Re: [Parseperl-discuss] determining object types w/o executing???

[Adam K. <ad...@ph...>]

Mark Ethan Trostler wrote:
> Howdy All,
>     A very strange conglomeration events has led me here....
> I'm very interested in attempting to determine what the type of an 
> object would be at runtime - without actually having to run the program 
> - say to determine available methods from a debugger (as mentioned in 
> the recent article on perl.com).

Because Perl is loosely types, you can't really do this. A variable 
could contain just about anything.

>     Clearly the code must be run to the point where the object is in the 
> code to then 'ref' it to find out it's type - then something like PPI 
> can be used to walk up the inheiritance tree collecting possible 
> subroutines.

If you already have the the code loaded, then something like 
Class::Inspector does a pretty good job of working out the available 
methods by walking the ISA tree.

>     Actually executing the program/script everytime just to determine 
> the type of the object is a bummer - as who knows what the side-effects 
> are of running & re-running the code over & over.

Right. It's generally considered dangerous.

>     Have you guys thought about how to tackle this?  It does seem the 
> code has to be re-run everytime you would want to know the type of an 
> object.
>     I have a bizarro solution involving attributed-'my' variables to 
> determine object types (replete w/code instrumentation via PPI) BUT 
> re-running the code everytime sucks.
>     Other then detecting whether the code has been modified before 
> having to re-run it again have you guys come up with something to 
> determine varaible types?
>     It can't be possible... right???

Technically no, it's impossible. But then PPI is about dealing with the 
impossible :)

To repeat something you're going to be repeating quite a bit when people 
ask "can I" questions, if YOU (as a person) can look at a piece of code 
in a document and know what it is, then you should be able to write code 
with PPI to work it out programmatically.

But take something like

package Foo;

sub foo {
	my $self = shift;
	my $param = shift;
	...
}

Can you tell me what those are? You can probably guess that $self is 
going to be an object of type 'Foo' (or a subclass). But what is $param? 
You can't tell.

There's a range of situations in which you can guess reasonably well, 
for example...

my $foo = My::Class->new;

But in general, Perl vars are untyped, so no you can't tell.

Adam K