Haskell - Get user input in do block and use it in if statement

Here is my code

main :: IO ()
main = do
    putStrLn "Pick a number?"
    putStrLn "From 1-5"
    numPick <- getLine
    putStrLn "Thank you for choosing a number."
    if numPick == 1 then
       do createProcess (proc "/usr/bin/ls" [])
    else
       do putStrLn "Do nothing"
    putStrLn "Were done here"

I want a user to pick a number and from the number that gets picked a system process is ran. I am new to Haskell does anyone know what am I doing wrong?

I am getting the following error when trying to compile.

hlsurl.hs:18:11: error:
    * Couldn't match type `()'
                     with `(Maybe Handle, Maybe Handle, Maybe Handle, ProcessHandle)'
      Expected type: IO
                       (Maybe Handle, Maybe Handle, Maybe Handle, ProcessHandle)
        Actual type: IO ()
    * In a stmt of a 'do' block: putStrLn "Do nothing"
      In the expression: do putStrLn "Do nothing"
      In a stmt of a 'do' block:
        if numPick == 1 then
            do createProcess (proc "/usr/bin/ls" [])
        else
            do putStrLn "Do nothing"
   |
18 |        do putStrLn "Do nothing"
   |           ^^^^^^^^^^^^^^^^^^^^^

Solution 1:

createProcess returns a number of handles associated with the process, while putStrLn returns nothing (IO ()). Because both parts of the if-else-expression need to be of the same type, you need to unify those function calls, e.g. by "swallowing" the createProcess's return value using void:

main :: IO ()
main = do
    putStrLn "Pick a number?"
    putStrLn "From 1-5"
    numPick <- getLine
    putStrLn "Thank you for choosing a number."
    if numPick == "1" then  -- also fixed "1", because `getLine` returns String
       void $ createProcess (proc "/usr/bin/ls" [])
    else
       putStrLn "Do nothing"
    putStrLn "Were done here"

The equivalent code without void is:

if numPick == "1" then do
   createProcess (proc "/usr/bin/ls" [])
   return ()
else
   putStrLn "Do nothing"