Why not allow mutation of the this binding?

Question

Hi Everyone,

I'm building a interpreter/compiler for a school project (well now its turning into a hobby project) and an instructor warned me not to allow mutation of the 'this' binding (he said it was gross and made a huge deal about it) but I never learned why this is so... dangerous or bad. I'm very curious about why this is so bad. I figured this sort of feature could be useful in some way or another.

I'm wondering if anyone familiar with building languages can tell me what sort of problems mutation on the 'this' binding can cause, and if they know of any cool or useful tricks that one could do if it actually was allowed.

Do any languages that you're aware of allow mutation of 'this'?

Thanks,

Answer 1

I can think of several reasons why it would be a bad idea.

1) 'this' is a pointer to the object instance on which the method call is invoked. Allowing changes to it could lead to memory access validations in the worst case.

2) The caller expects the method to work on the instance on which it was invoked. Changing 'this' to something else would produce unexpected results.

3) I can't think of anything that mutation of 'this' would allow that can't be achieved in a more standard, familiar way.

Answer 2

I'm surprised your instructor got so het up. It's just a change in the language definition. I don't think being able to change the meaning of this is useful or good language design, but I think as long as you stick to reference semantics it's a valid experiment.

The main reason not to allow this to refer to anything other than the receiver of the current method is that you will confound the expectations of anyone who has ever read or written an object-oriented program. That's not good design.