In JavaScript nested functions are very useful: closures, private methods and what have you..
What are nested PHP functions for? Does anyone use them and what for?
Here's a small investigation I did
<?php
function outer( $msg ) {
function inner( $msg ) {
echo 'inner: '.$msg.' ';
}
echo 'outer: '.$msg.' ';
inner( $msg );
}
inner( 'test1' ); // Fatal error: Call to undefined function inner()
outer( 'test2' ); // outer: test2 inner: test2
inner( 'test3' ); // inner: test3
outer( 'test4' ); // Fatal error: Cannot redeclare inner()
If you are using PHP 5.3 you can get more JavaScript-like behaviour with an anonymous function:
<?php
function outer() {
$inner=function() {
echo "test\n";
};
$inner();
}
outer();
outer();
inner(); //PHP Fatal error: Call to undefined function inner()
$inner(); //PHP Fatal error: Function name must be a string
?>
Output:
test
test
There is none basically. I've always treated this as a side effect of the parser.
Eran Galperin is mistaken in thinking that these functions are somehow private. They are simply undeclared until outer() is run. They are also not privately scoped; they do pollute the global scope, albeit delayed. And as a callback, the outer callback could still only be called once. I still don't see how it's helpful to apply it on an array, which very likely calls the alias more than once.
The only 'real world' example I could dig up is this, which can only run once, and could be rewritten cleaner, IMO.
The only use I can think of, is for modules to call a [name]_include method, which sets several nested methods in the global space, combined with
if (!function_exists ('somefunc')) {
function somefunc() { }
}
checks.
PHP's OOP would obviously be a better choice :)
[Rewritten according to the comment by #PierredeLESPINAY.]
It's not just a side-effect at all, but actually a very useful feature for dynamically modifying the logic of your program. It's from the procedural PHP days, but can come in handy with OO architectures too, if you want to provide alternative implementations for certain standalone functions in the most straightforward way possible. (While OO is the better choice most of the time, it's an option, not a mandate, and some simple tasks don't need the extra cruft.)
For example, if you dynamically/conditionally load plugins from your framework, and want to make the life of the plugin authors super easy, you can provide default implementations for some critical functions the plugin didn't override:
<?php // Some framework module
function provide_defaults()
{
// Make sure a critical function exists:
if (!function_exists("tedious_plugin_callback"))
{
function tedious_plugin_callback()
{
// Complex code no plugin author ever bothers to customize... ;)
}
}
}
Functions defined within functions I can't see much use for but conditionally defined functions I can. For example:
if ($language == 'en') {
function cmp($a, $b) { /* sort by English word order */ }
} else if ($language == 'de') {
function cmp($a, $b) { /* sort by German word order; yes it's different */ }
} // etc
And then all your code needs to do is use the 'cmp' function in things like usort() calls so you don't litter language checks all over your code. Now I haven't done this but I can see arguments for doing it.
All the above being said, one might simply create a nested function to replace some localized, repetitive code within a function (that will only be used inside the parent function). An anonymous function is a perfect example of this.
Some might say just create private methods (or smaller code blocks) in a class, but that is muddying the waters when an ultra-specific task (which is exclusive to the parent) needs to be modularized, but not necessarily available to the rest of a class. The good news is if it turns out that you do need that function somewhere else, the fix is rather elementary (move the definition to a more central location).
Generally speaking, using JavaScript as the standard by which to evaluate other C based programming languages is a bad idea. JavaScript is definitely its own animal when compared to PHP, Python, Perl, C, C++, and Java. Of course, there are lots of general similarities, but the nitty, gritty details (reference JavaScript: The Definitive Guide, 6th Edition, Chapters 1-12), when paid attention to, make core JavaScript unique, beautiful, different, simple, and complex all at the same time. That's my two cents.
Just to be clear, I'm not saying nested functions are private. Just that nesting can help avoid clutter when something trivial needs to be modularized (and is only needed by the parent function).
