these-0.8: Control/Monad/Trans/Chronicle.hs
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleInstances #-}
-----------------------------------------------------------------------------
-- | Module : Control.Monad.Chronicle
--
-- Hybrid error/writer monad class that allows both accumulating outputs and
-- aborting computation with a final output.
--
-- The expected use case is for computations with a notion of fatal vs.
-- non-fatal errors.
-----------------------------------------------------------------------------
module Control.Monad.Trans.Chronicle (
-- * The Chronicle monad
Chronicle, chronicle, runChronicle
-- * The ChronicleT monad transformer
, ChronicleT(..)
-- * Chronicle operations
, dictate, disclose, confess
, memento, absolve, condemn
, retcon
) where
import Control.Applicative
import Control.Monad
import Control.Monad.Fix
import Control.Monad.Trans
import Data.Default.Class
import Data.Functor.Apply (Apply(..))
import Data.Functor.Bind (Bind(..))
import Data.Functor.Identity
import Data.Semigroup
import Control.Monad.Error.Class
import Control.Monad.Reader.Class
import Control.Monad.RWS.Class
import Prelude
import Data.These
import Data.These.Combinators (mapHere)
-- --------------------------------------------------------------------------
-- | A chronicle monad parameterized by the output type @c@.
--
-- The 'return' function produces a computation with no output, and '>>='
-- combines multiple outputs with 'mappend'.
type Chronicle c = ChronicleT c Identity
chronicle :: These c a -> Chronicle c a
chronicle = ChronicleT . Identity
runChronicle :: Chronicle c a -> These c a
runChronicle = runIdentity . runChronicleT
-- --------------------------------------------------------------------------
-- | The `ChronicleT` monad transformer.
--
-- The 'return' function produces a computation with no output, and '>>='
-- combines multiple outputs with 'mappend'.
newtype ChronicleT c m a = ChronicleT { runChronicleT :: m (These c a) }
instance (Functor m) => Functor (ChronicleT c m) where
fmap f (ChronicleT c) = ChronicleT (fmap f <$> c)
instance (Semigroup c, Apply m) => Apply (ChronicleT c m) where
ChronicleT f <.> ChronicleT x = ChronicleT ((<.>) <$> f <.> x)
instance (Semigroup c, Applicative m) => Applicative (ChronicleT c m) where
pure = ChronicleT . pure . pure
ChronicleT f <*> ChronicleT x = ChronicleT (liftA2 (<*>) f x)
instance (Semigroup c, Apply m, Monad m) => Bind (ChronicleT c m) where
(>>-) = (>>=)
instance (Semigroup c, Monad m) => Monad (ChronicleT c m) where
return = ChronicleT . return . return
m >>= k = ChronicleT $
do cx <- runChronicleT m
case cx of
This a -> return (This a)
That x -> runChronicleT (k x)
These a x -> do cy <- runChronicleT (k x)
return $ case cy of
This b -> This (a <> b)
That y -> These a y
These b y -> These (a <> b) y
instance (Semigroup c) => MonadTrans (ChronicleT c) where
lift m = ChronicleT (That `liftM` m)
instance (Semigroup c, MonadIO m) => MonadIO (ChronicleT c m) where
liftIO = lift . liftIO
instance (Semigroup c, Monoid c, Applicative m, Monad m) => Alternative (ChronicleT c m) where
empty = mzero
(<|>) = mplus
instance (Semigroup c, Monoid c, Monad m) => MonadPlus (ChronicleT c m) where
mzero = confess mempty
mplus x y = do x' <- memento x
case x' of
Left _ -> y
Right r -> return r
instance (Semigroup c, MonadError e m) => MonadError e (ChronicleT c m) where
throwError = lift . throwError
catchError (ChronicleT m) c = ChronicleT $ catchError m (runChronicleT . c)
instance (Semigroup c, MonadReader r m) => MonadReader r (ChronicleT c m) where
ask = lift ask
local f (ChronicleT m) = ChronicleT $ local f m
reader = lift . reader
instance (Semigroup c, MonadRWS r w s m) => MonadRWS r w s (ChronicleT c m) where
instance (Semigroup c, MonadState s m) => MonadState s (ChronicleT c m) where
get = lift get
put = lift . put
state = lift . state
instance (Semigroup c, MonadWriter w m) => MonadWriter w (ChronicleT c m) where
tell = lift . tell
listen (ChronicleT m) = ChronicleT $ do
(m', w) <- listen m
return $ case m' of
This c -> This c
That x -> That (x, w)
These c x -> These c (x, w)
pass (ChronicleT m) = ChronicleT $ do
pass $ these (\c -> (This c, id))
(\(x, f) -> (That x, f))
(\c (x, f) -> (These c x, f)) `liftM` m
writer = lift . writer
-- this is basically copied from the instance for Either in transformers
-- need to test this to make sure it's actually sensible...?
