Next up for patterns: type patterns in switch

[Guy Steele]

> On Aug 13, 2020, at 8:19 AM, forax at univ-mlv.fr wrote:
> 
> . . .
> 
> I wonder if we find it natural only because we are used to use the keyword "default" inside a switch, . . .

I think that may be so; but given that it is so, I am happy to exploit that fact!

> I think i prefer using "default" (or any other keyword) only where it makes sense and doesn't allow "default" to be propagated.
> so
>   default Pair p: ...
> is ok but 
>   default Pair(Box(Frog f), Bag(Object o)): …
> should be written
>   case Pair(Box(Frog f), Bag(default Object o)): …

I think you intended that last line to read

	  case Pair(Box(default Frog f), Bag(default Object o)): …

and if so, I agree that this may be a better way to write it in the context I originally gave:

	switch (x) {
		case Pair(Box(Tadpole t), Bag(String s)): …
		case Pair(Box(Tadpole t), Bag(default Object o)): …
		case Pair(Box(default Frog f), Bag(String s)): …
		case Pair(Box(default Frog f), Bag(default Object o)): …    // I originally had "default Pair(Box(Frog f), Bag(Object o)): …” here
	}

But either way works, because of the subtle fact that if P: Pattern T, then Q is total over type T if and only if P(Q) is total over type P, so one can choose, on purely stylistic grounds, whether to use the “default” tag at the root of a pattern subtree that is total, or at all the relevant leaves, or for that matter at a relevant set of interior subtrees.