- newtype Wizard backend a = Wizard (MaybeT (Free backend) a)
- type PromptString = String
- data (f :+: g) w
- class (Functor sub, Functor sup) => sub :<: sup
- inject :: g :<: f => g (Free f a) -> Free f a
- class Run a b where
- runAlgebra :: b (a v) -> a v
- run :: (Functor f, Monad b, Run b f) => Wizard f a -> b (Maybe a)
- data Output w = Output String w
- data OutputLn w = OutputLn String w
- data Line w = Line PromptString (String -> w)
- data LinePrewritten w = LinePrewritten PromptString String String (String -> w)
- data Password w = Password PromptString (Maybe Char) (String -> w)
- data Character w = Character PromptString (Char -> w)
- data ArbitraryIO w = forall a . ArbitraryIO (IO a) (a -> w)
Wizard b a is a conversation with the user via back-end
b that will result in a data type
a, or may fail.
Wizard is made up of one or more "primitives" (see below), composed using the
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.
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.
Coproduct of two functors
Subsumption of two functors. You shouldn't define any of your own instances of this when writing back-ends, rely only on GeneralizedNewtypeDeriving.
|Functor f => f :<: f|
|ArbitraryIO :<: Haskeline|
|ArbitraryIO :<: BasicIO|
|Password :<: Haskeline|
|LinePrewritten :<: Haskeline|
|Character :<: Haskeline|
|Character :<: BasicIO|
|Character :<: Pure|
|Line :<: Haskeline|
|Line :<: BasicIO|
|Line :<: Pure|
|OutputLn :<: Haskeline|
|OutputLn :<: BasicIO|
|OutputLn :<: Pure|
|Output :<: Haskeline|
|Output :<: BasicIO|
|Output :<: Pure|
|WithSettings :<: Haskeline|
|(Functor f, Functor g, Functor h, :<: f g) => f :<: (:+: h g)|
|(Functor f, Functor g) => f :<: (:+: f g)|
Injection function for free monads, see "Data Types a la Carte" from Walter Swierstra,
A class for implementing actions on a backend. E.g Run IO Output provides an interpreter for the Output action in the IO monad.
|Run IO ArbitraryIO|
|Run IO Character|
|Run IO Line|
|Run IO OutputLn|
|Run IO Output|
|Run IO BasicIO|
|(Run b f, Run b g) => Run b (:+: f g)|
|Run (InputT IO) ArbitraryIO|
|Run (InputT IO) Password|
|Run (InputT IO) LinePrewritten|
|Run (InputT IO) Character|
|Run (InputT IO) Line|
|Run (InputT IO) OutputLn|
|Run (InputT IO) Output|
|Run (InputT IO) WithSettings|
|Run (InputT IO) Haskeline|
|Run (State PureState) Character|
|Run (State PureState) Line|
|Run (State PureState) OutputLn|
|Run (State PureState) Output|
|Run (State PureState) Pure|
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
A short tutorial on writing backends.
Backends consist of two main components:
- A monad,
M, in which the primitive actions are interpreted.
Runinstances specify an interpreter for each supported action, e.g
Run M Outputwill specify an interpreter for the
Outputprimitive 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
Functorinstance, and the
Runinstance are provided by generalized newtype deriving.
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
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)