Haskell' - class aliases
Martin Sulzmann
martin.sulzmann at gmail.com
Fri May 2 06:18:45 EDT 2008
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
>
>
>
>
>
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