Re: [Haskell-cafe] How to do this with associated types?

On Sun, Jul 25, 2010 at 1:53 PM, Alexey Karakulov wrote:

Suppose I have one piece of code like this:

...

class Result r e | r -> e where    failure :: e -> r a     success :: a -> r a

Maybe instance is discarding failure information:

...

instance Result Maybe e where     failure _ = Nothing     success x = Just x

I would argue that this shouldn't successfully compile. Your class declaration claims that given r, you can uniquely determine e. But you've effectively defined instance Result Maybe () instance Result Maybe Int ... many more instances ... which means that isn't the case. I wonder if there is a potential type safety failure in the compiler with instances of this form? -- ryan