[Haskell-cafe] Re: [Haskell] [ANN] Safe Lazy IO in Haskell

[Miguel Mitrofanov]

On 19 May 2009, at 09:06, Ryan Ingram wrote:

> This is a common problem with trying to use do-notation; there are
> some cases where you can't make the object an instance of Monad.  The
> same problem holds for Data.Set; you'd can write
>
> setBind :: Ord b => Set a -> (a -> Set b) -> Set b
> setBind m f = unions (map f $ toList m)
>
> but there is no way to use setBind for a definition of >>=

You can use a continuation trick.