I'm trying to understand multi-param typeclasses; a particular one (MonadError); and functional dependencies. For example, throwError in in the MonadError class definition: class Monad m => MonadError e m | m -> e where throwError :: e -> m a The concept of a multi-parameter class is a litte tricky. It's no longer so simple to say that "ONE type is AN instance of ONE class." Instead, it seems more appropriate to say that "in a context where e and m fulfill the criteria for class MonadError, the function throwError is available." In a sense, there is no single type that is an instance of MonadError. Is this the right way to put it? So I went looking for an instance of MonadError, in particular with use with Either. I couldn't find actual source code with an instance, but the Haddock documentation lists this: Instances: Error e => MonadError e (Either e) The code doesn't give the definition, but I suppose it would be: throwError e = Left e ??? Now I'm interested in understanding this functional dependency between m and e. For the compiler to decide that a particular instance's definition of throwError is available, it must decide that e is of class Error (it is given by class constraint) m is a Monad (Either e is such) and then this m -> e thing: I don't know how to put this into words Any explanation/clarification/correction appreciated. Thanks, Mike