I’m new to Haskell (as you can probably see in my code) and I wanted to make a simple Brainf### interpreter:

import Text.Read (readMaybe)
import Data.Maybe (fromJust)
import Data.List (findIndex)
import Data.Char (chr, ord)
import System.IO

findPairs :: [Int] -> [Char] -> [(Int, Int)] -> Int -> [(Int, Int)]
findPairs stack [] pairs _ = if stack == [] then pairs else error "Mismatched parentheses"
findPairs stack (x:xs) pairs index = findPairs (if x == '[' then index : stack else (if x == ']' then tail stack else stack)) xs (if x == ']' then (head stack, index) : pairs else pairs) (current + 1)

BrainfuckRec :: [Int] -> Int -> [Char] -> [(Int, Int)] -> Int -> IO ( )
BrainfuckRec arr ptr code pairs current = 
    if current == length code then "nn<Program terminated>" 
    else case code !! current of
        '+' -> BrainfuckRec ((take ptr arr) ++ [((arr !! ptr) + 1) `mod` 128] ++ drop (ptr + 1) arr) ptr code pairs (current + 1)
        '-' -> BrainfuckRec ((take ptr arr) ++ [if arr !! ptr == 0 then 127 else arr((arr !! ptr) - 1) `mod` 128] ++ (drop (ptr + 1) arr)) ptr code pairs (current + 1)
        '<' -> BrainfuckRec (if ptr > 0 then arr else 0 : arr) (if ptr > 0 then ptr - 1 else 0) code pairs (current + 1)
        '>' -> BrainfuckRec (if ptr == (length arr) - 1 then arr ++ [0] else arr) (ptr + 1) code pairs (current + 1)
        '[' -> BrainfuckRec arr ptr code pairs (fst (pairs !! (findIndex ((== current) . snd) pairs)))
        ']' -> BrainfuckRec arr ptr code pairs (snd (pairs !! (findIndex ((== current) . fst) pairs)))
        '.' -> do
            putChar(chr (arr !! ptr))
            return BrainfuckRec arr ptr code pairs (index + 1)
        ',' -> do
            input <- getLine
            let temp = case parseInt input of Just x  -> x
                                              Nothing -> ord (head input)
            return BrainfuckRec ((take ptr arr) ++ [temp] ++ drop (ptr + 1) arr) arr ptr code pairs (current + 1)

Brainfuck :: [Char] -> IO ( )
Brainfuck inputFile = Brainfuck [0] 0 (readFile inputFile ReadMode) (findPairs [] code [] 0) 0

parseInt :: [Char] -> Maybe Int
parseInt = readMaybe

but, at BrainfuckRec :: [Int] -> Int -> [Char] -> [(Int, Int)] -> Int -> IO ( ), I got this error:

• Invalid type signature: BrainfuckRec :: …
• A type signature should be of form ::

(It won’t show the text for some reason)

I tried asking ChatGPT to fix it, but it failed over and over. When I decided to ask this question, the code looked like this:

import Text.Read (readMaybe)
import Data.List (findIndex)
import Data.Char (chr, ord)

-- Match brackets for loops
findPairs :: [Int] -> [Char] -> [(Int, Int)] -> Int -> [(Int, Int)]
findPairs [] [] pairs _ = pairs
findPairs [] (']':_) _ idx = error ("Unmatched ']' at " ++ show idx)
findPairs stack [] pairs idx = error ("Unmatched '[' at " ++ show idx)
findPairs stack (x:xs) pairs idx
    | x == '['  = findPairs (idx : stack) xs pairs (idx + 1)
    | x == ']'  = case stack of
                    []      -> error ("Unmatched ']' at " ++ show idx)
                    (s:st)  -> findPairs st xs ((s, idx) : pairs) (idx + 1)
    | otherwise = findPairs stack xs pairs (idx + 1)

-- Recursive Brainfuck interpreter
BrainfuckRec :: [Int] -> Int -> [Char] -> [(Int, Int)] -> Int -> IO ()
BrainfuckRec arr ptr code pairs current = 
    if current == length code then putStrLn "nn<Program terminated>"
    else case code !! current of
        '+' -> BrainfuckRec ((take ptr arr) ++ [((arr !! ptr) + 1) `mod` 128] ++ drop (ptr + 1) arr) ptr code pairs (current + 1)
        '-' -> BrainfuckRec ((take ptr arr) ++ [if arr !! ptr == 0 then 127 else ((arr !! ptr) - 1) `mod` 128] ++ drop (ptr + 1) arr) ptr code pairs (current + 1)
        '<' -> BrainfuckRec arr (if ptr > 0 then ptr - 1 else 0) code pairs (current + 1)
        '>' -> BrainfuckRec (if ptr == length arr - 1 then arr ++ [0] else arr) (ptr + 1) code pairs (current + 1)
        '[' -> case findIndex ((== current) . snd) pairs of
                 Just idx -> BrainfuckRec arr ptr code pairs (if arr !! ptr == 0 then fst (pairs !! idx) else current + 1)
                 Nothing  -> error $ "Unmatched '[' at position " ++ show current
        ']' -> case findIndex ((== current) . fst) pairs of
                 Just idx -> BrainfuckRec arr ptr code pairs (if arr !! ptr /= 0 then snd (pairs !! idx) else current + 1)
                 Nothing  -> error $ "Unmatched ']' at position " ++ show current
        '.' -> do
            putChar (chr (arr !! ptr))
            BrainfuckRec arr ptr code pairs (current + 1)
        ',' -> getLine >>= input ->
            let temp = case parseInt input of
                            Just x  -> if x >= 0 && x < 128 then x else error "Invalid ASCII value!"
                            Nothing -> ord (head input)
            in BrainfuckRec ((take ptr arr) ++ [temp] ++ drop (ptr + 1) arr) ptr code pairs (current + 1)


-- Main Brainfuck interpreter
Brainfuck :: String -> IO ()
Brainfuck inputFile = do
    code <- readFile inputFile
    let pairs = findPairs [] code [] 0
    BrainfuckRec [0] 0 code pairs 0

-- Parse integer input
parseInt :: [Char] -> Maybe Int
parseInt = readMaybe

But the error is still there. What’s causing it? How do I fix it?