Alex | Following the declaration of "instance Monad []" | in the prelude, and puzzling over the absence of | its equivalent from Data.Set, I naively typed: | | instance Monad Set where | m >>= k = concatSets (mapSet k m) | return x = unitSet x | fail s = emptySet | | concatSets sets = foldl union emptySet (setToList sets) | instance (Eq b,Ord b) => Ord (Set b) where | compare set1 set2 = compare (setToList set1) (setToList set2) | | and got the following error: | | Could not deduce (Ord b) from the context (Monad Set) | arising from use of `concatSets' at dbMeta3.hs:242 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 see why this is no good, consider sequence :: Monad m => [m a] -> m [a] The code for sequence calls (>>=) a lot. You can't supply bindSet as the function to call because bindSet takes two Ord parameters at run-time, whereas ordinary bind does not. It's not just a quirk of the type system -- there is a real problem! Simon