I'd be interested to see examples that break on the implicit MV arg that don't already break on &key. Passing the arguments pseudo-array on, for example, shouldn't break. It just throws the responsibility for parsing the MV arg on to somebody else, which in non-MV-aware cases will typically ignore it and in MV-aware cases will handle it according to the protocol.
If you call a function FOO with &key parameters you're expecting it to be a PS function, and if function FOO calls other functions they don't care. You can make the same argument for calling a function BAR expecting multiple values. Except as you point out the multiple-values array will now be passed on to any functions whose value BAR returns, which might or might not lead to subtle bugs. So I am really opposed to this approach.
Vladimir
It strikes me that the implicit MV arg is a generalization of the &key mechanism where the key is not a symbol (string) but a sentinel object. The big break with current practice is that it's hidden rather than specified by the user at the source level. Makes me wonder if a different syntax than MULTIPLE-VALUE-BIND might help - something to make the underlying mechanism less unexpected...
To be sure, it's a hack and not at all what one would do with proper access to the internals. But I wonder if it's as good as we're likely to get by way of a correct implementation.
On Sun, Sep 2, 2012 at 6:28 PM, Vladimir Sedach vsedach@gmail.com wrote:
We may have crossed a wire here. When you said, "You can do that with a global table instead of setting a property on the function object," I thought you had in mind a global table keyed by function *name*, which is why I asked about lambdas since they have no names. JS won't let you use a function object as a key so one would have to concoct some naming scheme.
For my first prototype for the new MV mechanism, that's what I thought and used gensyms. But then I tried foo[<function object>] and that works in both FF and CL-JS. But looking at ECMAScript, property identifiers do indeed have to be JavaScript String objects (http://ecma-international.org/ecma-262/5.1/#sec-8.10).
Moreover, if it worked for MV, the sentinel idea could be used to tag anything else we wanted into the call. Seems like that could be pretty powerful.
Where does this break?
It wouldn't work for calling JavaScript functions that used the arguments pseudo-array, either for arbitrary arity or for passing arguments on. There's lots of JS functions around that do things like foo.apply(null, slice(arguments, x)) or whatever.
p.s. There's also a way to communicate exactly how many return values are desired, short-circuiting any computation that might be needed to generate the full VALUES list. (I think this point is independent of the above idea but I'll adapt the same examples.) Suppose we have:
In general you have to evaluate all the expressions given to values for their side-effects, so this would only save a few assignments.
Vladimir
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