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

[Rob Hoelz <rob@...>]

Thanks for the recommendations, Brian!  I agree that having a wantarray
function would probably be most flexible; I could also set a sensible
default.

On Wed, 28 Apr 2010 21:44:43 -0700
Brian Maher <brimworks@gmail.com> wrote:

> You could export several variations of your function.  For example:
> foo_scalar() and foo_array() would call the perl function foo() with
> wantarray to false and true respectively.  That could get ugly, unless
> you did something like a "type map" which is a description of perl
> function + contexts mapped to lua names.
> 
> Or you could make it be an optional parameter:
> 
>  foo({wantarray = true})
> 
> ...but that would mean all functions would need to take a table as the
> first argument, unless tables are not allowed to be passed to perl.
> 
> Or you could export it via some sort of "builder" strategy (not sure
> that this is the right term):
> 
>  perl("main::foo"):wantarray(true):call()
> 
> The perl function would take the name of the desired function as an
> argument and return an object that has a wantarray() method that can
> be called to change the calling context, and it would also have a
> call() method for actually making the subroutine call with a given set
> of parameters.  If wantarray() returns "self", then you can do method
> chaining like that above.
> 
> IMHO, the "builder" strategy is probably the most flexible in the
> long run.
> 
> Cheers,
> -Brian
> 
> On Wed, Apr 28, 2010 at 9:21 AM, Rob Hoelz <rob@hoelzro.net> wrote:
> > Hello lua-l,
> >
> > So, first some background:  I want to write a library that binds a
> > Perl interpreter to Lua.  Now, among the various challenges that
> > come along with this, the first one that comes to mind is Perl's
> > notion of context.
> >
> > For those of you who don't know Perl, it has this thing called
> > context.  Subroutines can be called in either void, scalar, or list
> > context.  A subroutine can detect this context and behave
> > differently if it likes.  For example:
> >
> > my $count = keys %hash; # returns the number of keys in a hash
> > my @keys = keys %hash; # returns a list of keys in a hash
> > keys %hash; # resets the internal iterator in a hash
> >
> > So, let's write up a quick example with my fake Lua-Perl bridge
> > (name suggestions would be appreciated):
> >
> > require 'perl'
> > perl.eval([[
> > sub foo {
> >  if(wantarray) { # wantarray is a function that returns the context
> > of the calling function return (1, 2);
> >  } else {
> >    return 3;
> >  }
> > }
> > ]])
> > local foo = perl.main.foo
> >
> > 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...
> >
> > Since foo on the Lua side is a lua_CFunction, it needs to know how
> > many return values are being asked for...I think this isn't
> > currently possible in Lua, but I figured I'd ask.
> >
> > Thanks,
> > Rob
> >
> 
> 
>

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