Class.hs

{-# OPTIONS -fallow-undecidable-instances #-}
-- Search for -fallow-undecidable-instances to see why this is needed

-----------------------------------------------------------------------------
-- |
-- Module      :  Control.Monad.Writer.Class
-- Copyright   :  (c) Andy Gill 2001,
--                (c) Oregon Graduate Institute of Science and Technology, 2001
-- License     :  BSD-style (see the file libraries/base/LICENSE)
--
-- Maintainer  :  libraries@haskell.org
-- Stability   :  experimental
-- Portability :  non-portable (multi-param classes, functional dependencies)
--
-- The MonadWriter class.
--
--      Inspired by the paper
--      /Functional Programming with Overloading and
--          Higher-Order Polymorphism/,
--        Mark P Jones (<http://web.cecs.pdx.edu/~mpj/pubs/springschool.html>)
--          Advanced School of Functional Programming, 1995.
-----------------------------------------------------------------------------

module Control.Monad.Writer.Class (
    MonadWriter(..),
    listens,
    censor,
  ) where

import Data.Monoid

-- ---------------------------------------------------------------------------
-- MonadWriter class
--
-- tell is like tell on the MUD's it shouts to monad
-- what you want to be heard. The monad carries this 'packet'
-- upwards, merging it if needed (hence the Monoid requirement)}
--
-- listen listens to a monad acting, and returns what the monad "said".
--
-- pass lets you provide a writer transformer which changes internals of
-- the written object.

class (Monoid w, Monad m) => MonadWriter w m | m -> w where
    tell   :: w -> m ()
    listen :: m a -> m (a, w)
    pass   :: m (a, w -> w) -> m a

listens :: (MonadWriter w m) => (w -> b) -> m a -> m (a, b)
listens f m = do
    ~(a, w) <- listen m
    return (a, f w)

censor :: (MonadWriter w m) => (w -> w) -> m a -> m a
censor f m = pass $ do
    a <- m
    return (a, f)