1 files changed, 31 insertions, 0 deletions

diff --git a/app/Data/Res.hs b/app/Data/Res.hs
new file mode 100644
index 0000000..e8c4ca4
--- /dev/null
+++ b/app/Data/Res.hs
@@ -0,0 +1,31 @@
+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