Question

In order to improve my Haskell skills I decided to go through some example code and try to rewrite in a different way.

Here is the initial function:

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

Here is the migrated function:

quicksort :: (Ord a) => [a] -> [a]  
quicksort [] = []  
quicksort (x:xs) = smallerSorted ++ [x] ++ biggerSorted 
    where smallerSorted = quicksort [a | a <- xs, a <= x]  
        biggerSorted = quicksort [a | a <- xs, a > x]  

However looks to me that it is not working.
Is there something wrong? Can functions using LET ported to WHERE?

This is my error output:

/Users/graph/Documents/Uni/Haskell/hey.hs:43:5:
    parse error on input `biggerSorted'
Failed, modules loaded: none.

Many thanks!

Answer 1

You need to match the indentation of the expressions in a where clause, e.g.

quicksort :: (Ord a) => [a] -> [a]  
quicksort [] = []  
quicksort (x:xs) = smallerSorted ++ [x] ++ biggerSorted 
    where smallerSorted = quicksort [a | a <- xs, a <= x]  
          biggerSorted = quicksort [a | a <- xs, a > x]  

or

quicksort :: (Ord a) => [a] -> [a]  
quicksort [] = []  
quicksort (x:xs) = smallerSorted ++ [x] ++ biggerSorted 
    where
        smallerSorted = quicksort [a | a <- xs, a <= x]  
        biggerSorted = quicksort [a | a <- xs, a > x]