On 18/02/07, Marc Weber
Is there a difference at all wehter specifying (Monad m) in the class declaration or not? I have to add it to the instance declaration anyway..
If you have, say: class Monad m => Foo m where ... Then it's illegal to say: instance Foo Data.Set.Set where ... Because Data.Set.Set doesn't instantiate Monad. Including the constraint in the class header forces every instance to satisfy that constraint in order for it to be a valid instance. In effect, Monad is a superclass of Foo. Similarly, you can't say: instance Foo m where ... Because, in general, m doesn't instatiate Monad. You have to use (Monad m => m) instead: instance Foo (Monad m => m) where ... Which is more normally written: instance Monad m => Foo m where ... Incidentally, you're here saying that: i) Every type T that instantiates Foo must also instantiate Monad (due to the constraint on the class head). ii) Every type T that instantiates Monad also instantiates Foo (due to the instance Monad m => Foo m). So Foo and Monad both have exactly the same member types. -- -David House, dmhouse@gmail.com