Control.Varying.Core

Value streams represent values that change over a given domain.

A stream takes some input (the domain e.g. time, place, etc) and when sampled using runVar - produces a value and a new value stream. This pattern is known as an automaton. varying uses this pattern as its base type with the additon of a monadic computation to create locally stateful signals that change over some domain.

Documentation

data Var m b c

The vessel of a value stream. A Var is a structure that contains a value that changes over some input. That input could be time (Float, Double, etc) or Events or Char - whatever. It's a kind of Mealy machine (an automaton) with effects.

Constructors

Var

Fields

• runVar :: b -> m (c, Var m b c)

  Given an input value, return a computation that effectfully produces an output value (a sample) and a Var for producing the next sample.

Instances

(Applicative m, Monad m) => Category * (Var m)

A very simple category instance.

  id = var id
  f . g = g ~> f

or

 f . g = f <~ g

It is preferable for consistency (and readability) to use 'plug left' (<~) and 'plug right' (~>) instead of (.) where possible.

Methods

id :: Var m a a

(.) :: Var m b c -> Var m a b -> Var m a c

(Applicative m, Monad m) => Arrow (Var m)

Vars are arrows, which means you can use proc notation.

v = proc a -> do
      ex <- intEventVar -< ()
      ey <- anotherIntEventVar -< ()
      returnA -< (+) <$> ex <*> ey

which is equivalent to

 v = (\ex ey -> (+) <$> ex <*> ey) <$> intEventVar <*> anotherIntEventVar

Methods

arr :: (b -> c) -> Var m b c

first :: Var m b c -> Var m (b, d) (c, d)

second :: Var m b c -> Var m (d, b) (d, c)

(***) :: Var m b c -> Var m b' c' -> Var m (b, b') (c, c')

(&&&) :: Var m b c -> Var m b c' -> Var m b (c, c')

(Applicative m, Monad m) => Functor (Var m b)

You can transform the sample value of any Var:

 fmap (*3) $ accumulate (+) 0

Will sum input values and then multiply the sum by 3.

Methods

fmap :: (a -> c) -> Var m b a -> Var m b c

(<$) :: a -> Var m b c -> Var m b a

(Applicative m, Monad m) => Applicative (Var m a)

Vars are applicative.

 (,) <$> pure True <*> var "Applicative"

Methods

pure :: b -> Var m a b

(<*>) :: Var m a (b -> c) -> Var m a b -> Var m a c

(*>) :: Var m a b -> Var m a c -> Var m a c

(<*) :: Var m a b -> Var m a c -> Var m a b

(Applicative m, Monad m, Floating b) => Floating (Var m a b)

Vars can be written as floats.

 let v = pi ~> accumulate (*) 0.0

which will attempt (and succeed) to multiply pi by zero every step.

Methods

pi :: Var m a b

exp :: Var m a b -> Var m a b

log :: Var m a b -> Var m a b

sqrt :: Var m a b -> Var m a b

(**) :: Var m a b -> Var m a b -> Var m a b

logBase :: Var m a b -> Var m a b -> Var m a b

sin :: Var m a b -> Var m a b

cos :: Var m a b -> Var m a b

tan :: Var m a b -> Var m a b

asin :: Var m a b -> Var m a b

acos :: Var m a b -> Var m a b

atan :: Var m a b -> Var m a b

sinh :: Var m a b -> Var m a b

cosh :: Var m a b -> Var m a b

tanh :: Var m a b -> Var m a b

asinh :: Var m a b -> Var m a b

acosh :: Var m a b -> Var m a b

atanh :: Var m a b -> Var m a b

(Applicative m, Monad m, Fractional b) => Fractional (Var m a b)

Vars can be written as fractionals.

 let v = 2.5 ~> accumulate (+) 0

which will add 2.5 each step.

Methods

(/) :: Var m a b -> Var m a b -> Var m a b

recip :: Var m a b -> Var m a b

fromRational :: Rational -> Var m a b

(Applicative m, Monad m, Num b) => Num (Var m a b)

Vars can be written as numbers.

 let v = 1 ~> accumulate (+) 0

which will sum the natural numbers.

Methods

(+) :: Var m a b -> Var m a b -> Var m a b

(-) :: Var m a b -> Var m a b -> Var m a b

(*) :: Var m a b -> Var m a b -> Var m a b

negate :: Var m a b -> Var m a b

abs :: Var m a b -> Var m a b

signum :: Var m a b -> Var m a b

fromInteger :: Integer -> Var m a b

(Applicative m, Monad m, Monoid b) => Monoid (Var m a b)

Vars can be monoids

let v = var (const "Hello ") `mappend` var (const "World!")