All of my php is OO, but I do see a use for nested functions, particularly when your function is recursive and not necessarily an object. That is to say, it does not get called outside of the function it is nested in, but is recursive and subsequently needs to be a function.
There's little point in making a new method for the express use of a single other method. To me that's clumsy code and sort-of not the point of OO. If you're never going to call that function anywhere else, nest it.
In webservice calling we found it a much lower overhead (memory and speed) dynamically including in a nested fashion, individual functions over libraries full of 1000s of functions. The typical call stack might be between 5-10 calls deep only requiring linking a dozen 1-2kb files dynamically was better than including megabytes. This was done just by creating a small util function wrapping requires. The included functions become nested within the functions above the call stack. Consider it in contrast to classes full of 100s of functions that weren't required upon every webservice call but could also have used the inbuilt lazy loading features of php.
if you are in php 7 then see this:
This implementation will give you a clear idea about nested function.
Suppose we have three functions(too(), boo() and zoo()) nested in function foo().
boo() and zoo() have same named nested function xoo(). Now in this code I have commented out the rules of nested functions clearly.
function foo(){
echo 'foo() is called'.'<br>';
function too(){
echo 'foo()->too() is called'.'<br>';
}
function boo(){
echo 'foo()->boo() is called'.'<br>';
function xoo(){
echo 'foo()->boo()->xoo() is called'.'<br>';
}
function moo(){
echo 'foo()->boo()->moo() is called'.'<br>';
}
}
function zoo(){
echo 'foo()->zoo() is called'.'<br>';
function xoo(){ //same name as used in boo()->xoo();
echo 'zoo()->xoo() is called'.'<br>';
}
#we can use same name for nested function more than once
#but we can not call more than one of the parent function
}
}
/****************************************************************
* TO CALL A INNER FUNCTION YOU MUST CALL OUTER FUNCTIONS FIRST *
****************************************************************/
#xoo();//error: as we have to declare foo() first as xoo() is nested in foo()
function test1(){
echo '<b>test1:</b><br>';
foo(); //call foo()
too();
boo();
too(); // we can can a function twice
moo(); // moo() can be called as we have already called boo() and foo()
xoo(); // xoo() can be called as we have already called boo() and foo()
#zoo(); re-declaration error
//we cannont call zoo() because we have already called boo() and both of them have same named nested function xoo()
}
function test2(){
echo '<b>test2:</b><br>';
foo(); //call foo()
too();
#moo();
//we can not call moo() as the parent function boo() is not yet called
zoo();
xoo();
#boo(); re-declaration error
//we cannont call boo() because we have already called zoo() and both of them have same named nested function xoo()
}
Now if we call test1() the output will be this:
test1:
foo() is called
foo()->too() is called
foo()->boo() is called
foo()->too() is called
foo()->boo()->moo() is called
foo()->boo()->xoo() is called
if we call test2() the output will be this:
test2:
foo() is called
foo()->too() is called
foo()->zoo() is called
zoo()->xoo() is called
But we cannot call both text1() and test2() at same time to avoid re-declaration error
For those that suggest that there is no practical use of nested functions. Yes they have use and this is an example.
Imagine that I have a file called my_file.php which is used to get an ajax result out of. But what if there are times that you don't want to get the result through ajax but you want to include it twice in the same page without conflicts?
Lets say ajax file my_file.php :
<?php
// my_file.php used for ajax
$ajax_json_in = 10;
function calculations($a, $b)
{ $result = $a + $b;
return $result;
}
$new_result = $ajax_json_in * calculations(1, 2);
$ajax_json_out = $new_result;
?>
Below example includes the above file twice without conflict. You may not want to ajax call it, because there are cases that you need to include it straight in the HTML.
<?php
// include the above file my_file.php instead of ajaxing it
function result1
{
$ajax_json_in = 20;
include("my_file.php");
return $ajax_json_out;
}
function result2
{
$ajax_json_in = 20;
include("my_file.php");
return $ajax_json_out;
}
?>
Including the file makes the file's functions nested. The file is used both for ajax calls and inline includes !!!
