Control.Auto.Effects

This module exports the preferred ways of interacting with the underlying Monad of the Auto type, including accessing, executing, and manipulating such effects.

Running effects

Continually

arrM

Arguments

:: (a -> m b)

monadic function

-> Auto m a b

Applies the given "monadic function" (function returning a monadic action) to every incoming item; the result is the result of executing the action returned.

Note that this essentially lifts a "Kleisli arrow"; it's like arr, but for "monadic functions" instead of normal functions:

arr  :: (a -> b)   -> Auto m a b
arrM :: (a -> m b) -> Auto m a b

arrM f . arrM g == arrM (f <=< g)

One neat trick you can do is that you can "tag on effects" to a normal Auto by using *> from Control.Applicative. For example:

>>> let a = arrM print *> sumFrom 0
>>> ys <- streamAuto a [1..5]
1                -- IO output
2
3
4
5
>>> ys
[1,3,6,10,15]    -- the result

Here, a behaves "just like" sumFrom 0...except, when you step it, it prints out to stdout as a side-effect. We just gave automatic stdout logging behavior!

effect

Arguments

:: m b

monadic action to contually execute.

-> Auto m a b

To get every output, executes the monadic action and returns the result as the output. Always ignores input.

This is basically like an "effectful" pure:

pure   :: b   -> Auto m a b
effect :: m b -> Auto m a b

The output of pure is always the same, and the output of effect is always the result of the same monadic action. Both ignore their inputs.

Fun times when the underling Monad is, for instance, Reader.

>>> let a = effect ask    :: Auto (Reader b) a b
>>> let r = evalAuto a () :: Reader b b
>>> runReader r "hello"
"hello"
>>> runReader r 100
100

If your underling monad has effects (IO, State, Maybe, Writer, etc.), then it might be fun to take advantage of *> from Control.Applicative to "tack on" an effect to a normal Auto:

>>> let a = effect (modify (+1)) *> sumFrom 0 :: Auto (State Int) Int Int
>>> let st = streamAuto a [1..10]
>>> let (ys, s') = runState st 0
>>> ys
[1,3,6,10,15,21,28,36,45,55]
>>> s'
10

Out Auto a behaves exactly like sumFrom 0, except at each step, it also increments the underlying/global state by one. It is sumFrom 0 with an "attached effect".

From inputs

effects :: Monad m => Auto m (m a) a

The input stream is a stream of monadic actions, and the output stream is the result of their executions, through executing them.

On Blips

arrMB :: Monad m => (a -> m b) -> Auto m (Blip a) (Blip b)

Maps one blip stream to another; replaces every emitted value with the result of the monadic function, executing it to get the result.

effectB :: Monad m => m b -> Auto m (Blip a) (Blip b)

Maps one blip stream to another; replaces every emitted value with the result of a fixed monadic action, run every time an emitted value is received.

execB :: Monad m => m b -> Auto m (Blip a) (Blip a)

Outputs the identical blip stream that is received; however, every time it sees an emitted value, executes the given monadic action on the side.

One-time effects

cache

Arguments

:: (Serialize b, Monad m)

=> m b

monadic action to execute and use the result of

-> Auto m a b

The very first output executes a monadic action and uses the result as the output, ignoring all input. From then on, it persistently outputs that first result.

Like execOnce, except outputs the result of the action instead of ignoring it.

Useful for loading resources in IO on the "first step", like a word list:

dictionary :: Auto IO a [String]
dictionary = cache (lines $ readFile "wordlist.txt")

execOnce

Arguments

:: Monad m

=> m b

monadic action to execute; result discared

-> Auto m a ()

Always outputs '()', but when asked for the first output, executes the given monadic action.

Pretty much like cache, but always outputs '()'.

cache_

Arguments

:: Monad m

=> m b

monadic action to execute and use the result of

-> Auto m a b

The non-resumable/non-serializable version of cache. Every time the Auto is deserialized/reloaded, it re-executes the action to retrieve the result again.

Useful in cases where you want to "re-load" an expensive resource on every startup, instead of saving it to in the save states.

dictionary :: Auto IO a [String]
dictionary = cache_ (lines $ readFile "dictionary.txt")

execOnce_

Arguments

:: Monad m

=> m b

monadic action to execute; result discared

-> Auto m a ()

The non-resumable/non-serializable version of execOnce. Every time the Auto is deserialized/reloaded, the action is re-executed again.

