[Bug 20019] Support subclassing ES6 Map

Tab Atkins Jr. jackalmage at gmail.com
Tue Nov 20 12:30:04 PST 2012

On Tue, Nov 20, 2012 at 10:57 AM, Mark S. Miller <erights at google.com> wrote:
> [+es-discuss]
> Speaking only for myself at this point -- I do not recall MultiMap
> previously being suggested to the committee.
> I think adding a MultiMap API to ES7 is a good idea. Neither Map nor
> MultiMap should be a subclass of the other, since neither is an LSP
> subtype of the other.

When properly designed, as long as you interact with it only through
Map methods, a MultiMap can be an LSP subtype of Map.

This means that get() should grab the first value, if one exists, or
undefined otherwise, while a separate method (getAll?) returns a
(potentially empty) list of all values.  set() should take only a
single value, and wipe out all other values currently attached to the
key.  (Or, be n-ary and *replace* values at corresponding indexes.
Given get()'s behavior, this is behaviorally indistinguishable from a
plain Map, even if multiple values already exist on a key.)  delete()
should wipe out all the values for a given key. (Plus, perhaps have a
2-arg version which specifies a value to kill.)

The tricky part is dealing with .size and the iterator methods.  You
need .size to reflect the number of keys, not pairs, to be consistent
with Map.  But then .size doesn't match the length of the iterators
returned by .items() or .values(), unless both of these are changed to
only return the first value by default.  (They can't return an array
of values, because that's not what Map does.)

> Since Map and Set will be in ES6 and MultiMap is trivially
> implementable from these, we can wait until we see some experimental
> implementations before standardizing. Hence the ES7 target.

Sure.  It just means that we'll have legacy in the DOM already as well.

> The issue of subclassing built-in types in general is interesting and
> important. Whatever we do for this in general, we should not need to
> make any special case for Map and MultiMap. In general, new built-in
> abstractions should act as much possible as if they were implemented
> by an ES6 class and exported by an ES6 module. (Likewise for old
> build-in abstractions, but less will likely be possible for these.)



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