Can a PHP Generator Recover From an Uncaught Exception? - php

Question

Consider this simple generator function in PHP.
function createAGenerator() {
echo 'Before First Yield',"\n";
yield 'First Yield';
echo 'Before Second Yield',"\n";
yield 'Second Yield';
echo 'Before Third Yield',"\n";
yield 'Third Yield';
}
If I throw into this generator using the generator object's throw method
$generator = createAGenerator();
try {
$generator->throw(new Exception('Throwing into a Generator'));
} catch(Exception $e) {
echo 'Caught Exception: ', $e->getMessage(), "\n";
}
echo 'Resuming Main Program Execution',"\n";
the generator function will re-throw the exception for me to catch. This all works as I expect.
However -- my generator now seems permanently stuck. If I try to move on to the next yield, or send a new value in, the generator appears to just return NULL. For example, the following program
<?php
function createAGenerator() {
echo 'Before First Yield',"\n";
yield 'First Yield';
echo 'Before Second Yield',"\n";
yield 'Second Yield';
echo 'Before Third Yield',"\n";
yield 'Third Yield';
}
$generator = createAGenerator();
try {
$generator->throw(new Exception('Throwing into a Generator'));
} catch(Exception $e) {
echo 'Caught Exception: ', $e->getMessage(), "\n";
}
echo 'Resuming Main Program Execution',"\n";
var_dump($generator->send('Some Value'));
var_dump($generator->current());
var_dump($generator->next());
var_dump($generator->current());
Returns the following output.
Before First Yield
Caught Exception: Throwing into a Generator
Resuming Main Program Execution
NULL
NULL
NULL
NULL
Is there a way for a generator to recover from this? Or does an uncaught exception in a generator "break" this current instance of the generator?

Answer 1

When you throw an exception, the generator will skip right to the end of the function. This is exactly what happens in a regular functions, if that function called something that threw an exception.
Your two options are:
To catch it in the generator (or use finally).
To not throw it at all
The idea of throwing an exception and use it as a "temporary" messaging system but resume normal operation is a bit weird. It sounds like a bit of an abuse of exceptions. Perhaps what you are trying to achieve can be accomplished without throwing an exception.
If you're trying to model an operation that has multiple steps and each can individually fail or succeed, one option might be to just yield the exception and not throw it.