Maude-help mailing list

[Michael Katelman <katelman AT uiuc.edu>]

On Sat, Jan 2, 2010 at 10:51 AM, Michael Katelman 
<katelman AT uiuc.edu>
 wrote:
> 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

I mean new equations for an equationally defined function where the
argument is one of the original constructors, not equations directly
on the constructors.

> 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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>>
>>
>