Switch attribute for unique and/or full Cases

[awygle]

It would be useful to be able to indicate that a Switch should cover all possible Cases, or at least that the given Cases should not overlap.

[jeanthom]

Somewhat related to #404 and #436

[whitequark]

Yeah, I think all of these should be implemented as lints with configurable severity.

[awygle]

I actually disagree that these should be lints. To my mind, a lint would make sense if the property we were looking for wasn't wholly contained to a particular language construct. In this case, the semantics of Switch(mode="full") vs Switch(mode="unique") vs Switch(mode="priority") (syntax descriptive only at this time) are both well-defined and entirely contained to the Switch context manager. Can you explain on your thinking as to why these would be lints?

[whitequark]

Can you explain on your thinking as to why these would be lints?

Two reasons:

1. Lints are something that can be removed without changing the semantics of the program at all. Not everything that fulfills this requirement must be a lint, but lints are generally more lightweight, so I default to those.
2. In my mind, people will want something along the lines of "please ensure all of the cases in my program/file/module are full", and lints would need to have a mechanism for inheriting them from outer scopes.

[awygle]

I see. I agree that 2 is a good reason to have a lint. I'll sit and think a bit about whether I want a lint also or a lint instead.

For 1, what do you mean by "lightweight"? Syntactically, at least the way I'm imagining them working (and yes, I know we haven't designed them yet), from a user perspective using a lint would be heavier-weight than passing a constructor argument (not to mention adding rightward lean), but maybe you have a different mental picture.

[whitequark]

For 1, what do you mean by "lightweight"?

In the sense that it doesn't introduce changes to the core language, not in terms of end-user syntax. I agree that syntactically a lint can end up being heavier.

[whitequark]

We discussed this on the meeting. The conclusion was that we want both the Switch-specific attribute (syntactically a kwarg) and a lint, because they serve different goals: Switch(mode="unique") explicitly declares intent for this specific Switch, whereas enabling a lint that warns (or errors) on non-unique switches implicitly analyzes your code for suspicious constructs in general.

Also, without the kwarg, disabling the lint for just a single Switch that may contain nested Switches introduces extreme rightward drift of 2 indents, which would likely significantly detract from the lint's usability. In other words, the kwarg completes the lint instead of competing with it.