Haskell main function

Question

module Main where

qsort :: Ord a => [a] -> [a]
qsort [] = []
qsort (x : xs) = qsort smaller ++ [x] ++ qsort larger
                 where
                   smaller = [a | a <- xs , a <= x]
                   larger  = [a | a <- xs , a >  x]

main = do qsort [1,3,2]

I get the following error

Couldn't match expected type `IO t0' with actual type `[a0]'         
In the expression: main
When checking the type of the function `main'

What am I doing wrong?

Answer 1

All functions within a do block must match the monadic value being returned. You could instead write

main = do
    print (qsort [1, 3, 2])

Because print returns an IO value. Similarly, if you were using the Maybe monad, you would have to do something like

-- lookup :: Eq k => k -> [(k, v)] -> Maybe v
-- listToMaybe :: [a] -> Maybe a

firstElementOf :: Eq q => k -> [(k, [v])] -> Maybe v
firstElementOf key assocMap = do
    v <- lookup key assocMap
    first <- listToMaybe v
    return first

This works because lookup and listToMaybe both return a Maybe, which is the return value of the overall do block as specified by the type signature of firstElementOf.

Looking at the type of qsort, it only returns [a], not IO something, so it can't be used directly inside main's do block. You could also assign it's returned value to a name using let:

main = do
    let result = qsort [1, 3, 2]
    print result