[pattern-switch] Opting into totality

[forax at univ-mlv.fr]

----- Mail original -----
> De: "Guy Steele" <guy.steele at oracle.com>
> À: "Remi Forax" <forax at univ-mlv.fr>, "Brian Goetz" <brian.goetz at oracle.com>
> Cc: "amber-spec-experts" <amber-spec-experts at openjdk.java.net>
> Envoyé: Vendredi 4 Septembre 2020 04:24:24
> Objet: Re: [pattern-switch] Opting into totality

> There is currently a fundamental unpleasant asymmetry caused by the arbitrary
> decision to require that all switch expressions be total but not all switch
> statements be total.
> 
> As we have added other options, the design space still has an unpleasant
> asymmetry (or lack of orthogonality) because of that original decision.
> 
> We have two choices: (a) recant that original decision, or (b) live with the
> asymmetry.
> 
> As I have already pointed out, (a) is certainly possible and completely
> consistent; all you have to do is return default values for the cases that are
> not covered.  Then the choice of switch-versus-expression is completely
> independent of the choice of regular-switch versus total-switch.  The likely
> result is that authors and IDEs will warn programmers that they should NEVER
> EVER use a non-total switch expression, so it seems silly to expand the design
> space to provide a combination of features that should never be used.  (“What,
> never?”  “Well, hardly ever.”)
> 
> The other choice, which is what Rémi and Brian have been discussing for the last
> week, is not whether to get rid of the asymmetry, but merely debating exactly
> what its shape should be.

yes

> 
> A possibility I don;’t think I have yet seen offered is that we should, as
> before, require switch expressions to be total, but also, in an incompatible
> move, require switch expressions to use whatever `total-switch` syntax is
> adopted.
> 
> In such a scenario, perhaps we really are trying to move the world to `snitch`
> after all, and we’re just arguing about the least disruptive spelling for it.

yep, it's a snitch move, but instead of calling it "when" or whatever,
we use a hyphenated keyword like "total-switch", "sealed-switch" or double words keyword like "sealed switch".

I think i'm not at ease with this proposed change because it goes against the design idea that leads to the current semantics of the switch expression,
the fact that a statement switch is partial while an expression switch is total.

> 
> —Guy

Rémi