It so happens that you can make a set data type that is a Monad, but it's
not exactly the best possible sets.
module SetMonad where
newtype Set a = Set { unSet :: [a] }
singleton :: a -> Set a
singleton x = Set [x]
unions :: [Set a] -> Set a
unions ss = Set $ concatMap unSet ss
member :: (Eq a) => a -> Set a -> Bool
member x s = x `elem` unSet s
instance Monad Set where
return = singleton
x >>= f = unions (map f (unSet x))
On Sat, Jan 8, 2011 at 9:28 PM, Peter Padawitz
Hi,
is there any way to instantiate m in Monad m with a set datatype in order to implement the usual powerset monad?
My straightforward attempt failed because the bind operator of this instance requires the Eq constraint on the argument types of m.
Peter
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