Data.Machine.Mealy

newtype MealyT f i o

Mealy is a finite state machine, where:

• f is the effect under which we evaluate,
• i is the input type and
• o is the output type.

Instances

• (Functor f) => Functor (MealyT f i)
• (Apply f) => Apply (MealyT f i)
• (Applicative f) => Applicative (MealyT f i)
• (Functor f) => Profunctor (MealyT f)
• (Functor f) => Strong (MealyT f)
• (Monad f) => Semigroup (MealyT f i o)
• (Monad f) => Monoid (MealyT f i o)
• (Monad f) => Semigroupoid (MealyT f)
• (Monad f) => Category (MealyT f)
• (Monad f) => Bind (MealyT f i)
• (Monad f) => Monad (MealyT f i)
• (Monad f) => Alt (MealyT f i)
• (Monad f) => Plus (MealyT f i)
• (Monad f) => Alternative (MealyT f i)
• (Monad f) => MonadPlus (MealyT f i)
• (MonadEffect f) => MonadEffect (MealyT f i)
• Lazy (MealyT f i o)

runMealyT :: forall f i o. MealyT f i o -> i -> f (Step f i o)

hoistMealyT :: forall f g i. Functor g => (f ~> g) -> (MealyT f i) ~> (MealyT g i)

Transforms a Mealy machine running in the context of f into one running in g, given a natural transformation from f to g.

data Step f i o

Step is the core for running machines. Machines can either stop via the Halt constructor, or emit a value and recursively construct the rest of the machine.

Constructors

• Emit o (MealyT f i o)
• Halt

type Source f o = MealyT f Unit o

Sources are 'initial nodes' in machines. They allow for data to be generated.

type Sink f i = MealyT f i Unit

Sinks are 'terminator nodes' in machines. They allow for an effectful computation to be executed on the inputs.

source :: forall f o. Functor f => f o -> Source f o

Wrap an effectful value into a source. The effect will be repeated indefinitely.

For example, generating ten instances of the value 1:

take 10 $ source (pure 1)

sink :: forall f i. Functor f => (i -> f Unit) -> Sink f i

Construct a machine which executes an effectful computation on its inputs.

For example, logging could be used as a sink:

take 10 $ source (pure 1) >>> sink logShow

stepMealy :: forall f i o. i -> MealyT f i o -> f (Step f i o)

Execute (unroll) a single step on a machine.

runMealy :: forall f. Monad f => MealyT f Unit Unit -> f Unit

Run a machine as an effectful computatation.

For example:

runMealy $ take 10 $ source (pure 1) >>> sink logShow

pureMealy :: forall f i o. Applicative f => (i -> Step f i o) -> MealyT f i o

Wrap a pure function into a machine. The function can either terminate via Halt, or Emit a value and then decide whether to Halt, continue with a different function, or (usually) wrap itself via pureMealy recursively.

For example, we can Halt on zero:

haltOn0 :: forall f. Applicative f => MealyT f Int Int
haltOn0 = pureMealy go
  where
    go 0 = Halt
    go n = Emit n (pureMealy haltOn0)

mealy :: forall f i o. (i -> f (Step f i o)) -> MealyT f i o

Wrap an effectful function into a machine. See pureMealy for an example using pure functions.

halt :: forall f i o. Applicative f => MealyT f i o

A machine which halts for any input.

take :: forall f i o. Applicative f => Int -> MealyT f i o -> MealyT f i o

Limit the number of outputs of a machine. After using up the n allotted outputs, the machine will halt.

drop :: forall f i o. Monad f => Int -> MealyT f i o -> MealyT f i o

Skip a number of outputs for a machine.

loop :: forall f i o. Monad f => MealyT f i o -> MealyT f i o

Loop a machine forever.

toUnfoldable :: forall f g i o. Unfoldable g => Comonad f => i -> MealyT f i o -> g o

Extract all the outputs of a machine, given some input.

zipWith :: forall f i a b c. Apply f => (a -> b -> c) -> MealyT f i a -> MealyT f i b -> MealyT f i c

Zip two machines together under some function f.

scanl :: forall f i a b. Functor f => (b -> a -> b) -> b -> MealyT f i a -> MealyT f i b

Accumulate the outputs of a machine into a new machine.

collect :: forall f i o. Functor f => MealyT f i o -> MealyT f i (List o)

Accumulates the outputs of a machine as a List.

singleton :: forall f i o. Applicative f => o -> MealyT f i o

Creates a machine which emits a single value before halting.

fromMaybe :: forall f i o. Applicative f => Maybe o -> MealyT f i o

Creates a machine which either emits a single value before halting (for Just), or just halts (in the case of Nothing).

fromArray :: forall f i o. Monad f => Array o -> MealyT f i o

Creates a machine which emits all the values of the array before halting.

msplit :: forall f i o. Applicative f => MealyT f i o -> MealyT f i (Maybe (Tuple o (MealyT f i o)))

Unwrap a machine such that its output is either Nothing in case it would halt, or Just the output value and the next computation.

interleave :: forall f i o. Monad f => MealyT f i o -> MealyT f i o -> MealyT f i o

Interleaves the values of two machines with matching inputs and outputs.

when :: forall f i a b. Monad f => MealyT f i a -> (a -> MealyT f i b) -> MealyT f i b

Given a machine and a continuation, it will pass outputs from the machine to the continuation as long as possible until one of them halts.

ifte :: forall f i a b. Monad f => MealyT f i a -> (a -> MealyT f i b) -> MealyT f i b -> MealyT f i b

If then else: given a machine producing a, a continuation f, and a machine producing b, generate a machine which will grab outputs from the first machine and pass them over to the continuation as long as neither halts. Once the process halts, the second (b) machine is returned.

wrapEffect :: forall f i o. Applicative f => f o -> MealyT f i o

Creates a machine which wraps an effectful computation and ignores its input.