下文的第一个逐行读取文件例子用三种方式实现;普通方法,迭代器和生成器,比较了他们的优缺点,很好,可以引用到自己的代码中 ,支持的php版本(PHP 5 >= 5.5.0)
后面的yield讲解,得逐行翻译理解
Generators provide an easy, boilerplate-free way of implementing iterators.
As an example, consider how you would implement the file()
function in userland code:
<span class="kw2">function</span> getLinesFromFile<span class="br0">(</span><span class="re0">$fileName</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">if</span> <span class="br0">(</span><span class="sy0">!</span><span class="re0">$fileHandle</span> <span class="sy0">=</span> <span class="kw3">fopen</span><span class="br0">(</span><span class="re0">$fileName</span><span class="sy0">,</span> <span class="st_h">'r'</span><span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span><span class="sy0">;</span> <span class="br0">}</span> <span class="re0">$lines</span> <span class="sy0">=</span> <span class="br0">[</span><span class="br0">]</span><span class="sy0">;</span> <span class="kw1">while</span> <span class="br0">(</span><span class="kw4">false</span> <span class="sy0">!==</span> <span class="re0">$line</span> <span class="sy0">=</span> <span class="kw3">fgets</span><span class="br0">(</span><span class="re0">$fileHandle</span><span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span> <span class="re0">$lines</span><span class="br0">[</span><span class="br0">]</span> <span class="sy0">=</span> <span class="re0">$line</span><span class="sy0">;</span> <span class="br0">}</span> <span class="kw3">fclose</span><span class="br0">(</span><span class="re0">$fileHandle</span><span class="br0">)</span><span class="sy0">;</span> <span class="kw1">return</span> <span class="re0">$lines</span><span class="sy0">;</span> <span class="br0">}</span> <span class="re0">$lines</span> <span class="sy0">=</span> getLinesFromFile<span class="br0">(</span><span class="re0">$fileName</span><span class="br0">)</span><span class="sy0">;</span> <span class="kw1">foreach</span> <span class="br0">(</span><span class="re0">$lines</span> <span class="kw1">as</span> <span class="re0">$line</span><span class="br0">)</span> <span class="br0">{</span> <span class="co1">// do something with $line</span> <span class="br0">}</span>
The main disadvantage of this kind of code is evident: It will read the whole file into a large array. Depending on how big the file is, this can easily hit the memory limit. This is not what you usually want. Instead you want to get the lines one by one. This is what iterators are perfect for.
Sadly implementing iterators requires an insane amount of boilerplate code. E.g. consider this iterator variant of the above function:
<span class="kw2">class</span> LineIterator implements Iterator <span class="br0">{</span> <span class="kw2">protected</span> <span class="re0">$fileHandle</span><span class="sy0">;</span> <span class="kw2">protected</span> <span class="re0">$line</span><span class="sy0">;</span> <span class="kw2">protected</span> <span class="re0">$i</span><span class="sy0">;</span> <span class="kw2">public</span> <span class="kw2">function</span> __construct<span class="br0">(</span><span class="re0">$fileName</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">if</span> <span class="br0">(</span><span class="sy0">!</span><span class="re0">$this</span><span class="sy0">-></span><span class="me1">fileHandle</span> <span class="sy0">=</span> <span class="kw3">fopen</span><span class="br0">(</span><span class="re0">$fileName</span><span class="sy0">,</span> <span class="st_h">'r'</span><span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">throw</span> <span class="kw2">new</span> RuntimeException<span class="br0">(</span><span class="st_h">'Couldn\'t open file "'</span> <span class="sy0">.</span> <span class="re0">$fileName</span> <span class="sy0">.</span> <span class="st_h">'"'</span><span class="br0">)</span><span class="sy0">;</span> <span class="br0">}</span> <span class="br0">}</span> <span class="kw2">public</span> <span class="kw2">function</span> <span class="kw3">rewind</span><span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw3">fseek</span><span class="br0">(</span><span class="re0">$this</span><span class="sy0">-></span><span class="me1">fileHandle</span><span class="sy0">,</span> <span class="nu0">0</span><span class="br0">)</span><span class="sy0">;</span> <span class="re0">$this</span><span class="sy0">-></span><span class="me1">line</span> <span class="sy0">=</span> <span class="kw3">fgets</span><span class="br0">(</span><span class="re0">$this</span><span class="sy0">-></span><span class="me1">fileHandle</span><span class="br0">)</span><span class="sy0">;</span> <span class="re0">$this</span><span class="sy0">-></span><span class="me1">i</span> <span class="sy0">=</span> <span class="nu0">0</span><span class="sy0">;</span> <span class="br0">}</span> <span class="kw2">public</span> <span class="kw2">function</span> valid<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="kw4">false</span> <span class="sy0">!