Manipulating underlying monads

"Sealing off" monadic Autos

sealState :: (Monad m, Serialize s) => Auto (StateT s m) a b -> s -> Auto m a b

Takes an Auto that works with underlying global, mutable state, and "seals off the state" from the outside world.

An 'Auto (StateT s m) a b' maps a stream of a to a stream of b, but does so in the context of requiring an initial s to start, and outputting a modified s.

Consider this example State Auto:

foo :: Auto (State s) Int Int
foo = proc x -> do
    execB (modify (+1)) . emitOn odd  -< x
    execB (modify (*2)) . emitOn even -< x
    st   <- effect get -< ()
    sumX <- sumFrom 0  -< x
    id    -< sumX + st

On every output, the "global" state is incremented if the input is odd and doubled if the input is even. The stream st is always the value of the global state at that point. sumX is the cumulative sum of the inputs. The final result is the sum of the value of the global state and the cumulative sum.

In writing like this, you lose some of the denotative properties because you are working with a global state that updates at every output. You have some benefit of now being able to work with global state, if that's what you wanted I guess.

To "run" it, you could use streamAuto to get a State Int Int:

>>> let st = streamAuto foo [1..10] :: State Int Int
>>> runState st 5
([  7, 15, 19, 36, 42, 75, 83,136,156,277], 222)

(The starting state is 5 and the ending state after all of that is 222)

However, writing your entire program with global state is a bad bad idea! So, how can you get the "benefits" of having small parts like foo be written using State, and being able to use it in a program with no global state?

Using sealState!

sealState       :: Auto (State s) a b -> s -> Auto' a b
sealState foo 5 :: Auto' Int Int

bar :: Auto' Int (Int, String)
bar = proc x -> do
    food <- sealState foo 5 -< x
    id -< (food, show x)

>>> streamAuto' bar [1..10]
[ (7, "1"), (15, "2"), (19, "3"), (36, "4"), (42, "5"), (75, "6") ...

We say that sealState f s0 takes an input stream, and the output stream is the result of running the stream through f, first with an initial state of s0, and afterwards with each next updated state.

This can be extended to sealing RandT from the MonadRandom package as well, as long as you hoistA first with StateT . runRandT.

sealState_ :: Monad m => Auto (StateT s m) a b -> s -> Auto m a b

The non-resuming/non-serializing version of sealState.

sealReader

Arguments

:: (Monad m, Serialize r)

=> Auto (ReaderT r m) a b

Auto run over Reader

-> r

the perpetual environment

-> Auto m a b

Takes an Auto that operates under the context of a read-only environment, an environment value, and turns it into a normal Auto that always "sees" that value when it asks for one.

>>> let a   = effect ask :: Auto (Reader b) a b
>>> let rdr = streamAuto' a [1..5] :: Reader b [b]
>>> runReader rdr "hey"
["hey", "hey", "hey", "hey", "hey"]

Useful if you wanted to use it inside/composed with an Auto that does not have a global environment:

bar :: Auto' Int String
bar = proc x -> do
    hey <- sealReader (effect ask) "hey" -< ()
    id -< hey ++ show x

>>> streamAuto' bar [1..5]
["hey1", "hey2", "hey3", "hey4", "hey5"]

Note that this version serializes the given r environment, so that every time the Auto is reloaded/resumed, it resumes with the originally given r environment, ignoring whatever r is given to it when trying to resume it. If this is not the behavior you want, use sealReader_.

sealReader_

Arguments

:: Monad m

=> Auto (ReaderT r m) a b

Auto run over Reader

-> r

the perpetual environment

-> Auto m a b

The non-resuming/non-serializing version of sealReader. Does not serialize/reload the r environment, so that whenever you "resume" the Auto, it uses the new r given when you are trying to resume, instead of loading the originally given one.

Unrolling/"reifying" monadic Autos

runStateA

Arguments

:: Monad m

=> Auto (StateT s m) a b

Auto run over a state transformer

-> Auto m (a, s) (b, s)

Auto whose inputs and outputs are a start transformer

Unrolls the underlying StateT of an Auto into an Auto that takes in an input state every turn (in addition to the normal input) and outputs, along with the original result, the modified state.

So now you can use any StateT s m as if it were an m. Useful if you want to compose and create some isolated Autos with access to an underlying state, but not your entire program.

Also just simply useful as a convenient way to use an Auto over State with stepAuto and friends.

