On Thu, Nov 24, 2011 at 2:17 AM, Evan Laforge <qdun...@gmail.com> wrote:
> I don't fully understand this but it looks interesting! I guess
> 'read' is a constructor for a sort of witness of the id, but I don't
> see how to get this value into the type parameter, or how Equal would
> be used in my mappend example. Could you help me figure out how it
> relates? Here's the closest I could get:
Did you try something like below?
data Thing id =
Thing { thing_id :: IString id
, thing_stuff :: [Int]
}
thing :: String -> [Int] -> Exists Thing
thing id stuff =
case istring id of
Exists id_ -> Exists (Thing id_ stuff)
instance Monoid (Thing id) where
mempty = Thing undefined []
mappend (Thing id1 stuff1) (Thing _ stuff2) =
Thing id1 (mappend stuff1 stuff2)
maybeAppend :: Exists Thing -> Exists Thing -> Maybe (Exists Thing)
maybeAppend (Exists t1) (Exists t2) =
case thing_id t1 `equal` thing_id t2 of
Just Refl -> Just $ Exists (t1 `mappend` t2)
Nothing -> Nothing
Cheers,
--
Felipe.
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