==</span> <span class="re0">$this</span><span class="sy0">-></span><span class="me1">line</span><span class="sy0">;</span> <span class="br0">}</span> <span class="kw2">public</span> <span class="kw2">function</span> <span class="kw3">current</span><span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="re0">$this</span><span class="sy0">-></span><span class="me1">line</span><span class="sy0">;</span> <span class="br0">}</span> <span class="kw2">public</span> <span class="kw2">function</span> <span class="kw3">key</span><span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="re0">$this</span><span class="sy0">-></span><span class="me1">i</span><span class="sy0">;</span> <span class="br0">}</span> <span class="kw2">public</span> <span class="kw2">function</span> <span class="kw3">next</span><span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">if</span> <span class="br0">(</span><span class="kw4">false</span> <span class="sy0">!==</span> <span class="re0">$this</span><span class="sy0">-></span><span class="me1">line</span><span class="br0">)</span> <span class="br0">{</span> <span class="re0">$this</span><span class="sy0">-></span><span class="me1">line</span> <span class="sy0">=</span> <span class="kw3">fgets</span><span class="br0">(</span><span class="re0">$this</span><span class="sy0">-></span><span class="me1">fileHandle</span><span class="br0">)</span><span class="sy0">;</span> <span class="re0">$this</span><span class="sy0">-></span><span class="me1">i</span><span class="sy0">++;</span> <span class="br0">}</span> <span class="br0">}</span> <span class="kw2">public</span> <span class="kw2">function</span> __destruct<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw3">fclose</span><span class="br0">(</span><span class="re0">$this</span><span class="sy0">-></span><span class="me1">fileHandle</span><span class="br0">)</span><span class="sy0">;</span> <span class="br0">}</span> <span class="br0">}</span> <span class="re0">$lines</span> <span class="sy0">=</span> <span class="kw2">new</span> LineIterator<span class="br0">(</span><span class="re0">$fileName</span><span class="br0">)</span><span class="sy0">;</span> <span class="kw1">foreach</span> <span class="br0">(</span><span class="re0">$lines</span> <span class="kw1">as</span> <span class="re0">$line</span><span class="br0">)</span> <span class="br0">{</span> <span class="co1">// do something with $line</span> <span class="br0">}</span>
As you can see a very simple piece of code can easily become very complicated when turned into an iterator. Generators solve this problem and allow you to implement iterators in a very straightforward manner:
<span class="kw2">function</span> getLinesFromFile<span class="br0">(</span><span class="re0">$fileName</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">if</span> <span class="br0">(</span><span class="sy0">!</span><span class="re0">$fileHandle</span> <span class="sy0">=</span> <span class="kw3">fopen</span><span class="br0">(</span><span class="re0">$fileName</span><span class="sy0">,</span> <span class="st_h">'r'</span><span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span><span class="sy0">;</span> <span class="br0">}</span> <span class="kw1">while</span> <span class="br0">(</span><span class="kw4">false</span> <span class="sy0">!==</span> <span class="re0">$line</span> <span class="sy0">=</span> <span class="kw3">fgets</span><span class="br0">(</span><span class="re0">$fileHandle</span><span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span> yield <span class="re0">$line</span><span class="sy0">;</span> <span class="br0">}</span> <span class="kw3">fclose</span><span class="br0">(</span><span class="re0">$fileHandle</span><span class="br0">)</span><span class="sy0">;</span> <span class="br0">}</span> <span class="re0">$lines</span> <span class="sy0">=</span> getLinesFromFile<span class="br0">(</span><span class="re0">$fileName</span><span class="br0">)</span><span class="sy0">;</span> <span class="kw1">foreach</span> <span class="br0">(</span><span class="re0">$lines</span> <span class="kw1">as</span> <span class="re0">$line</span><span class="br0">)</span> <span class="br0">{</span> <span class="co1">// do something with $line</span> <span class="br0">}</span>
The code looks very similar to the array-based implementation. The main difference is that instead of pushing values into an array the values are yield
ed.
Generators work by passing control back and forth between the generator and the calling code:
When you first call the generator function ($lines = getLinesFromFile($fileName)
) the passed argument is bound, but nothing of the code is actually executed. Instead the function directly returns a Generator
object. That Generator
object implements the Iterator
interface and is what is eventually traversed by the foreach
loop:
Whenever the Iterator::next()
method is called PHP resumes the execution of the generator function until it hits a yield
expression. The value of that yield
expression is what Iterator::current()
then returns.
