Re: goops - accessors, methods and generics

[Daniel Hartwig <mandyke@gmail.com>]

Hi

It seems you are expecting some CLOS behaviour in a language that can
not support it.  The accessors are generic functions, but each of your
modules creates a unique generic function, there is no implicit
namespace sharing in Scheme.  Define a base module with an appropriate
superclass or interface definition (generics); Scheme requires it.

On 22 February 2013 06:51, David Pirotte <david@altosw.be> wrote:
> Hello all,
>
> given the following 4 modules, I am facing what I consider an
> inconsistent goops behavior and have one problem which leads to my
> recurrent request of goops default behavior should be to

> [a] always
> create a generic function for accessors and methods that do not [yet]
> have one, *visible in the entire guile space [all modules]*

Note that <generic> is a superclass of <accessor>, so these are
already generic functions.  There is no global binding escalation in
Scheme.

Consider these situations followed by introducing any of your example
class definitions:
- module A binds ‘define’ to a non-procedure; or
- modules B and C bind ‘define’ to different procedures.

Your suggestion is appropriate in a language like Common Lisp with
dynamic scoping, and separate namespaces for function and non-function
bindings.  Not so appropriate for Scheme.

In your example the two classes share a common interface, but this
interface is never defined anywhere.  So if I have code that wants to
work with any widget, which module should be imported to get the
canonical interface definition?  Indeed, it will either have to be
created using a common superclass or manually defined generics (as you
later observe).  These widget implementations will then have to import
the base module to explicitly share in the interface.

> and [b] the
> default behavior should be '(merge-generics replace warn-override-core
> warn last) [but that at least that one I can set using :duplicate, I know]

The default behaviour is conservative with regards to namespace
separation.  If you desire this behaviour it seems best to explicitly
ask for it.  With CLOS a different default can be expected to apply
because bindings in the underlying language already work in a similar
way.

> what is more of a problem with the existing goops default, for me, is
> expressed in the mg-4.scm 'case': i can not make it work unless I
> manually create another module, manually making generics and make sure
> it is loaded before ...

In a language like Scheme this can not be avoided.  Namespaces must be
managed quite explicitly.

In your example, mg-1 and mg-2 are not sharing any bindings.  It is
not appropriate to assume that ‘dialog’ in either is related to the
other.  If they implement a common interface, as seems obvious in the
example, then they need to import the generic bindings for that from
a common base module.  Such a module should either define a superclass
or you can manually define the generics.

> or rename the accessors, which both solutions
> are really against my expectation and [long] CLOS practice: why should
> I have to manually do things which are inherent to oop [same name for
> slots pertaining to different classes is so common that i can not see
> any large application not having to do so, for semantic reasons].

GOOPS can not do everything the same as CLOS due to fundamental
differences between the underlying languages.

If the interface is the same, you have a candidate for superclass.
Either way, any code that hopes to use the (generic) interface needs
to import the bindings from somewhere, and that will be a module
containing either a superclass or a set of manually defined generics.

Regards