[m-dev.] proposal: user-defined equality predicates

[Tyson Richard DOWD]

> Hi,
> 
> I propose to extend Mercury to support user-defined equality predicates
> for user-defined types.  The following is a proposed new chapter for the
> Mercury Language Reference Manual describing this feature.  It is probably
> a bit short on rationale and examples, but please let me know what you think.

I'm concerned that it might not smoothly merge in with typeclasses. 
Does it?
> 
> -----------------------------------------------------------------------------
> 
> User-defined equality predicates
> ********************************
> 
>    When defining abstract data types, often it is convenient to use a
> non-canonical representation -- that is, one for which a single
> abstract value may have more than one different possible concrete
> representations.  For example, you may wish to implement an abstract
> type `set' by representing a set as an (unsorted) list.
> 
>      :- module set_as_unsorted_list.
>      :- interface.
>      :- type set(T).
>      
>      :- implementation.
>      :- import_module list.
>      :- type set(T) ---> set(list(T)).
> 
> In this example, the concrete representations `set([1,2])' and
> `set([2,1])' would both represent the same abstract value, namely the
> set containing the elements 1 and 2.

The `type(representation)' notation isn't really introduced clearly.

> 
>    For types such as this, which do not have a canonical representation,
> the standard definition of equality is the desired one; we want

I thought standard equality was syntactic? Why is this desired?

-- 
       Tyson Dowd           # 
                            #  Surreal humour isn't eveyone's cup of
     trd at cs.mu.oz.au        #  fur.
http://www.cs.mu.oz.au/~trd #