[Haskell-cafe] Set monad

[Lennart Augustsson]

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 <peter.padawitz at udo.edu>wrote:

> 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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