Fix REPL assignment

[xixixao]

Is no one ever using the REPL? The variable was never documented, but Node does this on its own, so you can access the last expression with both _ and __. No more annoying messages!!!

[GeoffreyBooth]

I don’t understand what’s going on here. See #4528, which you were a part of too.

We’re assigning to _ explicitly because Node does, in order to override it. Why would we assign to __?

[lydell]

So … the Node.js REPL has this feature that _ always refers to the value of the last expression, kind of like the “Ans” feature on some calculators (if you’ve ever used one of those).

As mentioned in a comment in the code, we add _ = before the inputted CoffeeScript code to force it to be an expression. So a variable name had to be chosen there. I guess _ was chosen since the Node.js REPL would clobber that variable anyway. However, newer versions of Node.js added the warning message whenever _ was assigned to.

So this PR simply uses another name (__) for the force-expression-assignment.

Everything works as before, with the exception that __ will now always be clobbered and we finally get rid of that annoying message.

I just tried this PR and it seems to be working fine:

$ ./bin/coffee
coffee> 1 + 1
2
coffee> _
2
coffee> __
2
coffee> 

[GeoffreyBooth]

Okay, @lydell if you think it’s okay then I’m fine with it. What about tests?

[lydell]

I’m not sure there very much to test? And cake test passes locally on this branch.

[GeoffreyBooth]

A test that __ does, in fact, pull the last value. Or maybe we don’t care about this enough to bother?

[lydell]

If @xixixao feels like writing a test then go for it, but if not it shouldn’t hold up this PR. There was no test before either.

[xixixao]

I'm not bothered.

[nigon135]

I'm not bothered