Generator methods, together with the IteratorAggregate
interface, can be used to easily implement traversable classes too:
<span class="kw2">class</span> Test implements IteratorAggregate <span class="br0">{</span> <span class="kw2">protected</span> <span class="re0">$data</span><span class="sy0">;</span> <span class="kw2">public</span> <span class="kw2">function</span> __construct<span class="br0">(</span><span class="kw3">array</span> <span class="re0">$data</span><span class="br0">)</span> <span class="br0">{</span> <span class="re0">$this</span><span class="sy0">-></span><span class="me1">data</span> <span class="sy0">=</span> <span class="re0">$data</span><span class="sy0">;</span> <span class="br0">}</span> <span class="kw2">public</span> <span class="kw2">function</span> getIterator<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">foreach</span> <span class="br0">(</span><span class="re0">$this</span><span class="sy0">-></span><span class="me1">data</span> <span class="kw1">as</span> <span class="re0">$key</span> <span class="sy0">=></span> <span class="re0">$value</span><span class="br0">)</span> <span class="br0">{</span> yield <span class="re0">$key</span> <span class="sy0">=></span> <span class="re0">$value</span><span class="sy0">;</span> <span class="br0">}</span> <span class="co1">// or whatever other traversation logic the class has</span> <span class="br0">}</span> <span class="br0">}</span> <span class="re0">$test</span> <span class="sy0">=</span> <span class="kw2">new</span> Test<span class="br0">(</span><span class="br0">[</span><span class="st_h">'foo'</span> <span class="sy0">=></span> <span class="st_h">'bar'</span><span class="sy0">,</span> <span class="st_h">'bar'</span> <span class="sy0">=></span> <span class="st_h">'foo'</span><span class="br0">]</span><span class="br0">)</span><span class="sy0">;</span> <span class="kw1">foreach</span> <span class="br0">(</span><span class="re0">$test</span> <span class="kw1">as</span> <span class="re0">$k</span> <span class="sy0">=></span> <span class="re0">$v</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">echo</span> <span class="re0">$k</span><span class="sy0">,</span> <span class="st_h">' => '</span><span class="sy0">,</span> <span class="re0">$v</span><span class="sy0">,</span> <span class="st0">"<span class="es1">\n</span>"</span><span class="sy0">;</span> <span class="br0">}</span>
Generators can also be used the other way around, i.e. instead of producing values they can also consume them. When used in this way they are often referred to as enhanced generators, reverse generators or coroutines.
Coroutines are a rather advanced concept, so it very hard to come up with not too contrived an short examples. For an introduction see an example on how to parse streaming XML using coroutines. If you want to know more, I highly recommend checking out a presentation on this subject.
Any function which contains a yield
statement is automatically a generator function.
The initial implementation required that generator functions are marked with an asterix modifier (function*
). This method has the advantage that generators are more explicit and also allows for yield-less coroutines.
The automatic detection was chosen over the asterix modifier for the following reasons:
function *&gen()
if (false) yield;
.
When a generator function is called the execution is suspended immediately after parameter binding and a Generator
object is returned.
The Generator
object implements the following interface:
final <span class="kw2">class</span> Generator implements Iterator <span class="br0">{</span> void <span class="kw3">rewind</span><span class="br0">(</span><span class="br0">)</span><span class="sy0">;</span> bool valid<span class="br0">(</span><span class="br0">)</span><span class="sy0">;</span> mixed <span class="kw3">current</span><span class="br0">(</span><span class="br0">)</span><span class="sy0">;</span> mixed <span class="kw3">key</span><span class="br0">(</span><span class="br0">)</span><span class="sy0">;</span> void <span class="kw3">next</span><span class="br0">(</span><span class="br0">)</span><span class="sy0">;</span> mixed send<span class="br0">(</span>mixed <span class="re0">$value</span><span class="br0">)</span><span class="sy0">;</span> mixed <span class="kw1">throw</span><span class="br0">(</span>Exception <span class="re0">$exception</span><span class="br0">)</span><span class="sy0">;</span> <span class="br0">}</span>
If the generator is not yet at a yield
statement (i.e. was just created and not yet used as an iterator), then any call to rewind
, valid
, current
, key
, next
or send
will resume the generator until the next yield
statement is hit.
Consider this example:
<span class="kw2">function</span> gen<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">echo</span> <span class="st_h">'start'</span><span class="sy0">;</span> yield <span class="st_h">'middle'</span><span class="sy0">;</span> <span class="kw1">echo</span> <span class="st_h">'end'</span><span class="sy0">;</span> <span class="br0">}</span> <span class="co1">// Initial call does not output anything</span> <span class="re0">$gen</span> <span class="sy0">=</span> gen<span class="br0">(</span><span class="br0">)</span><span class="sy0">;</span> <span class="co1">// Call to current() resumes the generator, thus "start" is echo'd.</span> <span class="co1">// Then the yield expression is hit and the string "middle" is returned</span> <span class="co1">// as the result of current() and then echo'd.</span> <span class="kw1">echo</span> <span class="re0">$gen</span><span class="sy0">-></span><span class="kw3">current</span><span class="br0">(</span><span class="br0">)</span><span class="sy0">;</span> <span class="co1">// Execution of the generator is resumed again, thus echoing "end"</span> <span class="re0">$gen</span><span class="sy0">-></span><span class="kw3">next</span><span class="br0">(</span><span class="br0">)</span><span class="sy0">;</span>
A nice side-effect of this behavior is that coroutines do not have to be primed with a next()
call before they can be used. (This is required in Python and also the reason why coroutines in Python usually use some kind of decorator that automatically primes the coroutine.)
