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