Existential Operator / Null Propagation Operator

Herby Vojčík herby at mailbox.sk
Tue Apr 7 19:09:32 UTC 2015



Kevin Smith wrote:
>
>  > Plus, it can be used for "normalizing" null/undefined to undefined:
>  >
>  >  var normalizedFoo = ?foo;
>  >
>  > Seems sort of nice that it is separated and there are no special
> operations for ?., ?(, ?[.
>
> I agree, that is nice.  But how does Nil get transformed into undefined?

While you do operations like call, construct, get on the reference 
(obtaining another reference), it shortcuts to return Nil. Whenever you 
are not in position to shortcut ref-to-ref (end of expression, for 
example), and you actually needs a value, it just converts to undefined.

?a.b()['foo'] => "smalltalk-like"
   (((('a' asRefIn: env) nilRefIfValueNullOrUndefined "ref, may be nil"
   at: 'b') "returns ref, may be Nil"
   callWithArguments: #()) "returns ref, may be Nil"
   at: 'foo') "returns ref, may be Nil")
   value "now, value is needed, ref needs to dereference"

Hopefully this sheds some light. If not, then I don't know how to 
explain it someone with better pedagogy skill must weight in.


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