Re: [Haskell-beginners] several questions: multi-param typeclasses, etc.

Chaddaï Fouché wrote:

On Sun, Nov 1, 2009 at 5:32 PM, Michael Mossey wrote:

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On the other hand, if you added

instance (Error e) => MonadError String (Either e)

and didn't include the functional dependency, the compiler would still run into a problem with overlapping instances and have no way to decide, which I presume is still an error.

Right, in this case it is true, but supposing the MonadError instance for Either was rather :

...

instance (Error e) => MonadError (Maybe String) (Either e)

There would be nothing a priori that would prevent you from writing another instance :

...

instance (Error e) => MonadError String (Either e)

I think I understand that you are saying it would create a messy situation to write this second instance, and the functional constraint causes the compiler to stop with an error if it sees that second instance. Is that true? Now I have a question that probably doesn't make sense, but what I'm really doing is picking your brain. I want to see how you would rephrase it: Let's assume for a moment that two different people are involved in this code. One of them writes the class definition. Another one writes an instance. In OO, which I am more familiar with, one person will write a class with a limited API in order to help put guarantees on the correct behavior of the class. In a sense, that person is saying: "I release my class to the world to do what it will, but before doing that I put some constraints on it so no one can distort my intentions." Is this functional dependency a similar situation? Does it make sense from the "point of view" of the author of the class definition? Or is it more a practical necessity? Thanks, Mike