So there is use in real life of nested functions.
Have a nice day.
I know this is an old post but fwiw I use nested functions to give a neat and tidy approach to a recursive call when I only need the functionality locally - e.g. for building hierarchical objects etc (obviously you need to be careful the parent function is only called once):
function main() {
// Some code
function addChildren ($parentVar) {
// Do something
if ($needsGrandChildren) addChildren ($childVar);
}
addChildren ($mainVar); // This call must be below nested func
// Some more code
}
A point of note in php compared with JS for instance is that the call to the nested function needs to be made after, i.e. below, the function declaration (compared with JS where the function call can be anywhere within the parent function
I have only really used this characteristic when it was useful to execute a small recursive function inside a primary, more categorical function, but didn't want to move it to a different file because it was fundamental to the behavior of a primary process. I realize there are other "best practice" ways of doing this, but I want to make sure my devs see that function every time they look at my parser, it's likely what they should modify anyway...
Nested functions are useful in Memoization (caching function results to improve performance).
<?php
function foo($arg1, $arg2) {
$cacheKey = "foo($arg1, $arg2)";
if (! getCachedValue($cacheKey)) {
function _foo($arg1, $arg2) {
// whatever
return $result;
}
$result = _foo($arg1, $arg2);
setCachedValue($cacheKey, $result);
}
return getCachedValue($cacheKey);
}
?>
Nested functions are useful if you want the nested function to utilize a variable that was declared within the parent function.
<?php
ParentFunc();
function ParentFunc()
{
$var = 5;
function NestedFunc()
{
global $var;
$var = $var + 5;
return $var;
};
echo NestedFunc()."<br>";
echo NestedFunc()."<br>";
echo NestedFunc()."<br>";
}
?>
I'm playing with PHP and some functional style programming.
I'm using the Functional-PHP library but question is generic to PHP (I'm using 7.2).
I try to create a callable from an imported function but what I get is
TypeError: Failed to create closure from callable: function 'pick' not found or invalid function name
Sample code:
use function Functional\pick;
class A
{
public function execute()
{
$pick1 = \Closure::fromCallable('pick');
}
}
PHP use statements define an alias for the rest of the file, but they won't affect a string referencing an imported function or class.
When you say
use function Functional\pick;
it means that in that file, you can call the Functional\pick function just using pick(...). But if you're using a string to reference it then PHP doesn't know to expand the alias.
The quickest way to resolve this is just to use the fully qualified function name when calling fromCallable:
$pick1 = \Closure::fromCallable('Functional\pick');
echo get_class($pick1);
Closure
Alternatively, if you really wanted to use the alias, you could wrap the call a level deeper with another anonymous function:
use function Functional\pick;
$pick1 = \Closure::fromCallable(function (...$args) { return pick(...$args); });
But that's a lot messier, in my opinion at least.
Edit: There's some decent discussion around this in this recent thread in php-externals
Sometimes, especially with callbacks functions or inheritance/implementation case, I don't want to use some arguments in method. But they are required by the method interface signature (and I can't change the signature, let's say it's something required via Composer). Example:
// Assuming the class implements an interface with this method:
// public function doSomething($usefull1, $usefull2, $usefull3);
public function doSomething($usefull, $useless_here, $useless_here) {
return something_with($usefull);
}
// ...
In some other languages (let's say Rust), I can ignore these arguments explicitly, which make the code (and intention) more readable. In PHP, it could be this:
public function doSomething($usefull, $_, $_) {
return something_with($usefull);
}
Is this possible in PHP? Did I missed something?
Side note: it's not only for trailing arguments, it could be anywhere in the function declaration
I think the best you can hope for is to give them unique names that will suggest that they will not be used in the call.
Perhaps:
function doSomething($usefull,$x1,$x2){
return something_with($usefull);
}
Or:
function doSomething($ignore1,$useful,$ignore2){
return something_with($useful);
}
PHP wants to have arguments accounted for and uniquely named.
