How to fix writing a list content in a file - haskell

Question

I have a function which contains a list. I want just to write that list content in a file from main after user input.
putTodo :: (Int, String) -> IO ()
putTodo (n, todo) = putStrLn (show n ++ ": " ++ todo)
prompt :: [String] -> IO ()
prompt todos = do
putStrLn "The list contains:"
mapM_ putTodo (zip [0..] todos)
putStrLn " "
command <- getLine
getCommand command todos
What I tried:
main = do
outh <- openFile "agenda.txt" WriteMode;
hPutStrLn outh prompt[]
-- hPutStrLn outh (show prompt[])
-- hPrint (show prompt[])
hClose outh;
Thank you.

Answer 1

Your code contains a couple of errors / problems:
prompt[] isn't valid (in main) - this should be prompt
hPutStrLn expects a String as its second argument, but you provide IO()
getCommand is not defined
What you need is:
a list of todos (possibly returned by a function)
a function that converts this list of todos to a string
hPutStrLn to print this string to the output file
Here's a simple version with a hard-coded list of todos (my Haskell isn't very advanced, so this could probably be done in a much more elegant way):
import System.IO
type Todo = (Int, String)
todoToString :: Todo -> String
todoToString (idx, name) = (show idx) ++ " : " ++ name
todosToString :: [Todo] -> String
todosToString todos = foldl (\acc t -> acc ++ "\n" ++ (todoToString t)) "" todos
allTodos :: [Todo]
allTodos = [(1, "Buy milk"), (2, "Program Haskell")]
main = do
outh <- openFile "agenda.txt" WriteMode;
hPutStrLn outh (todosToString allTodos);
hClose outh;