On Wed, 31 Mar 2004, Simon Peyton-Jones wrote:
I'm sorry to say that you just can't make Set an instance of Monad.
To make an instance of Monad for a type constructor T you must have a function
bindT :: forall a b. T a -> (a -> T b) -> T b
And there just *is* no such function for Set, because of the need for ordering. There's a less polymorphic function
bindSet :: forall a b. (Ord a, Ord b) => T a -> (a -> T b) -> T b
To translate the question from my problem to this one: What would be if Set would be declared within Ord context, i.e. data (Ord a) => Set a = ... ? Now it would be safe to use Set as Monad because Sets could only constructed from Ord types. But GHC doesn't accept this and an answer to my surprise was (see http://www.haskell.org//pipermail/haskell-cafe/2004-March/005985.html) "Constraints on datatype declarations are a misfeature of Haskell, and have no useful effect." Is this the final conclusion?