[parenscript-devel] A simpler way to do multiple value return? (Was: PS test failures in 7be9b45)

Vladimir Sedach vsedach at gmail.com
Mon Sep 3 14:55:59 UTC 2012

> 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.


> 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 at 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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