Fix allowing arithmetic expressions without spaces and no need for extra operator e.g. "4+-5". "4-5" works.

[harrybits]

Hello @arothuis! I realize this project is a small demonstration of ANTLR4, but I was interested in expanding the calculator's function and became annoyed with the strict formatting of the input. For example, I wanted to enter "6-4" instead of "6 + -4". No spaces to worry about. This PR accomplishes that objective. The trade-off is a slight muddling of your clear demo code.

This can be considered a hack, but it didn't seem too egregious and the benefit is nice. Also, a discussion of the fix could be beneficial to new users of ANTLR. Let me know what you think.

There are a few more minor edits to address compilation warnings.

[arothuis]

Hi there!

First of all, thanks for your work, your tests and your clear explanation. I see that the same issue has been reported by someone else here (#3), but I have only recently re-enabled notifications.

I'm not a fan of adding an expression to fix whitespace issues.
The problem we're seeing here is not about incorrectly handling spaces, it is rather about how I have chosen to define tokens and expressions in the grammar.

A more elegant approach could be as follows:

• define the NUMBER token to be positive only in the grammar
• add an expression for negation, i.e. '-' right=expression # Negation
• implement correct behaviors in CalculationListener::exitNegation (e.g. - popped) and CalculationVisitor::visitNegation (e.g. - whatever is on the right)

This approach would also allow -(1 + 2) to work.

Would you like to try this approach?
If not, I'm down to fix this whenever I have some time this week.
I'll create an issue for some other fixes that would make this a better example (#5).

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Context about this project:
I created this project to learn working with ANTLR so that I could point my students in the right direction. I have never worked with ANTLR and language grammars before, so mistakes were to be expected. Sadly, the assignment to work with ANTLR has been dropped from the curriculum, so I had no reason to update this example. I have contemplated archiving this repository, but I'd rather fix it so we have a decent example for people to study.

[harrybits]

I prefer the negation strategy. It's cleaner and more logical. I'll spend some more time on this for posterity.

[harrybits]

Implemented @arothuis 's strategy outlined in his comment above. Dropped my last commit containing the previous fix.

[arothuis-hu]

Nice! Thanks for all the work.
Merging!