I don't necessarily mind if we get to a world where member patterns can
be total. (On the other hand, I don't mind if we never do, either.)
The "dtors are total, others are partial" is a lump move, which is a
good-enough-for-now-and-maybe-forever approximation. If we are
considering allowing totality to be denoted elsewhere, adding the
ability to modify patterns later as total is an entirely reasonable
option to preserve, and I don't think we've closed that off.
That said, I don't think that means we can shift the axes as you
suggest. Deconstructors are different from instance patterns in far
more ways than just being total:
- Deconstructor patterns are not inherited
- Deconstructor patterns do not have names
- Deconstructor patterns cannot be overridden, or abstract
- Deconstructor patterns cannot have input parameters
Only the last still makes sense if you s/deconstructor/total/.
Constructors are special in the language, for better or worse; their
duals are too.
Now, you might say that the word "deconstructor" is fussy and that you
prefer to use pattern without a name/arg list:
pattern(BINDINGS) { ... } // dtor
pattern(BINDINGS) name(ARGS) { ... } // method pattern
That's bikeshed stuff, but its a possibility to discuss once the model
is nailed down. (The word "deconstructor" would be more palatable if we
used "constructor" for constructors, rather than the class name. The
C++ syntax "~C()" (un-C) would actually be good for deconstructors if it
weren't polluted by C++.)
We need to define override and method selection rules anyway; existing
rules for methods take modifiers into account (can't override instance
with static, or public with private), so totality constraints fit pretty
easily.
On 1/25/2021 7:32 AM, Remi Forax 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