Apart from the above the Generator
methods behave as follows:
rewind
: Throws an exception if the generator is currently after the first yield. (More in the “Rewinding a generator” section.)
valid
: Returns false
if the generator has been closed, true
otherwise. (More in the “Closing a generator” section.)
current
: Returns whatever was passed to yield
or null
if nothing was passed or the generator is already closed.
key
: Returns the yielded key or, if none was specified, an auto-incrementing key or null
if the generator is already closed. (More in the “Yielding keys” section.)
next
: Resumes the generator (unless the generator is already closed).
send
: Sets the return value of the yield
expression and resumes the generator (unless the generator is already closed). (More in the “Sending values” section.)
throw
: Throws an exception at the current suspension point in the generator. (More in the “Throwing into the generator” section.)
The newly introduced yield
keyword (T_YIELD
) is used both for sending and receiving values inside the generator. There are three basic forms of the yield
expression:
yield $key => $value
: Yields the value $value
with key $key
.
yield $value
: Yields the value $value
with an auto-incrementing integer key.
yield
: Yields the value null
with an auto-incrementing integer key.
The return value of the yield
expression is whatever was sent to the generator using send()
. If nothing was sent (e.g. during foreach
iteration) null
is returned.
To avoid ambiguities the first two yield
expression types have to be surrounded by parenthesis when used in expression-context. Some examples when parentheses are necessary and when they aren't:
<span class="co1">// these three are statements, so they don't need parenthesis</span> yield <span class="re0">$key</span> <span class="sy0">=></span> <span class="re0">$value</span><span class="sy0">;</span> yield <span class="re0">$value</span><span class="sy0">;</span> yield<span class="sy0">;</span> <span class="co1">// these are expressions, so they require parenthesis</span> <span class="re0">$data</span> <span class="sy0">=</span> <span class="br0">(</span>yield <span class="re0">$key</span> <span class="sy0">=></span> <span class="re0">$value</span><span class="br0">)</span><span class="sy0">;</span> <span class="re0">$data</span> <span class="sy0">=</span> <span class="br0">(</span>yield <span class="re0">$value</span><span class="br0">)</span><span class="sy0">;</span> <span class="co1">// to avoid strange (yield) syntax the parenthesis are not required here</span> <span class="re0">$data</span> <span class="sy0">=</span> yield<span class="sy0">;</span>
If yield
is used inside a language construct that already has native parentheses, then they don't have to be duplicated:
call<span class="br0">(</span>yield <span class="re0">$value</span><span class="br0">)</span><span class="sy0">;</span> <span class="co1">// instead of</span> call<span class="br0">(</span><span class="br0">(</span>yield <span class="re0">$value</span><span class="br0">)</span><span class="br0">)</span><span class="sy0">;</span> <span class="kw1">if</span> <span class="br0">(</span>yield <span class="re0">$value</span><span class="br0">)</span> <span class="br0">{</span> <span class="sy0">...</span> <span class="br0">}</span> <span class="co1">// instead of</span> <span class="kw1">if</span> <span class="br0">(</span><span class="br0">(</span>yield <span class="re0">$value</span><span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span> <span class="sy0">...</span> <span class="br0">}</span>
The only exception is the array()
structure. Not requiring parenthesis would be ambiguous here:
<span class="kw3">array</span><span class="br0">(</span>yield <span class="re0">$key</span> <span class="sy0">=></span> <span class="re0">$value</span><span class="br0">)</span> <span class="co1">// can be either</span> <span class="kw3">array</span><span class="br0">(</span><span class="br0">(</span>yield <span class="re0">$key</span><span class="br0">)</span> <span class="sy0">=></span> <span class="re0">$value</span><span class="br0">)</span> <span class="co1">// or</span> <span class="kw3">array</span><span class="br0">(</span><span class="br0">(</span>yield <span class="re0">$key</span> <span class="sy0">=></span> <span class="re0">$value</span><span class="br0">)</span><span class="br0">)</span>
Python also has parentheses requirements for expression-use of yield
. The only difference is that Python also requires parentheses for a value-less yield
(because the language does not use semicolons).
See also the "Alternative yield syntax considerations" section.
The languages that currently implement generators don't have support for yielding keys (only values). This though is just a side-effect as these languages don't support keys in iterators in general.
In PHP on the other hand keys are explicitly part of the iteration process and it thus does not make sense to not add key-yielding support. The syntax could be analogous to that of foreach
loops and array
declarations:
yield <span class="re0">$key</span> <span class="sy0">=></span> <span class="re0">$value</span><span class="sy0">;</span>
Furthermore generators need to generate keys even if no key was explicitly yielded. In this case it seems reasonable to behave the same as arrays do: Start with the key 0
and always increment by one. If in between an integer key which is larger than the current auto-key is explicitly yielded, then that will be used as the starting point for new auto-keys. All other yielded keys do not affect the auto-key mechanism.
