How to use "let" keyword to define multiple variables in Haskell

Question

I've the below code snippet in Haskell to implement the quicksort algorithm.

quicksort :: (Ord a) => [a] -> [a]
quicksort [] = []
quicksort (x:xs) =
let smaller = quicksort [a | a <- xs, a <= x]
    bigger  = quicksort [a | a <- xs, a > x]
in smaller ++ [x] ++ bigger

However, GHCi rejects the program telling me that line 5 has a syntax error. But I've checked the Haskell syntax for the "let" keyword and it seems OK.

Is there anyone who can help me with this problem? Thanks a lot.

Answer 1

You need to indent the let expression, since it is a continuation of the previous line.

quicksort :: (Ord a) => [a] -> [a]
quicksort [] = []
quicksort (x:xs) =
  let smaller = quicksort [a | a <- xs, a <= x]
      bigger  = quicksort [a | a <- xs, a > x]
  in smaller ++ [x] ++ bigger

Answer 2

Another way to do this You can write it in one line using braces and semicolons { ; ; ; }:

quicksort (x:xs) = let {smaller = quicksort [a | a <- xs, a <= x]; bigger = quicksort [a | a <- xs, a > x]} in smaller ++ [x] ++ bigger