Syntax error using where or let in binding in Hashkell-CodePudding

I'm newie to the Haskell world recently. And still, struggle with syntax. Both methods follow have syntax errors. What could be the problem? Version 1:

rev' :: [a] -> [a]
rev' [] = []
rev' [x] = [x]
rev' (x:xs) = let revHelper acc l = 
                    | acc [] = []
                    | revHelper acc (x':l') = revHelper (x':acc) l'
              in revHelper [] (x:xs)

Version 2:

rev' :: [a] -> [a]
rev' [] = []
rev' [x] = [x]
rev' (x:xs) = revHelper [] (x:xs)
    where revHelper acc l = 
            | acc [] = []
            | revHelper acc (x':l') = revHelper (x':acc) l'

Both have the same problem:

util.hs:47:21: error: parse error on input ‘|’
   |
47 |                     | acc [] = []
   |

Can anyone help me?

I'm trying implementation a tail recursive reverse function.

CodePudding user response：

You are using guards the wrong way. Likely you want to implement this with:

rev' :: [a] -> [a]
rev' [] = []
rev' [x] = [x]
rev' (x:xs) = let revHelper acc [] = []
                  revHelper acc (x':l') = revHelper (x':acc) l'
              in revHelper [] (x:xs)

This however is not entirely correct: you should return the accumulator, so:

rev' :: [a] -> [a]
rev' [] = []
rev' [x] = [x]
rev' (x:xs) = let revHelper acc [] = acc
                  revHelper acc (x':l') = revHelper (x':acc) l'
              in revHelper [] (x:xs)

Furthermore it is not necessary to here define base cases for rev', you can implement this as:

rev' :: [a] -> [a]
rev' = go []
    where go acc [] = acc
          go acc (x:xs) = go (x:acc) xs