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

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