<span class="kw2">function</span> gen<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> yield <span class="st_h">'a'</span><span class="sy0">;</span> yield <span class="st_h">'b'</span><span class="sy0">;</span> yield <span class="st_h">'key'</span> <span class="sy0">=></span> <span class="st_h">'c'</span><span class="sy0">;</span> yield <span class="st_h">'d'</span><span class="sy0">;</span> yield <span class="nu0">10</span> <span class="sy0">=></span> <span class="st_h">'e'</span><span class="sy0">;</span> yield <span class="st_h">'f'</span><span class="sy0">;</span> <span class="br0">}</span> <span class="kw1">foreach</span> <span class="br0">(</span>gen<span class="br0">(</span><span class="br0">)</span> <span class="kw1">as</span> <span class="re0">$key</span> <span class="sy0">=></span> <span class="re0">$value</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">echo</span> <span class="re0">$key</span><span class="sy0">,</span> <span class="st_h">' => '</span><span class="sy0">,</span> <span class="re0">$value</span><span class="sy0">,</span> <span class="st0">"<span class="es1">\n</span>"</span><span class="sy0">;</span> <span class="br0">}</span> <span class="co1">// outputs:</span> <span class="nu0">0</span> <span class="sy0">=></span> a <span class="nu0">1</span> <span class="sy0">=></span> b <span class="kw3">key</span> <span class="sy0">=></span> c <span class="nu0">2</span> <span class="sy0">=></span> d <span class="nu0">10</span> <span class="sy0">=></span> e <span class="nu0">11</span> <span class="sy0">=></span> f
This is the same behavior that arrays have (i.e. if gen()
instead simply returned an array with the yielded values the keys would be same). The only difference occurs when the generator yield non-integer, but numeric keys. For arrays they are cast, for generators the are not.
Generators can also yield by values by reference. To do so the &
modifier is added before the function name, just like it is done for return by reference.
This for example allows you to create classes with by-ref iteration behavior (which is something that is completely impossible with normal iterators):
class DataContainer implements IteratorAggregate { protected $data; public function __construct(array $data) { $this->data = $data; } public function &getIterator() { foreach ($this->data as $key => &$value) { yield <span class="re0">$key</span> <span class="sy0">=></span> <span class="re0">$value</span><span class="sy0">;</span> } } }
The class can then be iterated using by-ref foreach
:
<span class="re0">$dataContainer</span> <span class="sy0">=</span> <span class="kw2">new</span> DataContainer<span class="br0">(</span><span class="br0">[</span><span class="nu0">1</span><span class="sy0">,</span> <span class="nu0">2</span><span class="sy0">,</span> <span class="nu0">3</span><span class="br0">]</span><span class="br0">)</span><span class="sy0">;</span> <span class="kw1">foreach</span> <span class="br0">(</span><span class="re0">$dataContainer</span> <span class="kw1">as</span> <span class="sy0">&</span><span class="re0">$value</span><span class="br0">)</span> <span class="br0">{</span> <span class="re0">$value</span> <span class="sy0">*=</span> <span class="sy0">-</span><span class="nu0">1</span><span class="sy0">;</span> <span class="br0">}</span> <span class="co1">// $this->data is now [-1, -2, -3]</span>
Only generators specifying the &
modifier can be iterated by ref. If you try to iterate a non-ref generator by-ref an E_ERROR
is thrown.
Values can be sent into a generator using the send()
method. send($value)
will set $value
as the return value of the current yield
expression and resume the generator. When the generator hits another yield
expression the yielded value will be the return value of send()
. This is just a convenience feature to save an additional call to current()
.
Values are always sent by-value. The reference modifier &
only affects yielded values, not the ones sent back to the coroutine.
A simple example of sending values: Two (interchangeable) logging implementations:
<span class="kw2">function</span> echoLogger<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">while</span> <span class="br0">(</span><span class="kw4">true</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">echo</span> <span class="st_h">'Log: '</span> <span class="sy0">.</span> yield <span class="sy0">.</span> <span class="st0">"<span class="es1">\n</span>"</span><span class="sy0">;</span> <span class="br0">}</span> <span class="br0">}</span> <span class="kw2">function</span> fileLogger<span class="br0">(</span><span class="re0">$fileName</span><span class="br0">)</span> <span class="br0">{</span> <span class="re0">$fileHandle</span> <span class="sy0">=</span> <span class="kw3">fopen</span><span class="br0">(</span><span class="re0">$fileName</span><span class="sy0">,</span> <span class="st_h">'a'</span><span class="br0">)</span><span class="sy0">;</span> <span class="kw1">while</span> <span class="br0">(</span><span class="kw4">true</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw3">fwrite</span><span class="br0">(</span><span class="re0">$fileHandle</span><span class="sy0">,</span> yield <span class="sy0">.</span> <span class="st0">"<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy0">;</span> <span class="br0">}</span> <span class="br0">}</span> <span class="re0">$logger</span> <span class="sy0">=</span> echoLogger<span class="br0">(</span><span class="br0">)</span><span class="sy0">;</span> <span class="co1">// or</span> <span class="re0">$logger</span> <span class="sy0">=</span> fileLogger<span class="br0">(</span>__DIR__ <span class="sy0">.</span> <span class="st_h">'/log'</span><span class="br0">)</span><span class="sy0">;</span> <span class="re0">$logger</span><span class="sy0">-></span><span class="me1">send</span><span class="br0">(</span><span class="st_h">'Foo'</span><span class="br0">)</span><span class="sy0">;</span> <span class="re0">$logger</span><span class="sy0">-></span><span class="me1">send</span><span class="br0">(</span><span class="st_h">'Bar'</span><span class="br0">)</span><span class="sy0">;</span>
Exceptions can be thrown into the generator using the Generator::throw()
method. This will throw an exception in the generator's execution context and then resume the generator. It is roughly equivalent to replacing the current yield
expression with a throw
statement and resuming then. If the generator is already closed the exception will be thrown in the callers context instead (which is equivalent to replacing the throw()
call with a throw
statement). The throw()
method will return the next yielded value (if the exception is caught and no other exception is thrown).
