If anyone is interested, Typeclassopedia pointed me to Composing Monads by Jones and Duponcheel (1993), which contains exactly my implementation along with some other nice patterns for composing Monads via Traversable.sequence (called swap in the paper) and related operators. It would be interesting to see these ideas reimagined with modern type classes. You can find the paper here: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.138.4552 Regards, Hans On 10 okt 2013, at 18:25, Hans Höglund wrote:
I have been experimenting with compositions of monads carrying associated monoids (i.e. Writer-style) and discovered the following pattern:
---------------------------------------------------------------------- {-# LANGUAGE DeriveFunctor, DeriveFoldable, DeriveTraversable, GeneralizedNewtypeDeriving #-}
import Control.Monad import Control.Monad.Writer hiding ((<>)) import Data.Semigroup import Data.Foldable (Foldable) import Data.Traversable (Traversable) import qualified Data.Traversable as Traversable
newtype Foo m a = Foo (Writer m a) deriving (Monad, MonadWriter m, Functor, Foldable, Traversable)
newtype Bar m a = Bar { getBar :: [Foo m a] } deriving (Semigroup, Functor, Foldable, Traversable) instance Monoid m => Monad (Bar m) where return = Bar . return . return Bar ns >>= f = Bar $ ns >>= joinedSeq . fmap (getBar . f) where joinedSeq = fmap join . Traversable.sequence
runFoo (Foo x) = runWriter x runBar (Bar xs) = fmap runFoo xs ----------------------------------------------------------------------
That is, given a type that is Monadic and Traversable, we can define a list of the same type as a monad, whose binding action "glues together" the nested Monoid values. A trivial example:
---------------------------------------------------------------------- -- annotate all elements in bar tells :: String -> Bar String a -> Bar String a tells a (Bar xs) = Bar $ fmap (tell a >>) xs
-- a bar with no annotations x :: Bar String Int x = return 0
-- annotations compose with >>= y :: Bar String Int y = x <> tells "a" x >>= (tells "b" . return)
-- and with join z :: Bar String Int z = join $ tells "d" $ return (tells "c" (return 0) <> return 1)
-- runBar y ==> [(0,"b"),(0,"ab")] -- runBar z ==> [(0,"dc"),(1,"d")] ----------------------------------------------------------------------
However, I am concerned about the (Monad Bar) instance which seems ad-hoc to me, especially the use of sequence. Is there a more general pattern which uses a class other than Traversable? Any pointers would be much appreciated.
Regards, Hans