wizards-1.0.1: High level, generic library for interrogative user interfaces

Safe HaskellTrustworthy




newtype Wizard backend a Source

A Wizard b a is a conversation with the user via back-end b that will result in a data type a, or may fail. A Wizard is made up of one or more "primitives" (see below), composed using the Applicative, Monad and Alternative instances. The Alternative instance is, as you might expect, a maybe-style cascade. If the first wizard fails, the next one is tried. mzero can be used to induce failure directly.

The Wizard constructor is exported here for use when developing backends, but it is better for end-users to simply pretend that Wizard is an opaque data type. Don't depend on this unless you have no other choice.

Wizards are, internally, just a maybe transformer over a free monad built from some coproduct of functors, each of which is a primitive action.


Wizard (MaybeT (Free backend) a) 


Functor backend => Monad (Wizard backend) 
Functor backend => Functor (Wizard backend) 
(Monad (Wizard backend), Functor backend) => MonadPlus (Wizard backend) 
(Functor (Wizard backend), Functor backend) => Applicative (Wizard backend) 
(Applicative (Wizard backend), Functor backend) => Alternative (Wizard backend) 
(Monad (Wizard b), :<: ArbitraryIO b) => MonadIO (Wizard b) 

type PromptString = StringSource

A string for a prompt

data (f :+: g) w Source

Coproduct of two functors


Inl (f w) 
Inr (g w) 


(Run b f, Run b g) => Run b (:+: f g) 
(Functor (:+: h g), Functor f, Functor g, Functor h, :<: f g) => f :<: (:+: h g) 
(Functor (:+: f g), Functor f, Functor g) => f :<: (:+: f g) 
(Functor f, Functor g) => Functor (:+: f g) 

class (Functor sub, Functor sup) => sub :<: sup Source

Subsumption of two functors. You shouldn't define any of your own instances of this when writing back-ends, rely only on GeneralizedNewtypeDeriving.

inject :: g :<: f => g (Free f a) -> Free f aSource

Injection function for free monads, see "Data Types a la Carte" from Walter Swierstra, http://www.cs.ru.nl/~W.Swierstra/Publications/DataTypesALaCarte.pdf

class Run a b whereSource

A class for implementing actions on a backend. E.g Run IO Output provides an interpreter for the Output action in the IO monad.


runAlgebra :: b (a v) -> a vSource

run :: (Functor f, Monad b, Run b f) => Wizard f a -> b (Maybe a)Source

Run a wizard using some back-end.

Each of the following functors is a primitive action. A back-end provides interpreters for these actions using the Run class,

A short tutorial on writing backends.

Backends consist of two main components:

  1. A monad, M, in which the primitive actions are interpreted. Run instances specify an interpreter for each supported action, e.g Run M Output will specify an interpreter for the Output primitive action in the monad M.
  2. A newtype, e.g Backend a, which is a functor, usually implemented by wrapping a coproduct of all supported features. '(:<:)' instances, the Functor instance, and the Run instance are provided by generalized newtype deriving.

As an example, suppose I am writing a back-end to IO, like System.Console.Wizard.BasicIO. I want to support basic input and output, and arbitrary IO, so I declare instances for Run for the IO monad:

  instance Run IO Output      where runAlgebra (Output s w)        = putStr s   >> w
  instance Run IO OutputLn    where runAlgebra (OutputLn s w)      = putStrLn s >> w
  instance Run IO Line        where runAlgebra (Line s w)          = getLine    >>= w
  instance Run IO Character   where runAlgebra (Character s w)     = getChar    >>= w
  instance Run IO ArbitraryIO where runAlgebra (ArbitraryIO iov f) = iov        >>= f

And then I would define the newtype for the backend, which we can call MyIOBackend:

  newtype MyIOBackend a = MyIOBackend ((Output :+: OutputLn :+: Line :+: Character :+: ArbitraryIO) a)
                        deriving ( Functor, Run IO
                                 , (:<:) Output
                                 , (:<:) OutputLn
                                 , (:<:) Line
                                 , (:<:) Character
                                 , (:<:) ArbitraryIO

A useful convenience is to provide a simple identity function to serve as a type coercion:

  myIOBackend :: Wizard MyIOBackend a -> Wizard MyIOBackend a
  myIOBackend = id

One additional primitive action that I might want to include is the ability to clear the screen at a certain point. So, we define a new data type for the action:

  data ClearScreen w = ClearScreen w deriving Functor -- via -XDeriveFunctor

And a "smart" constructor for use by the user:

  clearScreen :: (ClearScreen :<: b) => Wizard b ()
  clearScreen = Wizard $ lift $ inject (ClearScreen (Pure ())) 

(These smart constructors all follow a similar pattern. See the source of System.Console.Wizard for more examples)

And then we define an interpreter for it:

  instance Run IO ArbitraryIO where runAlgebra (ClearScreen f) = clearTheScreen >> f

Now, we can use this as-is simply by directly extending our back-end:

  foo :: Wizard (ClearScreen :+: MyIOBackend)
  foo = clearScreen >> output "Hello World!"

Or, we could modify MyIOBackend to include the extension directly.

For custom actions that return output, the definition looks slightly different. Here is the definition of Line:

  data Line w = Line (PromptString) (String -> w) deriving Functor -- via -XDeriveFunctor

And the smart constructor looks like this:

  line :: (Line :<: b) => PromptString -> Wizard b String
  line s = Wizard $ lift $ inject (Line s Pure)