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[Francisco Durán <duran AT lcc.uma.es>]

Hi Pat,

I agree with you. I think Maude does not suffer from the expression  
problem. Order-sortedness gives a very expressive power, and makes  
"classifying" very precise.

I don't think however that I would buy your solution. If I wanted to  
define a new constructor for a sort S, I would import S in protecting  
mode, and define a supersort of it S'. Notice that in this way all  
functions on S still work, and I only need to worry about those I want  
to add. If I wanted to "extend" some of the previously defined  
functions to my new sort S', I'd just need to add the subsort  
overloaded declaration and cover the new cases. Notice that in this  
way you get the advantages of "closed types" for defining total  
functions at the time you have facilities for extending available types.

Happy new year,

Paco

El 31/12/2009, a las 22:49, pbrowne escribió:

> Michael,
> The expression problem can be described as the ability to add new
> variants (either constructors or methods) to a data type (or sort)
> without changing the existing code. The Haskell and OO language issues
> are well described at:
> http://stackoverflow.com/questions/870919/why-are-haskell-algebraic-data-types-closed
>
>
> With respect to Maude, here is my current *opinion*
> I *think* Maude supports subsort polymorphism that allows us to use
> elements of a subsort in the function’s rank (arity and co-arity).
> I *think* the Maude allows functions to be inherited and overridden  
> in a
> subsort (a  bit like overriding and inheritance in object oriented
> languages methods).
> Therefore I *think* that a Maude sort can be extended by subsorting  
> and
> adding a new method.
>
> I *think* that a new constructor could be added to an existing sort by
> importing in a using rather than protecting mode.
> So I (perhaps incorrectly) conclude that Maude does not suffer from  
> the
> expression problem.
>
>
> !!!!Happy new year to all on the Maude list!!!!
> Pat
>
>
>
> Michael Katelman wrote:
> > Pat,
> >
> > Could you clarify exactly what the "expression problem" is? I am
> > having difficulty following the wikipedia entry.
> >
> > Since you mentioned type classes (a la Haskell), perhaps it's  
> > relevant
> > to note that while ad-hoc overloading of functions is allowed in
> > Maude, the situation is somewhat different from Haskell's type
> > classes.  Haskell's type classes are used to support a sort of
> > restricted polymorphism where the type signature quantifies over a
> > subset of all types with the same overloaded operators, but the
> > "type-system" of Maude is not polymorphic; in the sense that the
> > signatures of functions cannot have universal quantifiers over
> > "types", like in Haskell.
> >
> > At least that's my take on how the two are related.
> >
> > -Mike
> >
> > On Thu, Dec 31, 2009 at 5:44 AM, pbrowne  
> > <Patrick.Browne AT comp.dit.ie> wrote:
> > > hi,
> > > I have a question concerning Wadler's expression problem[1] for
> > > algebraic data types. In relation to Haskells type classes is easy  
> > > to
> > > add a new operations on existing data types, it requires only the
> > > definition of a new function. All the old functions on those types
> > > continue to work unchanged. On the other hand, it is difficult to  
> > > add
> > > new structure to an existing data type: it requires the addition  
> > > of a
> > > new constructor for the existing data type.
> > >
> > > Questions:
> > > 1)Does expression problem exist in Maude?
> > > 2)If it does not exist in Maude, is there an illustrative example  
> > > available?
> > >
> > >
> > > Pat
> > >
> > > [1]http://en.wikipedia.org/wiki/Expression_Problem
> > >
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