Control.RMonad

Description

This module provides an alternative RMonad and RMonadPlus type class that allows for constraints on the contained type (a restricted monad) It makes use of associated datatypes (available in GHC 6.8).

To make your own type an instance of Monad, first define the Suitable type class for it. For example,

     instance Ord a => Suitable Set a where
        data Constraints Set a = Ord a => SetConstraints
        constraints _ = SetConstraints

You need to change Set to your own type, Ord a to your own constraints, and SetConstraints to some distinguished name (this name will not normally be visible to users of your type)

Next you can make an instance of RMonad and if appropriate RMonadPlus by defining the members in the usual way. When you need to make use of the constraint on the contained type, you will need to get hold of the constraint wrapped up in the Constraints datatype. For example here are the instances for Set:

    instance RMonad Set where
       return = Set.singleton
       s >>= f = let res = case constraints res of
                             SetConstraints -> Set.fold (a s' -> Set.union (f a) s') Set.empty s
                 in res
       fail _ = Set.empty

instance RMonadPlus Set where
       mzero = Set.empty
       mplus s1 s2 = let res = case constraints res of
                                  SetConstraints -> Set.union s1 s2
                     in res

Once you have made your type an instance of RMonad, you can use it in two ways. Firstly, import this module directly and use the NoImplicitPrelude extension so that do-syntax is rebound. Secondly, use the wrapper type in Control.RMonad.AsMonad which supports the normal Monad operations.