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 16:50, Daniel Fischer wrote:

Yes, the second appearance of 'constructors' is at an unspecified type.

instance (Enumerated a) => Target a where
   convert n
      | n < 0     = Nothing
      | otherwise = case drop n constructors of
                       (x:_) -> Just x
                       _ -> Nothing

would make it compile.

Neat trick.  It works: thanks!

But there'd be a risk that Target is unusable, depending on how instance resolution is

done.

Unusable?  How so?  Sorry, but I don't follow...

-Andy

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