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copied!<p>To do error handling, you don't necessary need the <code>Either(T)</code> monad. You can be perfectly fine by staying in <code>Applicative</code> via composition. Example (for fun using <code>AccValidation</code> which accumulates all errors):</p> <pre><code>import Control.Applicative import Control.Monad.Error import Data.Validation import Data.Bifoldable import Data.Functor.Compose -- Replicating OP example with a Dummy monad (Get is made Applicative in newer libs) data Dummy a = D a deriving Show instance Monad Dummy where return = D (D x) >>= f = f x instance Functor Dummy where fmap f (D x) = D (f x) getM1 :: ErrorT String Dummy Int getM1 = lift (D 1) -- Can do with Applicatives + (Acc)Validation too instance Applicative Dummy where pure = return (<*>) = ap getA :: Compose Dummy (AccValidation String) Int getA = Compose $ D (success 1) getE :: Compose Dummy (AccValidation String) Int getE = Compose $ D (failure "bad") -- Applicative composition can work either way getA2 :: Compose (AccValidation String) Dummy Int getA2 = Compose $ success (D 1) getE2 :: Compose (AccValidation String) Dummy Int getE2 = Compose $ failure "bad" main = do runMonadic $ (+) <$> getM1 <*> getM1 -- D "2" -- runApplicative $ (+) <$> getA <*> getA -- D "2" runApplicative $ (+) <$> getE <*> getA -- D "bad" runApplicative $ (+) <$> getE <*> getE -- D "badbad" -- runOtherApp $ (+) <$> getA2 <*> getA2 -- "D 2" runOtherApp $ (+) <$> getE2 <*> getE2 -- "badbad" where runMonadic = print . fmap (either id show) . runErrorT runApplicative = print . fmap (validate id show) . getCompose runOtherApp = print . validate id show . getCompose -- some helper mimicking @either@ of @Either@ validate :: (e -> c) -> (a -> c) -> AccValidation e a -> c validate f g = bifoldl (const f) (const g) undefined </code></pre>

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