instance (Semigroup c, MonadFix m) => MonadFix (ChronicleT c m) where
mfix f = ChronicleT (mfix (runChronicleT . f . these (const bomb) id (flip const)))
where bomb = error "mfix (ChronicleT): inner compuation returned This value"
-- | @'dictate' c@ is an action that records the output @c@.
--
-- Equivalent to 'tell' for the 'Writer' monad.
dictate :: (Semigroup c, Monad m) => c -> ChronicleT c m ()
dictate c = ChronicleT $ return (These c ())
-- | @'disclose' c@ is an action that records the output @c@ and returns a
-- @'Default'@ value.
--
-- This is a convenience function for reporting non-fatal errors in one
-- branch a @case@, or similar scenarios when there is no meaningful
-- result but a placeholder of sorts is needed in order to continue.
disclose :: (Default a, Semigroup c, Monad m) => c -> ChronicleT c m a
disclose c = dictate c >> return def
-- | @'confess' c@ is an action that ends with a final output @c@.
--
-- Equivalent to 'throwError' for the 'Error' monad.
confess :: (Semigroup c, Monad m) => c -> ChronicleT c m a
confess c = ChronicleT $ return (This c)
-- | @'memento' m@ is an action that executes the action @m@, returning either
-- its record if it ended with 'confess', or its final value otherwise, with
-- any record added to the current record.
--
-- Similar to 'catchError' in the 'Error' monad, but with a notion of
-- non-fatal errors (which are accumulated) vs. fatal errors (which are caught
-- without accumulating).
memento :: (Semigroup c, Monad m) => ChronicleT c m a -> ChronicleT c m (Either c a)
memento m = ChronicleT $
do cx <- runChronicleT m
return $ case cx of
This a -> That (Left a)
That x -> That (Right x)
These a x -> These a (Right x)
-- | @'absolve' x m@ is an action that executes the action @m@ and discards any
-- record it had. The default value @x@ will be used if @m@ ended via
-- 'confess'.
absolve :: (Semigroup c, Monad m) => a -> ChronicleT c m a -> ChronicleT c m a
absolve x m = ChronicleT $
do cy <- runChronicleT m
return $ case cy of
This _ -> That x
That y -> That y
These _ y -> That y
-- | @'condemn' m@ is an action that executes the action @m@ and keeps its value
-- only if it had no record. Otherwise, the value (if any) will be discarded
-- and only the record kept.
--
-- This can be seen as converting non-fatal errors into fatal ones.
condemn :: (Semigroup c, Monad m) => ChronicleT c m a -> ChronicleT c m a
condemn (ChronicleT m) = ChronicleT $ do
m' <- m
return $ case m' of
This x -> This x
That y -> That y
These x _ -> This x
-- | @'retcon' f m@ is an action that executes the action @m@ and applies the
-- function @f@ to its output, leaving the return value unchanged.
--
-- Equivalent to 'censor' for the 'Writer' monad.
retcon :: (Semigroup c, Monad m) => (c -> c) -> ChronicleT c m a -> ChronicleT c m a
retcon f m = ChronicleT $ mapHere f `liftM` runChronicleT m