Nerd Sniping: What does the code do at the end of the page?(neil.fraser.name)

26 pointsby RiderOfGiraffes17 anni fa32 comments

neil.fraser.name

Nerd Sniping: What does the code do at the end of the page?

http://neil.fraser.name/news/2009/10/27/

11 comments

It returns false (also in Ruby) --- the "finally" (or "ensure") clause runs after the function body, but before the function epilogue.

It's not really a "paradox" so much as it is very bad style to "return" from a "finally".

Edit

My partner Dave has a much better trick question:

    def foo
        begin
            return false
        ensure
            next
        end
    end

"You win", I told him. "Nah, Google won. But nobody really wins if you write code like that."

wvl17 anni fa

Here's a good article that explains what happens: http://blog.leshill.org/blog/2009/11/17/ensure-with-explicit...

ahlatimer17 anni fa

The last trick question doesn't work in Ruby, at least not Ruby 1.8.7. Gives me a "LocalJumpError: unexpected next".

I realize that's not the point, but I figured I'd mention it anyway.

[deleted]17 anni fa

notauser17 anni fa

Huh, interesting. The code supplied is:

try { return true; } finally { return false; }

Which returns false. I guess this is because of the guarantee that the finally block will be run, no matter what. The JS interpreter must be catching the return and holding it pending the execution of the finally block.

Checking with

try { return true; } finally { alert('hi'); }

Shows this to be true. (It both returns true AND alerts 'hi'.)

pkulak17 anni fa

I see it that the "finally" is run right before the return. So, the statement "true" would be evaluated, then control would go to the "finally" block, where the method would return false.

pinstriped_dude17 anni fa

Its a tricky question, but not something I would use to find a "keeper". From the Java tutorial - "The finally block _always_ executes when the try block exits." After the interpreter executes the "return true;" statement, it exits the try block. And then, if you know that "The finally block _always_ executes when the try block exits.", you know that the code block will return false;

blasdel17 anni fa

It's not the answer that makes this a "keeper" screen, but the candidate's progression of facial expressions.

deekayteez17 anni fa

in .NET its illegal - "Control cannot leave the body of a finally clause"

Jupe17 anni fa

It appears MS thought this through. The contrived example is but ONE way a mediocre programmer could really munge this stuff up if the language is not specific about protecting exit points from a finall{} block.

echaozh17 anni fa

What about throwing exceptions in the finally block? Isn't it an exit point too?

When you call non-trivial 3rd party code from your finally block, how do you ensure it doesn't throw? Or what do you do if it does? Indefinitely nested try blocks?

nollidge17 anni fa

Indeed. I can't think of a reason as to why any language would even allow it.

Jach17 anni fa

I know it's horrible in practice, and I really can't see a valid use for this particular case, but forbidding things like this (or other things like multiple inheritance) just because it can be abused has never seemed like a good idea to me.

nollidge17 anni fa

It's not just that it can be abused, though - every use of it is an abuse. Finally{} is not for decisions and manipulating values and program logic, it's for cleaning up resources.

bantic17 anni fa

I wonder about the merits of this sort of thing as an interview screen, though. There's not necessarily a strong correlation between knowledge of language esoterica and other more important qualities like perseverance and raw intelligence.

xsmasher17 anni fa

The test is not whether you know the correct answer, but whether your brow furrows when you read the code. If you don't find the snippet interesting you fail.

The ultimate screen would be to follow the candidate home and see if they Google or test the answer.

blasdel17 anni fa

The point is not to assess language knowledge, but disposition. Even if the candidate is unamused, they should still instantly get what the interviewer found punny about it.

zephjc17 anni fa

it's not so mysterious if you know what the intent of a 'finally' block is.

tptacek17 anni fa

I agree; the trickiest thing about it is the confusing relationship between the syntax for exception handling and the syntax for cleanup blocks, which are only vaguely related concepts.

akkartik17 anni fa

That code uses one of several gotchas in java: http://www.cs.arizona.edu/projects/sumatra/hallofshame

lurkinggrue17 anni fa

Error: Norman coordinate.

j_baker17 anni fa

It's also worth noting that this won't work in Python before 2.5 (or maybe 2.4).

josephkern17 anni fa

You mean after 2.5 right?

try: return True finally: return False

-- There's an error in your program: * 'return' outside function ...

Just sayin'

paulbaumgart17 anni fa

It works if you put it in a function (2.5.1):

  >>> def test():
  ...  try: return True
  ...  finally: return False
  ... 
  >>> test()
  False

Jach17 anni fa

2.6.2 as well. I liked this little snippet, though I grumbled a little on the inside and would be appalled if I ever found something like that in serious code...

kevindication17 anni fa

In 2.3 it does. Why wouldn't it?

j_baker17 anni fa

I swear I read that this behavior changed somehow in Python 2.5. But I'm not sure how. But it does at least appear to work in python 2.4.

kevindication17 anni fa

In 2.5 they made try/except/finally possible.

dpcan17 anni fa

A couple things can happen I guess, it depends on the interpreting language.

huhtenberg17 anni fa

It's just a dumbed down version of Duff's device :-)

docmach17 anni fa

How is this related to Duff's device?