Simple text menu in Haskell

Question

I would like to know what is the best solution to create simple menu with functionality described below (pseudo code) just like I'm used to:

while (true) {
    x = readLine();
    case (x):
         x == "1" then do sth1 function
         x == "2" then do sth2 function
}

Or maybe any other ideas on how to make a menu not in the pattern described above?

Answer 1

There's a few cool packages for high level ways to construct command line systems in general:

1. ui-command: A framework for friendly commandline programs
2. haskeline: A command-line interface for user input, written in Haskell.
3. HCL: High-level library for building command line interfaces.

I particularly like ui-command, as it is an entire framework for your command line tools: It will dispatch to handler functions that you provide for each command, and also provide command-specific help to the user.

The goal is a polished feeling, rather than a hackish feeling.

Answer 2

Something like

menu :: IO ()
menu = do
      putStrLn . unlines $ map concatNums choices
      choice <- getLine
      case validate choice of
         Just n  -> execute . read $ choice
         Nothing -> putStrLn "Please try again"

      menu
   where concatNums (i, (s, _)) = show i ++ ".) " ++ s

validate :: String -> Maybe Int
validate s = isValid (reads s)
   where isValid []            = Nothing
         isValid ((n, _):_) 
               | outOfBounds n = Nothing
               | otherwise     = Just n
         outOfBounds n = (n < 1) || (n > length choices)

choices :: [(Int, (String, IO ()))]
choices = zip [1.. ] [
   ("DoSomething", foo)
 , ("Quit", bar)
 ]

execute :: Int -> IO ()
execute n = doExec $ filter (\(i, _) -> i == n) choices
   where doExec ((_, (_,f)):_) = f

foo = undefined
bar = undefined

You could probably split the enumerating in "choices" so you only have the descriptions and functions inside it, a little bit of separation, but this works. Evaluating the "menu" function will let you choose what to do!

Answer 3

Here's another example that is a little more menu-like, in that it reads single characters in reacts directly, without requiring the user to press enter.

import System.IO
import System.Exit

import Control.Monad


main = forever (printMenu >> readChoice >>= menuAction)

printMenu = putStr "\np)rint 'Hello, world!'\ne)xit\nyour choice: " >> hFlush stdout

readChoice = hSetBuffering stdin NoBuffering >> hSetEcho stdin False >> getChar

menuAction 'p' = putStrLn "\nHello, world!"
menuAction 'e' = exitSuccess
menuAction _ = hPutStrLn stderr "\nInvalid choice."