Re: [Haskell-cafe] Noob question and sequence of operations (print then do something else)

24 Sep 2007


      Sorry, I was actually trying to use this as an example for something more
complicated I am trying to do.  In this example, why would the inferred type
be "IO ()"

aa :: String -> String
aa instr = do
  putStrLn "abc"
  putStrLn "abc"
  return "Az"

  Couldn't match expected type `[t]' against inferred type `IO ()'
    In the expression: putStrLn "abc"
    In a 'do' expression: putStrLn "abc"
    In the expression:
    do putStrLn "abc"
       putStrLn "abc"
       return "Az"

On 9/24/07, Sam Hughes <hughes@rpi.edu> wrote:
...
John Wicket wrote:
...
I am still in an imperative way of thinking.  In this example here; how
would I call "putStrLn" and then set the function with a value.  Eg:
aa :: String -> IO ()
aa instr = do
  putStrLn "abc"
  putStrLn "abc"
  return "123"
--- The error I am getting.
Couldn't match expected type `()' against inferred type `[Char]'
    In the first argument of `return', namely `"123"'
    In the expression: return "123"
    In the expression:
    do putStrLn "abc"
       putStrLn "abc"
       return "123"
Your type signature is wrong.  If you want an IO action whose return
value is a String, say so:
aa :: String -> IO String