The guidelines for Ruby are to add a bang (!) to any methods that mutate the object rather than returning a new one. That's not strictly followed, though, but for most of the commonly used standard library bits, you can be fairly certain that that is the case.
In practice, even in the standard library, this isn't followed often enough to rely on. Here's a (possibly incomplete, since I'm writing this on the fly) list of bangless mutating methods just from Array:
As an even more extreme example, IO contains precisely one bang-method, despite the fact that probably 75% of IO's instance methods are destructive.
The general rule seems to be that if there's a mutating and non-mutating version of the same method, the mutating one will get a bang, but when there's a mutating method with no counterpart, it might get a bang but probably won't.
The guideline is: if your method does something that the programmer should think twice about or shouldn't use without proper knowledge (e.g. didn't read the docs), use !. An incomplete case of usages:
- There is a safer alternative (e.g. mutating vs. non-mutating or skipped validation)
- It should only be called once in a process (e.g. Padrino.start!)
- It is non-reversible (many statemachine libraries use action! as the way to invoke state transitions, which might not be reversible)
This doesn't mean that every method needs to be suffixed by ! if it does something destructive. `delete` in the context of an ORM is standard, so it doesn't have a bang. The whole point of `pop` is to manipulate the receiver: no point in warning about it. IO is always destructive, so ! doesn't make sense either.
