1 files changed, 27 insertions, 22 deletions

diff --git a/app/Data/Iban.hs b/app/Data/Iban.hs
index a42e192..45343ec 100644
--- a/app/Data/Iban.hs
+++ b/app/Data/Iban.hs
@@ -15,26 +15,31 @@ mkIban :: T.Text -> Either String Iban
 mkIban t = validateIban t >> return (Iban t)
 
 validateIban :: T.Text -> Either String ()
-validateIban t = AP.parseOnly ibanP t
+validateIban = AP.parseOnly $ do
+  countryCode <- AP.count 2 AP.letter
+  checkDigits <- AP.count 2 AP.digit
+  chars <- AP.many1 (AP.letter <|> AP.digit)
+  endOfInput
+  if length chars < 30
+    then
+      if valid countryCode checkDigits chars
+        then return ()
+        else fail $ "IBAN checksum does not match (" <> countryCode <> checkDigits <> chars <> ")"
+    else fail "IBAN has more than 34 characters"
   where
-    ibanP = do
-      countryCode <- AP.count 2 ibanLetter
-      checkDigits <- AP.count 2 ibanDigit
-      chars <- AP.many1 ibanChar
-      endOfInput
-      if length chars < 30
-        then
-          if valid countryCode checkDigits chars
-            then return ()
-            else fail $ "IBAN checksum does not match (" ++ T.unpack t ++ ")"
-        else fail "IBAN has more than 34 characters"
-      where
-        ibanChar = ibanDigit <|> ibanLetter
-        ibanDigit = toInteger . digitToInt <$> AP.digit
-        ibanLetter = letterToInt <$> AP.letter
-        letterToInt c = toInteger (ord (toUpper c) - ord 'A' + 10)
-        charsToInteger = foldl' (\acc d -> if d >= 10 then acc * 100 + d else acc * 10 + d) 0
-        ibanToInteger countryCode checkDigits chars =
-          charsToInteger chars * 1000000 + charsToInteger countryCode * 100 + charsToInteger checkDigits
-        valid countryCode checkDigits chars =
-          ibanToInteger countryCode checkDigits chars `mod` 97 == 1
+    letterToInt c = ord (toUpper c) - ord 'A' + 10
+    charsToInteger =
+      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)
+              | otherwise -> error "unreachable"
+        )
+        0
+    ibanToInteger countryCode checkDigits chars =
+      charsToInteger chars * 1000000 + charsToInteger countryCode * 100 + charsToInteger checkDigits
+    valid countryCode checkDigits chars =
+      ibanToInteger countryCode checkDigits chars `mod` 97 == 1