A different semantics for WeakMap#get default value

Mark S. Miller erights at google.com
Thu Jan 19 09:08:43 PST 2012

On Wed, Jan 18, 2012 at 11:37 PM, Andreas Rossberg <rossberg at google.com>wrote:

> On 19 January 2012 07:51, Mark S. Miller <erights at google.com> wrote:
> > Everyone on this thread, is there any need expressed in this thread that
> is
> > not satisfied by InfiniteMap?
> Looks good, except that I don't understand why you want a lazyFactory
> function instead of a simple initialValue -- if you recommend
> lazyFactory having no observable side effect, then (with no arguments)
> it will be a constant function, and you could as well use a plain
> value.
> On the other hand, a function would be useful if it also got the key
> as an argument, so that it can manufacture values in a key-dependent
> manner.

I think providing the key, or the key and defaultValue as Andrea suggests
is a fine idea. However, even without these, the answer to your question is
"fresh mutable state" as in

    function lazyFactory() { return Set(); }
or similarly
    function lazyFactory() { return []; }

This addresses the only concrete use cases I've seen mentioned in this
thread -- using nested single-key single-valued collections to emulate

a) multi-valued collections, i.e., MultiMaps, as single-valued collections
from keys to sets of values, or

b) multi-keyed collections, i.e., a two dimensional map, as a map from a
first key to a collection mapping from a second key (array index) to a

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