Maude-help mailing list

[Michael Katelman <katelman AT uiuc.edu>]

On Fri, Jan 1, 2010 at 6:16 PM, Francisco Durán 
<duran AT lcc.uma.es>
 wrote:
> 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.

Perhaps it's also worth mentioning, since the example cited was
OO-style inheritance, that if you extend a data type as above and add
new equations for constructors in the original definition, they will
not override the original ones like those of a child class in Java.
Essentially, Maude could choose to apply either one at its discretion.
The usual requirement to ensure that this operates correctly is
confluence.

-Mike

>
> 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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>>
>>
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