On Jun 19, 2012, at 10:31 AM, Brendan Eich wrote:
> Brendan Eich wrote:
>> Allen Wirfs-Brock wrote:
>>> I still stand by my alternative semantics as being a way to address the
>>> more important use case without have ?()
>> No, the basis cases differ and that is a bug in your alternative.
>>
>> foo?.bar means take bar from foo or undefined if foo == null.
>>
>> foo?.bar() means take bar from foo and throw if foo == null else try to
>> invoke the result if typeof "function", else undefined.
>>
>> This is an off-by-one bug.
>
> It also uses ?. to mean . and magically applies the ? to the later ().
>
> Or if the semantics suppresses a TypeError both for the "take bar" and
> "invoke it" parts, then it's mixing two things in one under syntax located in
> the wrong place for the second suppression. Mainly, suppression should be
> precise and at the point of application -- where the dot goes if getting or
> setting, where the left paren goes if invoking.
I don't agree, probably because I'm look at ?. as an operator that evaluates to
a special kind of Reference. Like with References in general, it is the
ultimate consumer (usually via GetValue/SetValue, but not always) of the
Reference that actually access the value or throws an error if appropriate.
If we say that the value Missing represents a Reference to a known to be
non-existant value, then the semantics of LHS?.prop can be summarized as:
LHS ==> Result
-------------------------------------------
Unresolvable Ref Missing
undefined/null Missing
Missing Missing
other non-Ref ConditionalRef(GetValue(LHS),prop)
Reference Missing or
ConditionalRef(GetValue(LHS),prop)
(both normal and Conditional)
This differs from LHS.prop whose corresponding table would be
LHS ==> Result
-------------------------------------------
Unresolvable throw
undefined/null throw
Missing throw
other non-Ref Reference(GetValue(LHS),prop)
Reference throw or Reference(GetValue(LHS),prop)
(both normal and Conditional)
In addition, GetValue(V) is extended with these cases:
V ==> Result
-------------------------------------------
Missing undefined
ConditionalRef undefined if referenced property doesn't exist,
otherwise same as normal Reference
Just like with all References, it is the consumer of a
Missing/ConditionalReference value that determines what to do with. Most
consumers unconditionally call GetValue however, some like typeof and delete
explicitly deal with the Reference in some other manner.
All I have done is add function call to the list of operators that have some
Reference sensitive semantics. It would have special handling for
Missing/ConditionalRefs. I could also add "Missing" semantics to delete, but
it really isn't very important there.
>
> Also, if you agree that the ?( case in CS is less significant, then we
> shouldn't add it in haste or mangle ?. semantics to try to cover it.
No, I didn't say that. I said that I suspect that
f?()
is less significant than
foo?.bar?()
I suspect that (if you could say them in JS) we would find that
g(foo?.bar)
and
g(foo?.baz?())
are about equally significant and that people will write (or typo)
foo?.bar()
with the expectation that it means
foo?.bar?()
The risk is in adding precisely CoffeeScript ?. in haste without thinking
through implications of not also having ?(). It may mean a different variant
of ?. semantics would work better in our case.
Allen
>
> /be
>
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