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Programming language: Haskell
License: BSD 3-clause "New" or "Revised" License
Tags: User Interfaces    

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wizards is an Haskell library designed for the quick and painless development of interrogative programs, which revolve around a "dialogue" with the user, who is asked a series of questions in a sequence much like an installation wizard.

Everything from interactive system scripts, to installation wizards, to full-blown shells can be implemented with the support of wizards.

It is developed transparently on top of a free monad (see Swierstra's excellent paper on this topic), which separates out the semantics of the program from the wizards interface. A variety of backends exist, including a full featured backend for Haskeline, a debug-friendly simpler implementation in terms of System.IO primitives, and a completely pure implementation modelled as a function from an input string to output. It is also possible to write your own backends, or extend the existing back-ends with new features.

While both built-in IO backends operate on a console, there is no reason why wizards cannot also be used for making GUI wizard interfaces.

Below are installation instructions and some educational examples.

Information on how to write backends or extend backends, as well as structured API documentation is available on Hackage:


(Or, you can just run cabal haddock to generate the documentation from the source).


To install with cabal from hackage, just go:

cabal install wizards

Otherwise, to install from source:

git clone https://github.com/liamoc/wizards.git
cd wizards
runhaskell Setup.hs configure
runhaskell Setup.hs build
runhaskell Setup.hs install

Or, if you have cabal, you can replace runhaskell Setup.hs with cabal there.


A value of type Wizard b a is a conversation with the user via back-end b that will result in a value of type a, or fail. Monad, Applicative and Alternative instances are defined. Code can also be written monomorphically for a specific back-end:

foo :: Wizard Haskeline Int

Or polymorphically for many back-ends like so:

foo :: (Output :<: b, Line :<: b) => Wizard b Int

This describes a Wizard that will result in an Int that runs on any back-end that supports capabilities for Output and Line.

Below are a series of educational examples. You'll probably need to run them with -XOverlappingInstances. If you want more structured documentation, please refer to the API documentation on Hackage (or generate it with cabal haddock).

{-# LANGUAGE OverlappingInstances, TypeOperators, FlexibleContexts #-}  
import System.Console.Haskeline  
import System.Console.Wizard  
import System.Console.Wizard.Haskeline --  
import System.Console.Wizard.BasicIO   -- choose a backend, Haskeline recommended.  
import System.Console.Wizard.Pure      --  
import Control.Applicative  
import Control.Monad  
import Control.Monad.Trans  
import Data.Monoid

Student Records

This example demonstrates use of the Applicative instance to build up data structures, retry, inRange, defaultTo, and parseRead.

Suppose we have a Student data type, that contains a name and a class number (which we shall say must be in the interval [1,5]).

type Name = String  
type Class = Int   
data Student = Student Name Class deriving (Show)

A Name must be a non-empty string. If the user enters an empty string, we will prompt them again:

nameWizard :: (Line :<: b) => Wizard b Name
nameWizard = retry $ nonEmpty $ line "Name: "

A Class must be between 1 and 5. If the user enters nothing, we will default to 1. If they enter an invalid string, they will be prompted again:

classWizard :: (Line :<: b) => Wizard b Class
classWizard = retry 
            $ inRange (1,5) 
            $ parseRead 
            $ nonEmpty (line "Class[1]: ") `defaultTo` "1"

We can now populate a Student data type using the Applicative instance of Wizard b.

studentWizard :: (Line :<: b) => Wizard b Student                        
studentWizard = Student <$> nameWizard <*> classWizard

And run our wizard with the Haskeline back-end:

main :: IO ()
main = runInputT defaultSettings (run $ haskeline $ studentWizard)   
   >>= print  

Or with the Basic IO back-end:

main :: IO ()
main = (run $ basicIO $ studentWizard) >>= print  


This example demonstrates masked input, failure (using Alternative), and retryMsg, as well as simple use of validator and outputLn.

Ask for a password three times, then fail:

passwordW :: (Password :<: b, OutputLn :<: b) => String -> Wizard b ()  
passwordW realPassword =   
    w = do validator (== realPassword) $ password "Enter password: " (Just '*') 
           outputLn "The secret is 42"  
  in w <|> w <|> w <|> outputLn "Password rejected. Goodbye!"  

Here we use validator to check if the user has entered the correct password, and, if so, print out a secret message.

Or, for unlimited tries, we can use the retryMsg function (or just retry):

passwordW2 :: (Password :<: b, OutputLn :<: b) => String -> Wizard b ()  
passwordW2 realPassword = (retryMsg "Incorrect password." 
                       $ validator (== realPassword) 
                       $ password "Enter password: " (Just '*'))
                      >> outputLn "The secret is 42"            

To run this in the Haskeline back-end, we can simply use it as follows:

main :: IO ()
main = void $  runInputT defaultSettings $ run $ haskeline $ passwordW "rosebud"

The Basic IO back-end, however, doesn't support password input. We can extend it to simply read a line of unmasked text for password input (i.e ignoring the mask character) easily; by importing the relevant Shim module:

-- don't import this with Haskeline, or Overlapping instances will muck up your password input
import System.Console.Wizard.Shim.Password

And running it with the extension, like so:

main = void $ run $ (passwordW "rosebud" :: Wizard (Password :+: BasicIO) ())

Counting sticks (custom parsers)

This example demonstrates using custom parse functions.

Suppose we have a parser that picks up sticks:

parseSticks :: String -> Maybe Int  
parseSticks [] = Just 0  
parseSticks ('|':r) = fmap (+1) $ parseSticks r  
parseSticks (_:_) = Nothing

We can equip a wizard with this parser using the parser modifier:

sticksW = (do s <- parser parseSticks (line "Enter sticks!: ")
              outputLn $ "I found " ++ show s ++ " sticks!")
          <|> outputLn "I found something that wasn't a stick and got confused."

main :: IO ()
main = void $ runInputT defaultSettings $ run $ haskeline $ sticksW      

This will run the parseSticks parser on the user input, and, if it succeeds, output the number of sticks parsed. If it fails, it will output an error message.

Extended features

The Haskeline and BasicIO backends (or any backends that supports the ArbitraryIO capability) also support embedding arbitrary IO actions inside wizards through a MonadIO instance. For example:

missilesW :: (ArbitraryIO :<: b, Character :<: b) => Wizard b ()  
missilesW = do retry $ validator (== 'x') $ character "Press 'X' to fire the missiles"
               liftIO $ fireTheMissiles
 where fireTheMissiles = putStrLn "FIRE!"              

Another backend-specific feature unique to the Haskeline backend allows setting Haskeline settings through a wizard modifier, for example:

specialHistory :: (WithSettings :<: b, Line :<: b, Output :<: b) => Wizard b ()
specialHistory = withSettings (defaultSettings {historyFile = Just "histfile"})
               $ line "Answers to this question are recorded in histfile" >>= output