Can pattern methods be total ?

Tagir Valeev amaembo at
Mon Jan 25 16:54:59 UTC 2021


I think, marking a set of patterns as exhaustive (e.g.
Optional.of+Optional.empty) is on the radar. And saying that the set
of a single pattern is exhaustive is exactly the same as saying that
the pattern is total. So probably the same syntax should be reused and
no need to invent something special for 'total' marking.

With best regards,
Tagir Valeev.

On Mon, Jan 25, 2021 at 7:33 PM Remi Forax <forax at> wrote:
> The last document sent by Brian split patterns in two categories, deconstructor and method patterns.
> I would like to propose another way to see patterns, along the axis of partial vs total pattern.
> A total pattern is a pattern that always succeed and bind the bindings.
> A partial pattern is a pattern that may succeed or not and if it succeed bind the bindings.
> Currently Brian proposes that all destructors are total patterns and all method patterns are partial patterns.
> While i agree for the former, as a user I will be surprising if "instanceof Point(var x, var y)" can fail not because it's not a Point but because the deconstruction did not succeed.
> I disagree with the later because having total method patterns is useful, by example you may want to be able to see a Point in cartesian coordinates or in polar coordinates. You can not use a deconstructor here because in both cases the bindings are two doubles, so you need a name to disambiguate between them.
> Something like
>   class Point {
>     ...
>     total pattern [double, double] polar() { ... }
>     total pattern [double, double] cartesian() { ... }
>   }
> Here "total" means that the compiler can reject patterns with the same prefix that both use different total nested patterns on the same class. It also means that the compiler will enforce that body on the pattern method always "assign" values to the bindings (also allowing infinte loops and throws).
> If we go in that direction, we also need to introduce a rule for overriding, if a pattern method is declared total, all its overriden methods must be marked total (enforced by both the compiler and the VM).
> So if a pattern method is not total it can be overriden by a pattern method marked total but the reverse is not possible.
> Note that in term of compilation strategy this may requires a bridge method so we may decide to have a stricter rule, i.e. a pattern method that override a pattern method must have the same "totality".
> Another reason to know if a pattern is total or not is when we will introduce patterns at local variables declaration site.
>   Point(var x, var y) = point;
> I think only total patterns should be allowed here.
> So
>   Point.polar(var x, var y) = point;
> is ok but
>   Optional.of(var value) = optional;
> is not, because optional can be Optional.empty().
> So can pattern methods be total ?
> Rémi

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