Question

Two module Up.hs and Down.hs

module Up (isSortedUp) where
isSortedUp x y z = if x>y && y>z then return(True) else return(False)


module Down (isSortedDown) where
isSortedDown x y z = if x<y && y<z then return(True) else return(False)

And the main program Main.hs

import System.Environment
import Up
import Down
main = do
  args<-getArgs
  let a = read(args !! 0)
  let b = read(args !! 1)
  let c = read(args !! 2)
  if (isSortedUp a b c || isSortedDown a b c) 
    then putStrLn "True" 
    else putStrLn "False"

During compilation I get the following error:

Couldn't match expected type `Bool' with actual type `m0 Bool'
In the return type of a call of `isSortedUp'
In the first argument of `(||)', namely `isSortedUp a b c '
In the expression: (isSortedUp a b c || isSortedDown a b c)

Answer 1

You seem to be confused about return. It is not a keyword for returning values like in other programming languages. In Haskell, return is a function that promotes a pure value to a monadic one (e.g. an Int to an IO Int). You don’t use it for non-monadic code.

isSortedUp x y z = if x>y && y>z then True else False

Also, instead of writing if foo then True else False, you can simply write foo:

isSortedUp x y z = x>y && y>z

Your main function can also be simplified a little using pattern matching and the fact that print on booleans prints "True" or "False".

main = do
  (a:b:c:_) <- getArgs
  print (isSortedUp a b c || isSortedDown a b c)