[Haskell-cafe] Assorted AT fun and games

[Brandon S Allbery KF8NH]

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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.

- -- 
brandon s. allbery     [linux,solaris,freebsd,perl]      allbery at kf8nh.com
system administrator  [openafs,heimdal,too many hats]  allbery at ece.cmu.edu
electrical and computer engineering, carnegie mellon university      KF8NH
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