Safe Haskell	Trustworthy
Language	Haskell98

System.Console.Wizard.Internal

Synopsis

Documentation

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.

Constructors

Wizard (MaybeT (Free backend) a)

Instances

Functor backend => Monad (Wizard backend) Source
Functor backend => Functor (Wizard backend) Source
Functor backend => Applicative (Wizard backend) Source
Functor backend => Alternative (Wizard backend) Source
Functor backend => MonadPlus (Wizard backend) Source

type PromptString = String Source

A string for a prompt

data (f :+: g) w infixr 9 Source

Coproduct of two functors

Constructors

Inl (f w)
Inr (g w)

Instances

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

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.

Minimal complete definition

inj

Instances

Functor f => f :<: f Source
ArbitraryIO :<: Haskeline Source
ArbitraryIO :<: BasicIO Source
Password :<: Haskeline Source
LinePrewritten :<: Haskeline Source
Character :<: Haskeline Source
Character :<: BasicIO Source
Character :<: Pure Source
Line :<: Haskeline Source
Line :<: BasicIO Source
Line :<: Pure Source
OutputLn :<: Haskeline Source
OutputLn :<: BasicIO Source
OutputLn :<: Pure Source
Output :<: Haskeline Source
Output :<: BasicIO Source
Output :<: Pure Source
WithSettings :<: Haskeline Source
(Functor f, Functor g, Functor h, (:<:) f g) => f :<: ((:+:) h g) Source
(Functor f, Functor g) => f :<: ((:+:) f g) Source

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

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 where Source

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

Methods

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

Instances

Run IO BasicIO Source
(Run b f, Run b g) => Run b ((:+:) f g) Source
Run (InputT IO) WithSettings Source
Run (InputT IO) Haskeline Source
Run (State PureState) Pure Source

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,

data Output w Source

Constructors

Output String w

Instances

Functor Output Source
Output :<: Haskeline Source
Output :<: BasicIO Source
Output :<: Pure Source

data OutputLn w Source

Constructors

OutputLn String w

Instances

Functor OutputLn Source
OutputLn :<: Haskeline Source
OutputLn :<: BasicIO Source
OutputLn :<: Pure Source

data Line w Source

Constructors

Line PromptString (String -> w)

Instances

Functor Line Source
Line :<: Haskeline Source
Line :<: BasicIO Source
Line :<: Pure Source

data LinePrewritten w Source

Constructors

LinePrewritten PromptString String String (String -> w)

Instances

Functor LinePrewritten Source
LinePrewritten :<: Haskeline Source

data Password w Source

Constructors

Password PromptString (Maybe Char) (String -> w)

Instances

Functor Password Source
Password :<: Haskeline Source

data Character w Source

Constructors

Character PromptString (Char -> w)

Instances

Functor Character Source
Character :<: Haskeline Source
Character :<: BasicIO Source
Character :<: Pure Source

data ArbitraryIO w Source

Constructors

forall a . ArbitraryIO (IO a) (a -> w)

Instances

Functor ArbitraryIO Source
ArbitraryIO :<: Haskeline Source
ArbitraryIO :<: BasicIO Source

A short tutorial on writing backends.

Backends consist of two main components:

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.
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)