[GUI] State, Attributes and Some basic widgets

[Alastair Reid]

Wolfgang Thaller wrote:
>> [...] The question is, to what extent do we use type classes to
>> achieve this, etc...  I would advocate specifc types for specific
>> widget classes (e.g.  Button, ScrollBar, etc.), just because it's
>> the Haskell Way ;-) .

Glynn Clements <glynn.clements@virgin.net> writes:
> Unfortunately, it's not the "typical GUI toolkit" way; at least,
> both Xt and Qt have an OO-style class hierarchy. While they differ
> on the exact hierarchy, both have a generic "Widget" class, with
> operations which apply to all widgets.

Since Haskell has a different type system from OO languages, we simply
cannot do things the 'typical GUI toolkit' way.  Instead, we have to
find a way to encode as much as possible of the foreign type system in
Haskell's type system.

One way to do this is the one Wolfgang advocates: we provide a bunch
of separate datatypes for each leaf node in the OO class hierarchy, we
provide a class for each non-leaf node (or 'abstract class' if you
prefer) in the OO class hierarchy and we use inheritance between the
Haskell classes and instance declarations to encode the inheritance
relations in the OO class hierarchy.  This approach suffers a bit from
being tedious to specify all the instances you need for each datatype
but otherwise works pretty well.

There are other ways, of course, and we should explore them.  Perhaps
you could make a concrete proposal Glynn?

--
Alastair Reid                 alastair@reid-consulting-uk.ltd.uk  
Reid Consulting (UK) Limited  http://www.reid-consulting-uk.ltd.uk/alastair/