Haskell' - class aliases

[Martin Sulzmann]

Any chance to express this in terms of a formal (constraint rewrite 
framework).
For example, the Haskell rule,  do *not* display implied superclasses,  
can be
specified as follows. Consider the special case of

class Eq a
class Eq a => Ord a

Eq a, Ord a <=> Ord a

The above rule only applies *after* type inference took place.

Martin



John Meacham wrote:
> This isn't really a response to your email, but I have been mulling the
> last few hours away from a computer and wanted to get this stream of
> conciousness out when it is fresh in my mind.
>
> The more I think about it, I think 'superclass' is just the wrong
> terminology for dealing with class aliases. Superclass implies a strict
> partial order on classes, which just isn't the case for class aliases,
> for instance
>
>   
>> class alias Foo a => Foo a = Bar a where ...
>>     
>
> Has a defined (if not very useful) meaning with class aliases, but is
> really odd if you consider 'Foo a' a "superclass". So, I think the
> following terminology should be used:
>
>    Context of --+
>    alias        |     The alias -+    +--- The expansion of the alias
>                 |                |    |
>                 v                v    v
>   
>> class alias (S1 a .. Sn a) => A a = (C1 a ... Cn a) where
>>      fd1 = ....
>>      ....
>>      fdn = ....
>>     
>             ^
>             +----  The defaults of the alias
>
>
>
> given this, the expansion of 'A a' in any context other than an instance
> head is
>
>   
>> A a --> reduce(S1 a .. Sn a, C1 a ... Cn a)
>>     
>
> where reduce is standard entailment reduction on class contexts (like (Eq
> a,Ord a, Eq a) reduces to (Ord a))
>
> This expansion is carried out iteratively on all class aliases  until a
> fixed point is reached, then all class aliases are deleted from the
> result and the remaining context is the final result. (This will always
> terminate due to there being a finite number of class aliases that can
> be pulled into the expansion)
>
>
> likewise, for instance declarations:
>
>
>   
>> instance A a where ...
>>     
>
> --> 
>
>   
>> foreach C in C1 .. Cn:
>>    instance (S1 a ... Sn a) => C a where ...
>>     
>
> I left out the default methods here. I need to think about them a bit
> more to come up with a formal expansion as it is a bit trickier (to
> typeset if nothing else), but I hope this is somewhat more clear for
> some...
>
>         John
>
>
>
>
>