module Main where
import Data.List
-- quicksort of any list
qsort [] = []
qsort (x:xs) = qsort(filter(<x) xs) ++ [x] ++ qsort(filter(>=x) xs)
-- optimized quicksort, uses middle element as pivot
qsortOpt [] = []
qsortOpt x = qsortOpt less ++ [pivot] ++ qsortOpt greater
where
pivot = x !! ((length x) `div` 2)
less = filter (<pivot) (delete pivot x)
greater = filter (>=pivot) (delete pivot x)
main = do
putStr "Enter a list: "
l <- readLn
print (qsortOpt l)
-- end of code
Why do I get
ERROR "qsort.hs":17 - Unresolved top-level overloading
*** Binding : main
*** Outstanding context : (Read b, Show b, Ord b)
?
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