An example of the functionality:
<span class="kw2">function</span> gen<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">echo</span> <span class="st0">"Foo<span class="es1">\n</span>"</span><span class="sy0">;</span> try <span class="br0">{</span> yield<span class="sy0">;</span> <span class="br0">}</span> catch <span class="br0">(</span>Exception <span class="re0">$e</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">echo</span> <span class="st0">"Exception: {<span class="es4">$e->getMessage</span>()}<span class="es1">\n</span>"</span><span class="sy0">;</span> <span class="br0">}</span> <span class="kw1">echo</span> <span class="st0">"Bar<span class="es1">\n</span>"</span><span class="sy0">;</span> <span class="br0">}</span> <span class="re0">$gen</span> <span class="sy0">=</span> gen<span class="br0">(</span><span class="br0">)</span><span class="sy0">;</span> <span class="re0">$gen</span><span class="sy0">-></span><span class="kw3">rewind</span><span class="br0">(</span><span class="br0">)</span><span class="sy0">;</span> <span class="co1">// echos "Foo"</span> <span class="re0">$gen</span><span class="sy0">-></span><span class="kw1">throw</span><span class="br0">(</span><span class="kw2">new</span> Exception<span class="br0">(</span><span class="st_h">'Test'</span><span class="br0">)</span><span class="br0">)</span><span class="sy0">;</span> <span class="co1">// echos "Exception: Test"</span> <span class="co1">// and "Bar"</span>
Rewinding to some degree goes against the concept of generators, as they are mainly intended as one-time data sources that are not supposed to be iterated another time. On the other hand, most generators probably *are* rewindable and it might make sense to allow it. One could argue though that rewinding a generator is really bad practice (especially if the generator is doing some expensive calculation). Allowing it to rewind would look like it is a cheap operation, just like with arrays. Also rewinding (as in jumping back to the execution context state at the initial call to the generator) can lead to unexpected behavior, e.g. in the following case:
<span class="kw2">function</span> getSomeStuff<span class="br0">(</span>PDOStatement <span class="re0">$stmt</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">foreach</span> <span class="br0">(</span><span class="re0">$stmt</span> <span class="kw1">as</span> <span class="re0">$row</span><span class="br0">)</span> <span class="br0">{</span> yield doSomethingWith<span class="br0">(</span><span class="re0">$row</span><span class="br0">)</span><span class="sy0">;</span> <span class="br0">}</span> <span class="br0">}</span>
Here rewinding would simply result in an empty iterator as the result set is already depleted.
For the above reasons generators will not support rewinding. The rewind
method will throw an exception, unless the generator is currently before or at the first yield. This results in the following behavior:
<span class="re0">$gen</span> <span class="sy0">=</span> createSomeGenerator<span class="br0">(</span><span class="br0">)</span><span class="sy0">;</span> <span class="co1">// the rewind() call foreach is doing here is okay, because</span> <span class="co1">// the generator is before the first yield</span> <span class="kw1">foreach</span> <span class="br0">(</span><span class="re0">$gen</span> <span class="kw1">as</span> <span class="re0">$val</span><span class="br0">)</span> <span class="br0">{</span> <span class="sy0">...</span> <span class="br0">}</span> <span class="co1">// the rewind() call of a second foreach loop on the other hand</span> <span class="co1">// throws an exception</span> <span class="kw1">foreach</span> <span class="br0">(</span><span class="re0">$gen</span> <span class="kw1">as</span> <span class="re0">$val</span><span class="br0">)</span> <span class="br0">{</span> <span class="sy0">...</span> <span class="br0">}</span>
So basically calling rewind
is only allowed if it wouldn't do anything (because the generator is already at its initial state). After that an exception is thrown, so accidentally reused generators are easy to find.
Generators cannot be cloned.
Support for cloning was included in the initial version, but removed in PHP 5.5 Beta 3 due to implementational difficulties, unclear semantics and no particularly convincing use cases.
When a generator is closed it frees the suspended execution context (as well as all other held variables). After it has been closed valid
will return false
and both current
and key
will return null
.
A generator can be closed in two ways:
return
statement (or the end of the function) in a generator or throwing an exception from it (without catching it inside the generator).
