Re: [Xen-devel] [PATCH 1 of 4] xl: idl: Abolish keyed union types

[Gianni Tedesco <gianni.tedesco@xxxxxxxxxx>]

On Thu, 2011-01-06 at 17:56 +0000, Ian Jackson wrote:
> Gianni Tedesco writes ("[Xen-devel] [PATCH 1 of 4] xl: idl: Abolish keyed 
> union types"):
> > xl: idl: Abolish keyed union types
> > 
> > Since the IDL file has become useful for generating language
> > bindings it has become apparent that the KeyedUnion type has no
> > straightforward translation to scripting languages which have no
> > notion of unions.
> 
> Uhh?  Most scripting languages' aggregate types are practically
> nothing _but_ unions!

Not really sure I follow this.

> The correct translation of a keyed union would probably be an
> aggregate (eg, in Python, a dictionary) containing an entry for the
> key and entries for whatever version of the union it involved.

Hrm, I don't think we can easily do it this way. We create full fledged
python types to correspond to the C types currently. Probably the more
natural construction for unions would be basically the C equivalent of:

class Foo:
        def __init__(self):
                self.__hidden = xl.thing_pv()
        def __getattr__(self, k):
                return self.__hidden.__getattr__(k)
        def __setattr__(self, k, v):
                if k == 'hvm':
                        if v:
                                self.__hidden = xl.thing_hvm()
                                # copy all attributes from before
                        elif v:
                                self.__hidden = xl.thing_pv()
                                # copy all attributes from before
                return self.__hidden.__setattr__(k, v)

so that:
        x = Foo()
        x.pv_thing = True
        x.pv_thing
        >>> True
        x.hvm = True
        x.pv_thing
        >>> Exception: AttributeError
        x.hvm_thing = 'thing'

In other words, frob the getter/setter table whenever the key is set. I
could probably implement this if required as a pre-requisite for the
rest of the series but it may take a day or two.

Alternatively, I have some further patches to do with the Python binding
which does some more sophisticated stuff and I could re-introduce the
KeyedUnion later?

> > Turns out this is only used in domain_build_info which is hardly a memory
> > criticial structure.
> 
> However I don't have a strong opinion about the desirability of using
> a tagged union for this particular structure.

Hence my casualness in abolishing it.

Gianni


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