On Nov 11, 2011, at 11:07 AM, Allen Wirfs-Brock wrote:
> On Nov 11, 2011, at 10:52 AM, Axel Rauschmayer wrote:
>
>> Got it, related to what you solve in generic classes with This-types
>> (covariance vs. contravariance...).
>>
>> // assume |this| is the source collection
>> if (this[Array.derivedArrayKey]) A = this[Array.derivedArrayKey](); else A
>> = [ ];
>>
>> Another possibility:
>>
>> if (this[Array.derivedArrayKey]) {
>> A = this[Array.derivedArrayKey]();
>> } else if (typeof this.constructor === "function") {
>> A = new this.constructor();
>
> In Smalltalk the default implementation of species is:
> species
> ^self class
>
> which is pretty much the moral equivalent of: this.constructor.
>
> However, that might introduce backwards compatibility issues for existing
> code which has constructors but currently always produces Array instances
> from these functions.
Right, this was what Alex Russell and I realized and sighed about, after first
hoping we could use this.constructor.
>
>> } else {
>> A = [ ];
>> }
>>
>> Creating a subclass this way (i.e., using @derivedArrayKey) produces code
>> whose meaning is more obvious (to me) than the code below.
>
> I just wanted to dash out the most directly extensions of Jake's original
> code that would exhibit the concept. Ultimately, it's a matter of defining a
> method using which ever formulation you prefer (and which gets adopted into
> the language)
We should probably think about a systematic naming convention (and location
convention) for these built-in private name objects.
/be
>
>>
>>>>> function createArraySubclass(proto,...values) {
>>>>> return proto <| [...values].{
>>>>> [Array.derivedArrayKey](){return proto<| [ ]}
>>>>> }
>>>>> }
>
> Allen
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