On Dec 29, 2007 11:34 AM, Allison Randal <[EMAIL PROTECTED]> wrote:
> Klaas-Jan Stol wrote:
> >
> > Earlier I read over this, thinking: there's probably a good reason for
> that
> > (the need for being contiguous), but I haven't figured it out yet.
> > For sake of documentation, could you please explain why this is so?
> >
> > so, why would it not be possible to write this:
> >
> > sub foo :multi
> > .param pmc x :invocant('Integer')
> > .param pmc y
> > .param pmc z :invocant('Integer')
> > ..
> >
> > and
> >
> > sub foo :multi
> > .param pmc x :invocant('Integer')
> > .param pmc y
> > .param pmc z :invocant('String')
> >
> > The former "foo" can be thought of as having signature "Integer, *,
> Integer"
> > and the second "Integer, *, String", (where * means any type, don't
> care).
> >
> > I don't see a particular problem. Not that I need to be able to do this,
> > just wondering why this rule is so.
>
> From both a parsing and self-documentation perspective, it's clearer to
> explicitly say:
>
> .sub foo :multi
> .param pmc x :invocant('Integer')
> .param pmc y :invocant('ANY')
> .param pmc z :invocant('String')
> .param pmc a
>
> than it is to simply leave 'y' blank.
>
> The behavior of the parameters 'y' and 'a' are a bit different. The 'a'
> parameter is only considered at the very start to knock out signatures
> that couldn't possibly fit (wrong number of required parameters). The
> 'y' parameter is a full participant in multi-dispatch, and contributes
> to the overall score of the signature for a particular call. An
> "Integer, Integer, Integer" signature may be considered a better match
> for a call than "Integer, *, Integer" (when the call is all Integers),
> because it's more specific to the particular types passed.
>
I think the penny dropped here :-) I'll write down here how I see it, also
so that I don't forget, and this text can be rephrased into documentation.
Assume that :invocant markers do not need to be contiguous, then:
Essentially, you don't know how many :invocants there are for a specific
:multi. For instance:
.sub foo :multi
.param pmc a :invocant('Integer')
.param pmc b
.param pmc c
..
.end
Is this sub a multi that takes 1 invocant (of type Integer) or maybe an
Integer parameter and 1 'any' parameter (don't care which type), or maybe 2
don't-care types.
In order to solve that, adding a :invocant('any') (* a special marker would
be more convenient, maybe someone would like to define a class 'any' (you
never know :-) but that's beside the pointnow *)
By adding these markers, what effectively happens is that all :invocant
parameters that are considered the :multi signature, are grouped together,
anyway. So, this is the reason that :invocant-marked params must be grouped
together.
kjs
