2 files changed, 36 insertions, 5 deletions

diff --git a/app/Data/Iban.hs b/app/Data/Iban.hs
index 45343ec..412577a 100644
--- a/app/Data/Iban.hs
+++ b/app/Data/Iban.hs
@@ -4,6 +4,8 @@ import Control.Applicative ((<|>))
 import Data.Attoparsec.Text as AP
 import Data.Char
   ( digitToInt,
+    isAscii,
+    isDigit,
     ord,
     toUpper,
   )
@@ -24,7 +26,7 @@ validateIban = AP.parseOnly $ do
     then
       if valid countryCode checkDigits chars
         then return ()
-        else fail $ "IBAN checksum does not match (" <> countryCode <> checkDigits <> chars <> ")"
+        else fail $ "IBAN checksum does not match (" ++ countryCode ++ checkDigits ++ chars ++ ")"
     else fail "IBAN has more than 34 characters"
   where
     letterToInt c = ord (toUpper c) - ord 'A' + 10
@@ -32,10 +34,8 @@ validateIban = AP.parseOnly $ do
       foldl'
         ( \acc -> \case
             d
-              | '0' <= d && d <= '9' -> acc * 10 + toInteger (digitToInt d)
-              | 'A' <= d && d <= 'Z'
-                  || 'a' <= d && d <= 'z' ->
-                  acc * 100 + toInteger (letterToInt d)
+              | isDigit d -> acc * 10 + toInteger (digitToInt d)
+              | isAscii d -> acc * 100 + toInteger (letterToInt d)
               | otherwise -> error "unreachable"
         )
         0
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