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module Data.Res where
import Control.Applicative
import Data.String (IsString (fromString))
data Res e r = Ok r | Err e
instance Functor (Res e) where
fmap f (Ok v) = Ok (f v)
fmap _ (Err e) = Err e
instance Applicative (Res e) where
pure = Ok
(Ok f) <*> (Ok v) = Ok (f v)
(Err e) <*> _ = Err e
_ <*> (Err e) = Err e
instance Monad (Res e) where
(Ok v) >>= f = f v
(Err e) >>= _ = Err e
instance IsString e => MonadFail (Res e) where
fail = Err . fromString
instance IsString e => Alternative (Res e) where
empty = fail "mzero"
m1@(Ok _) <|> _ = m1
(Err _) <|> m2 = m2
liftEither :: Either e r -> Res e r
liftEither = either Err Ok
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