When used with State s, it turns an Auto (State s) a b into an Auto' (a, s) (b, s).

runReaderA

Arguments

:: Monad m

=> Auto (ReaderT r m) a b

Auto run over global environment

-> Auto m (a, r) b

Auto receiving environments

Unrolls the underlying ReaderT of an Auto into an Auto that takes in the input "environment" every turn in addition to the normal input.

So you can use any ReaderT r m as if it were an m. Useful if you want to compose and create some isolated Autos with access to an underlying environment, but not your entire program.

Also just simply useful as a convenient way to use an Auto over Reader with stepAuto and friends.

When used with Reader r, it turns an Auto (Reader r) a b into an Auto' (a, r) b.

runWriterA :: (Monad m, Monoid w) => Auto (WriterT w m) a b -> Auto m a (b, w)

Unrolls the underlying WriterT w m Monad, so that an Auto that takes in a stream of a and outputs a stream of b will now output a stream (b, w), where w is the accumulated log of the underlying Writer at every step.

foo :: Auto (Writer (Sum Int)) Int Int
foo = effect (tell 1) *> effect (tell 1) *> sumFrom 0

>>> let fooWriter = streamAuto foo
>>> runWriter $ fooWriter [1..10]
([1,3,6,10,15,21,28,36,45,55], Sum 20)

foo increments an underlying counter twice every time it is stepped; its "result" is just the cumulative sum of the inputs.

When we "stream" it, we get a [Int] -> Writer (Sum Int) [Int]...which we can give an input list and runWriter it, getting a list of outputs and a "final accumulator state" of 10, for stepping it ten times.

We can write and compose own Autos under Writer, using the convenience of a shared accumulator, and then "use them" with other Autos:

bar :: Auto' Int Int
bar = proc x -> do
  (y, w) <- runWriterA foo -< x
  blah <- blah -< w

And now you have access to the underlying accumulator of foo to access. There, w represents the continually updating accumulator under foo, and will be different/growing at every "step".

runTraversableA

Arguments

:: (Monad f, Traversable f)

=> Auto f a b

Auto run over traversable structure

-> Auto m a (f b)

Auto returning traversable structure

Unrolls the underlying Monad of an Auto if it happens to be Traversable ('[]', Maybe, etc.).

It can turn, for example, an Auto [] a b into an Auto' a [b]; it collects all of the results together. Or an Auto Maybe a b into an Auto' a (Maybe b).

This might be useful if you want to make some sort of "underyling inhibiting" Auto where the entire computation might just end up being Nothing in the end. With this, you can turn that possibly-catastrophically-failing Auto (with an underlying Monad of Maybe) into a normal Auto, and use it as a normal Auto in composition with other Autos...returning Just if your computation succeeded.

Hoists

hoistA

Arguments

:: (Monad m, Monad m')

=> (forall c. m c -> m' c)

monad morphism; the natural transformation

-> Auto m a b

-> Auto m' a b

Swaps out the underlying Monad of an Auto using the given monad morphism "transforming function", a natural transformation.

Basically, given a function to "swap out" any m a with an m' a, it swaps out the underlying monad of the Auto.

This forms a functor, so you rest assured in things like this:

hoistA id == id
hoistA f a1 . hoistA f a2 == hoistA f (a1 . a2)

generalizeA :: Monad m => Auto' a b -> Auto m a b

Generalizes an Auto' a b to an Auto m a b' for any Monad m, using hoist.

Working with IO

catchA

Arguments

:: Exception e

=> Auto IO a b

Auto over IO, expecting an exception of a secific type.

-> Auto IO a (Either e b)

Wraps a "try" over an underlying IO monad; if the Auto encounters a runtime exception while trying to "step" itself, it'll output a Left with the Exception. Otherwise, will output left.

Note that you have to explicitly specify the type of the exceptions you are catching; see Control.Exception documentation for more details.

TODO: Possibly look into bringing in some more robust tools from monad-control and other industry established error handling routes? Also, can we modify an underlying monad with implicit cacting behavior?

Constructing monadic Autos from other monads

fromState :: (Serialize s, Monad m) => (a -> StateT s m b) -> s -> Auto m a b

Turns an a -> StateT s m b arrow into an Auto m a b, when given an initial state. Will continually "run the function", using the state returned from the last run.

fromState_ :: Monad m => (a -> StateT s m b) -> s -> Auto m a b

Non-seralizing/non-resuming version of fromState. The state isn't serialized/resumed, so every time the Auto is resumed, it starts over with the given initial state.