Safe Haskell | Trustworthy |
---|---|
Language | Haskell98 |
- 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)
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.
Wizard
s are, internally, just a maybe transformer over a free monad built from some coproduct of functors,
each of which is a primitive action.
type PromptString = String Source
A string for a prompt
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.
inj
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) |
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
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 v Source
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 :: (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,
Line PromptString (String -> w) |
data LinePrewritten w Source
LinePrewritten PromptString String String (String -> w) |
Password PromptString (Maybe Char) (String -> w) |
Character PromptString (Char -> w) |
data ArbitraryIO w Source
forall a . ArbitraryIO (IO a) (a -> w) |
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.gRun M Output
will specify an interpreter for theOutput
primitive action in the monad M.
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)