Methods

mempty :: Var m a b

mappend :: Var m a b -> Var m a b -> Var m a b

mconcat :: [Var m a b] -> Var m a b

Creating value streams

You can create a pure value stream by lifting a function (a -> b) with var:

addsOne :: Monad m => Var m Int Int
addsOne = var (+1)

var is equivalent to arr.

You can create a monadic value stream by lifting a monadic computation (a -> m b) using varM:

getsFile :: Var IO FilePath String
getsFile = varM readFile

You can create either with the raw constructor. You can also create your own combinators using the raw constructor, as it allows you full control over how value streams are stepped and sampled:

delay :: Monad m => b -> Var m a b -> Var m a b
delay b v = Var $ a -> return (b, go a v)
    where go a v' = Var $ a' -> do (b', v'') <- runVar v' a
                                    return (b', go a' v'')

var :: Applicative a => (b -> c) -> Var a b c

Lift a pure computation into a Var.

varM :: Monad m => (a -> m b) -> Var m a b

Lift a monadic computation into a Var.

mkState

Arguments

:: Monad m

=> (a -> s -> (b, s))

state transformer

-> s

intial state

-> Var m a b

Create a Var from a state transformer.

Composing value streams

You can compose value streams together using ~> and <~. The "right plug" (~>) takes the output from a value stream on the left and "plugs" it into the input of the value stream on the right. The "left plug" does the same thing in the opposite direction. This allows you to write value streams that read naturally.

(<~) :: Monad m => Var m b c -> Var m a b -> Var m a c infixl 1

Same as ~> with flipped parameters.

(~>) :: Monad m => Var m a b -> Var m b c -> Var m a c infixr 1

Connects two Vars by chaining the first's output into the input of the second. This is the defacto Var composition method and in fact . is an alias of <~, which is just ~> flipped.

Adjusting and accumulating

delay :: Monad m => b -> Var m a b -> Var m a b

Delays the given stream by one sample using the argument as the first sample. This enables the programmer to create streams that depend on themselves for values. For example:

let v = 1 + delay 0 v in testVar_ v

accumulate :: Monad m => (c -> b -> c) -> c -> Var m b c

Accumulates input values using a folding function and yields that accumulated value each sample.

Sampling value streams (running and other entry points)

The easiest way to sample a stream is to run it in the desired monad with runVar. This will produce a sample value and a new stream.

do (sample, v') <- runVar v inputValue

Much like Control.Monad.State there are other entry points for running value streams like evalVar, execVar. There are also extra control structures such as loopVar and whileVar.

evalVar :: Functor m => Var m a b -> a -> m b

Iterate a Var once and return the sample value.

execVar :: Functor m => Var m a b -> a -> m (Var m a b)

Iterate a Var once and return the next Var.

loopVar :: Monad m => a -> Var m a a -> m a

Loop over a Var that produces its own next input value.

loopVar_ :: (Functor m, Monad m) => Var m () a -> m ()

Loop over a Var that takes no input value.

whileVar

Arguments

:: Monad m

=> (a -> Bool)

The predicate to evaluate samples.

-> a

The initial input/sample value.

-> Var m a a

The Var to iterate

-> m a

The last sample

Iterate a Var that produces its own next input value until the given predicate fails.

whileVar_ :: Monad m => (a -> Bool) -> Var m () a -> m a

Iterate a Var that requires no input until the given predicate fails.

Testing value streams

testVar :: (Read a, Show b) => Var IO a b -> IO ()

A utility function for testing Vars that require input. The input must have a Read instance. Use this in GHCI to step through your Vars by typing the input and hitting return.

testVar_ :: Show b => Var IO () b -> IO ()

A utility function for testing Vars that don't require input. Use this in GHCI to step through your Vars using the return key.

testWhile_ :: Show a => (a -> Bool) -> Var IO () a -> IO ()

A utility function for testing Vars that don't require input. Runs a Var printing each sample until the given predicate fails.

vtrace :: (Applicative a, Show b) => Var a b b

Trace the sample value of a Var and pass it along as output. This is very useful for debugging graphs of Vars.

vstrace :: (Applicative a, Show b) => String -> Var a b b

Trace the sample value of a Var with a prefix and pass the sample along as output. This is very useful for debugging graphs of Vars.

vftrace :: Applicative a => (b -> String) -> Var a b b

Trace the sample value after being run through a "show" function. This is very useful for debugging graphs of Vars.