Edit: If you want to avoid declaring variables that you won't use (but you know they are being sent), try func_get_args() and list(). This should make the code lean, clean, readable. (Demo)
function test(){
// get only use argument 2
list(,$useful,)=func_get_args();
echo $useful;
}
test('one','two','three'); // outputs: two
Assign default value for optional parameter.
function doSomething($usefull,$useless1=null,$useless2=null){
return something_with($usefull);
}
Now....
Parameter 1 is required
Parameter 2 is optional
Parameter 3 is optional
Call function like..
doSomething($data);
doSomething($data,$anotherData);
doSomething($data,$anotherData,$anotherData1);
Your concrete object does not fit the interface completly, so you can just add a adapter class between them. So the interface stay as it is and your object just get what it really need.
class Adapter extends CustomInterface
{
function doSomething($ignore1,$useful,$ignore2){
return $customClass->something_with($useful);
}
}
I was just going through the code of this observer pattern HERE, and i came across the below line of code:
abstract class AbstractObserver {
abstract function update(AbstractSubject $subject_in);
}
The very first function in that snippet actually , now i am very new to PHP, I know javascript , but i am not so good in PHP , now what on earth are these parameters passed to the abstract function:
AbstractSubject $subject_in
?? What is this ?
Its a single parameter that it's typed, meaning that that function can only recieve an instance of AbstractSubject as the parameter.
Note that the words are not separated by a comma, but a space, meaning that the first word (AbstractSubject) defines the type of the parameter (second word, $subject_in) the method may receive, if you pass a value of any other type than an instance of AbstractSubject, you will get an exception raised.
So, an implementation of the given method should look something like this:
class MyObserver extends AbstractObserver
{
public function update(AbstractSubject $subject_in)
{
//Your implementation of the method goes here.
}
}
I am interested in something google couldn't really help me with...
I know that its possible to use anonymous functions and also store functions in a variable in PHP like that
$function = function myFoo() { echo "bar"; }
and call it using the variable: $function();
So far so good but what if I have a function or method declared somewhere but not saved on intializing?
I have a function that shall expect a callable, of course call_user_func() can help me here but then I need to pass the method name in my callback handler which I think is pretty unsafe because in the moment I add the callback I cant say if it really is a function and exists when I store it.
Thatswhy I would like to realize the following szenario:
This function:
function anyFunction() {
echo "doWhatever...!";
}
should be saved in a variable at a later point in time:
$myOtherFunction = get_registered_func("anyFunction");
I know get_registered_func() doesnt exist but I want to know if this is possible somehow!
With this I could now have another function
function setCallback(callable $theCallbackFunction) { }
And use it like this:
setCallback($myOtherFunction);
This would have a great advantage that an exception / a fatal is thrown when the parameter is no function or does not exist.
So in short, is there a way to store a previously defined, already existing function or method in a variable?
PHP's callable meta type can be:
a string
an array
an anonymous function
Only the latter one is somewhat "type safe", i.e. if you get an anonymous function you know it's something you can call. The other two options are merely formalised informal standards (if that makes sense) which are supported by a few functions that accept callbacks; they're not actually a type in PHP's type system. Therefore there's basically no guarantee you can make about them.
You can only work around this by checking whether the callable you got is, well, callable using is_callable before you execute them. You could wrap this into your own class, for example, to actually create a real callable type.
I see no reason why this shouldn't be possible, other than there not being a PHP function to do it. The anonymous function syntax is newly introduced in PHP, I wouldn'be surprised if it was still a little rough around the edges.
You can always wrap it:
function wrap_function ($callback) {
if (is_callable($callback) === false) {
throw new Exception("nope");
}
return function () {
call_user_func($callback, func_get_args());
}
}
$foo = new Foo();
$ref = wrap_function(array($foo, "bar"));
Not a good way, but maybe this helps you:
function foo($a) {
echo $a;
}
$callFoo = 'foo';
$callFoo('Hello World!'); // prints Hello