Re: [Haskell-cafe] Ambiguous type variable with subclass instance (also: is there a better way to do this?)

[Andy Gimblett]

On 17 Sep 2009, at 18:01, Ryan Ingram wrote:
Here's a way that works more closely to your original version:

instance Enumerated a => Target a where
   convert n
       | n >= 0 && n < numConstrs = Just (constrs !! n)
       | otherwise = Nothing
    where
       constrs = constructors
       numConstrs = length constrs

Aha - that's great, and it works without OverlappingInstances (but  
still with FlexibleInstances and UndecidableInstances - should that  
worry me?)

Just making sure constructors is only referenced once is the key, it  
seems.

Thanks!

-Andy
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