If the generator contains (relevant) finally
blocks those will be run. If the generator is force-closed (i.e. by removing all references) then it is not allowed to use yield
in the finally
clause (a fatal error will be thrown). In all other cases yield
is allowed in finally
blocks.
The following resources are destructed while closing a generator:
execute_data
)
$this
object.
EX(object)
).
foreach
loop variables which are still alive (taken from brk_cont_array
).
Currently it can happen that temporary variables are not cleaned up properly in edge-case situations. Exceptions are also subject to this problem: https://bugs.php.net/bug.php?id=62210. If that bug could be fixed for exceptions, then it would also be fixed for generators.
This is a list of generators-related error conditions:
yield
outside a function: E_COMPILE_ERROR
return
with a value inside a generator: E_COMPILE_ERROR
Generator
class: E_RECOVERABLE_ERROR
(analogous to Closure
behavior)
E_ERROR
(this is just a placeholder until Etienne's arbitrary-keys patch lands)
Exception
Exception
Exception
E_NOTICE
(analogous to return
behavior)
E_ERROR
(analogous to return
behavior)
E_NOTICE
(analogous to return
behavior)
This list might not be exhaustive.
You can find a small micro benchmark at https://gist.github.com/2975796. It compares several ways of iterating ranges:
xrange
)
RangeIterator
)
urange
)
range
)
For large ranges generators are consistently faster; about four times faster than an iterator implementation and even 40% faster than the native range
implementation.
For small ranges (around one hundred elements) the variance of the results is rather high, but from multiple runs it seems that in this case generators are slightly slower than the native implementation, but still faster than the iterator variant.
The tests were run on a Ubuntu VM, so I'm not exactly sure how representative they are.
A question that has come up a few times during discussion: Why not use callback functions, instead of generators? For example the above getLinesFromFile
function could be rewritten using a callback:
<span class="kw2">function</span> processLinesFromFile<span class="br0">(</span><span class="re0">$fileName</span><span class="sy0">,</span> callable <span class="re0">$callback</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">if</span> <span class="br0">(</span><span class="sy0">!</span><span class="re0">$fileHandle</span> <span class="sy0">=</span> <span class="kw3">fopen</span><span class="br0">(</span><span class="re0">$fileName</span><span class="sy0">,</span> <span class="st_h">'r'</span><span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span><span class="sy0">;</span> <span class="br0">}</span> <span class="kw1">while</span> <span class="br0">(</span><span class="kw4">false</span> <span class="sy0">!==</span> <span class="re0">$line</span> <span class="sy0">=</span> <span class="kw3">fgets</span><span class="br0">(</span><span class="re0">$fileHandle</span><span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span> <span class="re0">$callback</span><span class="br0">(</span><span class="re0">$line</span><span class="br0">)</span><span class="sy0">;</span> <span class="br0">}</span> <span class="kw3">fclose</span><span class="br0">(</span><span class="re0">$fileHandle</span><span class="br0">)</span><span class="sy0">;</span> <span class="br0">}</span> processLinesFromFile<span class="br0">(</span><span class="re0">$fileName</span><span class="sy0">,</span> <span class="kw2">function</span><span class="br0">(</span><span class="re0">$line</span><span class="br0">)</span> <span class="br0">{</span> <span class="co1">// do something</span> <span class="br0">}</span><span class="br0">)</span><span class="sy0">;</span>
This approach has two main disadvantages:
Firstly, callbacks integrate badly into the existing PHP coding paradigms. Having quadruply-nested closures is something very normal in languages like JavaScript, but rather rare in PHP. Many things in PHP are based on iteration and generators can nicely integrate with this.
