not have to know (as much about) if they are coding inside a namespace or not.

Notes: By throwing an E_NOTICE when a fallback occurs, the performance issues become more manageable, but it would reduce the feasibility of overloading. Also note that if functions (and constants) would be removed from namespaces, then some of the disadvantages would be removed as functions (and constants) would always directly reference the global namespace. Also if instead of checking autoload before global, one could first check global before falling back to autoload. This prevents performance issues for classes, but would raise issues with the load order similar to functions/constants, but means things would be consistent in the sense that overloading requires defining the relevant identifiers ahead of use for everything (classes/functions/constants).

Advantages
  1. Does not require fully qualified names for all global identifiers
  2. Simple rule, everything falls back
  3. Ability to overload any kind of global identifier
Disadvantages
  1. There is overhead for the fallback
  2. Additionally there is a possible performance bomb with autoload
  3. Overloading global identifiers requires ensuring that all relevant files are loaded or unexpected behavior might occur

Do not fall back to the global namespace

Assumption: People are willing to spend more time on updating their legacy code that they migrate to namespaces and adapt their coding style to fully qualify global identifiers when working within namespaces.

Advantages
  1. No risk for people relying on behavior that does the same but with more overhead
  2. No risk for ambiguity
Disadvantages
  1. Require fully qualified names (or use statements) for all global identifiers

More about namespaces

Changelog

- from 1.0 to 1.1: added variant with 1) ns 2) global 3) autoload - from 1.0 to 1.0.1: tweaked examples - from 0.9 to 1.0: added some examples, added note about use statement, fixed some language issues

rfc/namespaceresolution.txt · Last modified: 2013/10/29 18:40 by nikic