Fw: Finding out the number of requested return values for a Lua-Perl bridge

[Rob Hoelz <rob@...>]

Forgot to send to lua-l.

Begin forwarded message:

Date: Thu, 29 Apr 2010 13:52:21 -0500
From: Rob Hoelz <rob@hoelzro.net>
To: Duncan Cross <duncan.cross@gmail.com>
Subject: Re: Finding out the number of requested return values for a
Lua-Perl  bridge


On Thu, 29 Apr 2010 19:18:54 +0100
Duncan Cross <duncan.cross@gmail.com> wrote:

> On Thu, Apr 29, 2010 at 6:41 PM, Rob Hoelz <rob@hoelzro.net> wrote:
> > Duncan-
> >
> > Thanks for pointing out my 0-indexing; I must have Perl on the
> > brain! =P
> >
> > The reason I wrote the last example as I did is because if I am
> > able to detect how many return values my function is expected to
> > return, I will force scalar context for one return value and list
> > context for more than one.  Here's an example of a function being
> > called in both contexts, but returning one value:
> >
> > my $a = foo;
> > my ( $a ) = foo;
> >
> > -Rob
> >
> > On Thu, 29 Apr 2010 18:37:00 +0100
> > Duncan Cross <duncan.cross@gmail.com> wrote:
> >
> >> On Wed, Apr 28, 2010 at 5:21 PM, Rob Hoelz <rob@hoelzro.net> wrote:
> >> > If I were to call foo in Perl, I could do it several ways:
> >> >
> >> > my $a = foo; # $a is 3
> >> > my @values = foo; # $values[0] is 1, $values[1] is 2
> >> > my ( $a, $b ) = foo; # $a is 1, $b is two
> >> > my ( $a ) = foo; # $a is 1, the second return value is discarded
> >> >
> >> > The equivalent statements in Lua:
> >> >
> >> > local a = foo()
> >> > local values = { foo() }
> >> > local a, b = foo()
> >> > local a = ({ foo() })[0] -- not sure how else to do this,
> >> > really...
> >>
> >> Hi Rob,
> >>
> >> I don't know anything about Perl, but I think I can still usefully
> >> comment on this last equivalent statement, that you're not sure how
> >> else to do. All you need to do is:
> >>
> >> local a = foo()
> >>
> >> ...and the second value will be silently discarded. You don't need
> >> to create a temporary table to do this. (Also, don't forget Lua
> >> tables index from 1 rather than 0, unless you have patched your
> >> version of Lua, so ({ foo() })[0] will always evaluate to nil
> >> anyway.)
> >>
> >> -Duncan
> >
> >
> 
> Hi Rob,
> 
> Ah, okay, I understand now - what you were trying to say in that
> example is a bit more subtle than I realised. That's what comes from
> not reading things through properly :) Sorry for the noise.
> 
> To contribute, how about a changing the bridge to expect a special
> extra first parameter when calling these functions? true to return a
> list, false to return a scalar, or one of a set of strings. You could
> use this directly if that is not too much trouble, or wrap the
> functions using something like this:
> 
>   function listFunc(func)  return function(...)  return
> func(true, ...);  end function scalarFunc(func)  return
> function(...)  return func(false, ...);  end
> 
>   fooList = listFunc(foo)
>   fooScalar = scalarFunc(foo)
> 
>   count = fooScalar()
>   a, b = fooList()
> 
> It may not be quite as nice syntactically as Brian's suggestion, but
> possibly easier to do.
> 
> -Duncan

Another thought:  I could do what Pluto does and access Lua's private
internal state by asking for the Lua headers' location at compile
time.  Not very nice, but it would get the job done.

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