A concrete example, which was actually my initial motivation to write the generators patch:
<span class="kw2">protected</span> <span class="kw2">function</span> getTests<span class="br0">(</span><span class="re0">$directory</span><span class="sy0">,</span> <span class="re0">$fileExtension</span><span class="br0">)</span> <span class="br0">{</span> <span class="re0">$it</span> <span class="sy0">=</span> <span class="kw2">new</span> RecursiveDirectoryIterator<span class="br0">(</span><span class="re0">$directory</span><span class="br0">)</span><span class="sy0">;</span> <span class="re0">$it</span> <span class="sy0">=</span> <span class="kw2">new</span> RecursiveIteratorIterator<span class="br0">(</span><span class="re0">$it</span><span class="sy0">,</span> RecursiveIteratorIterator<span class="sy0">::</span><span class="me2">LEAVES_ONLY</span><span class="br0">)</span><span class="sy0">;</span> <span class="re0">$it</span> <span class="sy0">=</span> <span class="kw2">new</span> RegexIterator<span class="br0">(</span><span class="re0">$it</span><span class="sy0">,</span> <span class="st_h">'(\.'</span> <span class="sy0">.</span> <span class="kw3">preg_quote</span><span class="br0">(</span><span class="re0">$fileExtension</span><span class="br0">)</span> <span class="sy0">.</span> <span class="st_h">'$)'</span><span class="br0">)</span><span class="sy0">;</span> <span class="re0">$tests</span> <span class="sy0">=</span> <span class="kw3">array</span><span class="br0">(</span><span class="br0">)</span><span class="sy0">;</span> <span class="kw1">foreach</span> <span class="br0">(</span><span class="re0">$it</span> <span class="kw1">as</span> <span class="re0">$file</span><span class="br0">)</span> <span class="br0">{</span> <span class="co1">// read file</span> <span class="re0">$fileContents</span> <span class="sy0">=</span> <span class="kw3">file_get_contents</span><span class="br0">(</span><span class="re0">$file</span><span class="br0">)</span><span class="sy0">;</span> <span class="co1">// parse sections</span> <span class="re0">$parts</span> <span class="sy0">=</span> <span class="kw3">array_map</span><span class="br0">(</span><span class="st_h">'trim'</span><span class="sy0">,</span> <span class="kw3">explode</span><span class="br0">(</span><span class="st_h">'-----'</span><span class="sy0">,</span> <span class="re0">$fileContents</span><span class="br0">)</span><span class="br0">)</span><span class="sy0">;</span> <span class="co1">// first part is the name</span> <span class="re0">$name</span> <span class="sy0">=</span> <span class="kw3">array_shift</span><span class="br0">(</span><span class="re0">$parts</span><span class="br0">)</span><span class="sy0">;</span> <span class="co1">// multiple sections possible with always two forming a pair</span> <span class="kw1">foreach</span> <span class="br0">(</span><span class="kw3">array_chunk</span><span class="br0">(</span><span class="re0">$parts</span><span class="sy0">,</span> <span class="nu0">2</span><span class="br0">)</span> <span class="kw1">as</span> <span class="re0">$chunk</span><span class="br0">)</span> <span class="br0">{</span> <span class="re0">$tests</span><span class="br0">[</span><span class="br0">]</span> <span class="sy0">=</span> <span class="kw3">array</span><span class="br0">(</span><span class="re0">$name</span><span class="sy0">,</span> <span class="re0">$chunk</span><span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span><span class="sy0">,</span> <span class="re0">$chunk</span><span class="br0">[</span><span class="nu0">1</span><span class="br0">]</span><span class="br0">)</span><span class="sy0">;</span> <span class="br0">}</span> <span class="br0">}</span> <span class="kw1">return</span> <span class="re0">$tests</span><span class="sy0">;</span> <span class="br0">}</span>
This is a function which I use to provide test vectors to PHPUnit. I point it to a directory containing test files and then split up those test files into individual tests + expected output. I can then use the result of the function to feed some test function via @dataProvider
.
The problem with the above implementation obviously is that I have to read all tests into memory at once (instead of one-by-one).
How can I solve this problem? By turning it into an iterator obviously! But if you look closer, this isn't actually that easy, because I'm adding new tests in a nested loop. So I would have to implement some kind of complex push-back mechanism to solve the problem. And - getting back on topic - I can't use callbacks here either, because I need a traversable for use with @dataProvider
. Generators on the other hand solve this problem very elegantly. Actually, all you have to do to turn it into a lazy generator is replace $tests[] =
with yield
.
The second, more general problem with callbacks is that it's very hard to manage state across calls. The classic example is a lexer + parser system. If you implement the lexer using a callback (i.e. lex(string $sourceCode, callable $tokenConsumer)
) you would have to figure out some way to keep state between subsequent calls. You'd have to build some kind of state machine, which can quickly get really ugly, even for simple problems (just look at the hundreds of states that a typical LALR parser has). Again, generators solve this problem elegantly, because they maintain state implicitly, in the execution state.
Andrew proposed to use a function-like syntax for yield
instead of the keyword notation. The three yield
variants would then look as follows:
yield()
yield($value)
yield($key => $value)
The main advantage of this syntax is that it would avoid the strange parentheses requirements for the yield $value
syntax.
One of the main issues with the pseudo-function syntax is that it makes the semantics of yield
less clear. Currently the yield
syntax looks very similar to the return
syntax. Both are very similar in a function, so it is desirable to keep them similar in syntax too.
Generally PHP uses the keyword $expr
syntax instead of the keyword($expr)
syntax in all places where the statement-use is more common than the expression-use. E.g. include $file;
is usually used as a statement and only very rarely as an expression. isset($var)
on the other hand is normally used as an expression (a statement use wouldn't make any sense, actually).
As yield
will be used as a statement in the vast majority of cases the yield $expr
syntax thus seems more appropriate. Furthermore the most common expression-use of yield
is value-less, in which case the parentheses requirements don't apply (i.e. you can write just $data = yield;
).
So the function-like yield($value)
syntax would optimize a very rare use case (namely $recv = yield($send);
), at the same time making the common use cases less clear.
The current implementation can be found in this branch: https://github.com/nikic/php-src/tree/addGeneratorsSupport.
I also created a PR so that the diff can be viewed more easily: https://github.com/php/php-src/pull/177