valhalla-dev Digest, Vol 7, Issue 66

Thomas W at
Tue Jan 20 01:15:19 UTC 2015

Hi Gavin, Maurizio, Brian,

[Resent from private, thanks Maurizio:]

"Meet rule" is potentially required for this idea of OO dispatch due to the
"open" nature of the subtype/ method declaration universe. By contrast, I
am assuming that specialization space must be completely defined &
fulfilled in each & every method implementation.

No interaction between separate partial spatial coverages, therefore no
need for a meet rule.

[Gavin said:]
> The “meet” terminology comes from maths: given a partially ordered set S,
the meet of a subset of S is the greatest lower bound (if it exists).
> In OO type systems, it is typically used in the context of overloading:
e.g. if you have overloads whose argument types have a common lower
> bound (i.e. a common subtype) then you must have an overload with
argument types of that common lower bound.
> For example, Fortress:

[Brian said:]
> Except that's exactly how method overload selection works in Java today
[1]; by contrast, there are no left-to-right precedence mechanisms.
> So while you might consider the left-to-right rule "better", when taken
in the context of how the language already works (and how developers
> I think this has to be rejected.

This may be conflating two separate issues.  My focus would be on
circumscribing matching into a well-defined & explicitly dimensioned space,
which can be done -- making it a well-structured construct with a "closed
space" which can thus be evaluated/determined independently, rather than an
"open space" where interaction with other declarations/ constructs becomes

The single-dimensional example would be 'switch' statements -- 'case'
clauses live within their 'switch' container, and do not interact.

The other issue is to use a precedence rule within a single system, rather
than a "meet rule" between systems. L-to-R most specific was merely the
first suggestion off the top of my head. Other matching precedence rules
(such as the current Java method resolution rules) would obviously be worth


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