## What happens during begin; return; ensure?

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I was looking into the internals of a gem, and ran into the following pattern:

def foo
begin
return 'foo'
ensure
puts 'bar'
end
end
foo


This will print the strings 'foo' and 'bar' to STDOUT. How does this happen? My naive understanding would be that when you encounter a return statement, then you pop the call off the stack and return the value specified by return. I would imagine that if this were the case, then you would not then be able to execute the ensure block. Obviously the ensure block is executed. I'm curious as to how this is happening. How is this behavior implemented?

This is just the way it's defined in MRI, more specifically in the function rb_ensure in eval.c (I snipped the actual implementation, but essentially it just generates a Ruby execution context, executes the block in that context and then makes sure that all ensure blocks are run one after the other:

* Equivalent to <code>begin .. ensure .. end</code>.
*
* Calls the function \a b_proc with \a data1 as the argument,
* then calls \a e_proc with \a data2 when execution terminated.
* \return The return value of \a b_proc if no exception occurred,
*   or \c Qnil if otherwise.
* \sa rb_rescue
* \sa rb_rescue2
* \sa rb_protect
* \ingroup exception
*/
VALUE
rb_ensure(VALUE (*b_proc)(ANYARGS), VALUE data1, VALUE (*e_proc)(ANYARGS), VALUE data2)
{
...
}


If you're interested in how the parser decides to call this faction, look at parse.y it's part of the rules for bodystmt.

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when you encounter a return statement then you

When MRI encounters the return statement, it does something. Some interpreters might print "FOO" to the printer; that is located in your boss’ room :)

pop the call off the stack and return the value specified by return

Almost. When MRI encounters the return statement, it calls the ensure block if it exists and then pops the call off the stack.

The thing is there is no mandatory rule that return must do something special, graved in stone. Ruby interpreter (and some others) does it that way.

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return assigns a value to the method, even if the execution goes trough ensure block.

For example:

def foo
begin
puts txt = 'foo'
return txt
ensure
puts 'bar'
end
puts txt = 'baz'
return txt
end

p foo


Puts "foo" and "bar" but not "baz". Also foo method returns the string "foo".

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• That is how ensure is designed to work. That is the whole point. Are you expecting particular implementation details of ensure? That probably depends on the Ruby implementation.
• The question of the day: what would happen if you return from ensure?