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dmwit/Entry.hs

Created February 19, 2019 15:54
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Entry.hs
{-# LANGUAGE FlexibleInstances #-}
import Data.Functor.Compose
import Control.Monad
joinEntry :: Maybe [Maybe [a]] -> Maybe [a]
joinEntry = fmap join . join . fmap sequenceA
instance Monad (Compose Maybe []) where
-- or use liberal applications of coerce to avoid the calls to Compose and getCompose
x >>= f = Compose . joinEntry . getCompose . fmap (getCompose . f) $ x
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