On 17 January 2013 18:00, Mark S. Miller <[email protected]> wrote:
> I still have this position on classes. But I no longer buy that
> pessimistic conclusion about WeakMaps. Consider how WeakMaps would be
> used by the expansion of classes-with-private. Just 'cause it's on the
> top of my head, below I use the old representation of one WeakMap per
> class providing access to a record of all the private state. For the
> same reason, I'll use the encapsulation of the Purse example without
> any of the numeric checks.
>
> class Purse {
> constructor(private balance) {
> getBalance() { return balance; }
> makePurse() { return Purse(0); }
> deposit(amount, srcPurse) {
> private(srcPurse).balance -= amount;
> balance += amount;
> }
> }
Hm, I'm afraid I don't fully understand that example. There seems to
be a missing closing brace for the constructor, and I don't know what
the free occurrences of 'balance' are referring to. Also, the second
line of the deposit function seems to be missing in the expansion.
>
> expansion
>
> let Purse = (function() {
> let amp = WeakMap();
> function Purse(balance) {
> amp.set(this, Object.seal({
> get balance() { return balance; },
> set balance(newBalance) { balance = newBalance; }
> }));
> }
> Purse.prototype = {
> getBalance: function() { return balance; },
> makePurse: function() { return Purse(0); },
> deposit: function(amount, srcPurse) {
> amp.get(srcPurse).balance -= amount;
> }
> }
> return Purse;
> })();
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