[Haskell-cafe] Assorted AT fun and games

Andrew Coppin andrewcoppin at btinternet.com
Sun Aug 15 13:06:58 EDT 2010

Brandon S Allbery KF8NH wrote:
> Hash: SHA1
> On 8/15/10 09:00 , Andrew Coppin wrote:
>>  class (Vector (Point x)) => HasSpace x where
>>    type Point x :: *
> (...)
>> And now things get *really* interesting. Consider this:
>>  data Foo x = Foo !x !(Point x)
>> Surprisingly, GHC accepts this. This despite the rather obvious fact that
>> "Point x" can exist if and only if "x" has a HasSpace instance. And yet, the
>> type definition appears to say that "x" is simply an unconstrained type
>> variable. Intriguing...
> Maybe I'm missing something in all the type machinery I elided, but it looks
> to me like you have that backwards:  HasSpace x requires Point x but not
> vice versa.  Your actual usage may require the reverse association, but the
> definition of Foo won't be modified by that usage --- only applications of
> that definition.

Well, since Point is part of the definition of HasSpace, and therefore 
Point x is defined only if an instance HasSpace x exists. I'm not sure 
how that's "Point x doesn't require HasSpace x".

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