feat: Natural instead of Ints for nonnegativity

src/Language/Brainfuck.hs

@@ -1,5 +1,4 @@
 {-# LANGUAGE ImportQualifiedPost #-}
-{-# LANGUAGE ViewPatterns #-}
 {-# LANGUAGE DerivingStrategies #-}
 {-# LANGUAGE LambdaCase #-}
 module Language.Brainfuck (parse, ParseFailure, render) where
@@ -16,10 +15,11 @@ import Data.Vector.Mutable qualified as MutableVector
 
 import Language.Brainfuck.Instruction ( Instruction(..) )
 import qualified Data.List as List
+import Numeric.Natural (Natural)
 
 data ParseFailure
-  = UnexpectedClosingBracket Int -- closing bracket position
-  | UnmatchedOpenBracket Int -- opening bracket position
+  = UnexpectedClosingBracket Natural -- closing bracket position
+  | UnmatchedOpenBracket Natural -- opening bracket position
   deriving stock (Show, Eq)
 
 -- | Convert a Text to a list of instructions, discard all comments.
@@ -36,14 +36,14 @@ parse text = runST $ do
 
     Right (instructions, rest) -> case rest of
       Text.Empty -> Right instructions
-      _ -> Left (UnexpectedClosingBracket $ Text.length text - Text.length rest)
+      _ -> Left (UnexpectedClosingBracket . fromIntegral $ Text.length text - Text.length rest)
 
     Left failure -> Left failure
 
   where
 
     go :: Text -> MVector s Instruction -> Int -> ST s (Either ParseFailure (Vector Instruction, Text))
-    go (Text.Empty  ) instructions index = do
+    go Text.Empty instructions index = do
       let populatedSlice = MutableVector.take index instructions
       frozen <- Vector.force <$!> Vector.freeze populatedSlice
       pure . Right $ (frozen, Text.empty)
@@ -69,7 +69,7 @@ parse text = runST $ do
           innerResult <- go cs innerVector 0
           case innerResult of
             Right (body, ']' Text.:< rest) -> recognizeInstruction (Loop body) rest
-            Right _ -> pure $ Left (UnmatchedOpenBracket $ Text.length text - Text.length t)
+            Right _ -> pure $ Left (UnmatchedOpenBracket . fromIntegral $ Text.length text - Text.length t)
             _ -> pure innerResult
         _   -> go cs instructions index