Get the return values from another function - Haskell

Question

I'm trying to get the values from this function, so, when i call that function "prompt" a send a message and this function return an value according with the supposed question that i made.

But i can't receive that value, so why ? I'm noob with haskell.

module Main (main) where
import System.IO (stdout, hSetBuffering, BufferMode(NoBuffering))
type State = Double

main::IO()
prompt::Read a =>  String ->  IO a


main = do putStrLn "Testing"
          v <- prompt "Whats your name?"
          return ()

prompt str = do putStrLn str
                valor <- readLn
                return valor

Answer 1

Your problem, I believe, is confined to your type signatures.

prompt :: Read a => String -> IO a

This type signature says that prompt is a function which takes a string, does some I/O magic, and then returns a value of type a, for any readable type a the user wishes. However, your implementation gets a string from the user and returns it.

If your intention is that prompt return a string, simply change your type.

prompt :: Read a => String -> IO a

If your intention truly is that prompt return a value of some arbitrary user-specified type, you need to resolve the ambiguity when you actually read the value.

prompt :: Read a => String -> IO a
prompt str = do putStrLn str
                valor <- readLn
                return valor

You'll get an ambiguity error in main, since Haskell can't figure out what type you want v to be. This can be resolved by explicitly specifying.

v <- prompt "Whats your name?" :: IO String

(or, equivalently, IO Int for an integer, IO Double for a double, or any other readable type)

However, more than likely you intended that prompt always return a string. So I recommend the first solution, as it will be more intuitive to users.