New exception classes

[marcandre]

Prefer the use of exceptions from the standard library over introducing new exception classes.

I feel this is not quite accurate.

Indeed, a gem should use standard library exceptions in case the gem is called with invalid inputs (e.g. TypeError if called with an Integer instead of a String, ArgumentError, IndexError, etc.). Typically these are never rescued.

But when something proper to the gem happens, instead of raising a RuntimeError it should have its own subclasses of StandardError for that gem (with potentially a base class for the gem). E.g. ::Parser::SyntaxError, ::Parser::ClobberingError. Typically these may be rescued by the dependents of the gem.

May I amend the guide?

[bbatsov]

The intended meaning is to prefer the standard library exceptions when introducing custom ones won't really add any benefits.

Indeed, a gem should use standard library exceptions in case the gem is called with invalid inputs (e.g. TypeError if called with an Integer instead of a String, ArgumentError, IndexError, etc.). Typically these are never rescued.

File IO exceptions also come to mind (e.g. IOError). That being said - when the standard library doesn't cut it, it should be obvious one needs to introduce additional exception types.

In general I agree that the guideline can be improved, so feel free to take a stab at it.