101 lines
3.5 KiB
Elm
101 lines
3.5 KiB
Elm
-- This library is a way to package up dynamic behavior. It makes it easier to
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-- dynamically create dynamic components. See the [original release
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-- notes](/blog/announce/version-0.5.0.elm) on this library to get a feel for how
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-- it can be used.
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module Automaton where
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import open Basics
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import Signal (lift,foldp,Signal)
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import open List
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import Maybe (Just, Nothing)
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data Automaton a b = Step (a -> (Automaton a b, b))
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-- Run an automaton on a given signal. The automaton steps forward
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-- whenever the input signal updates.
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run : Automaton a b -> b -> Signal a -> Signal b
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run auto base inputs =
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let step a (Step f, _) = f a
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in lift (\(x,y) -> y) (foldp step (auto,base) inputs)
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-- Step an automaton forward once with a given input.
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step : a -> Automaton a b -> (Automaton a b, b)
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step a (Step f) = f a
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-- Compose two automatons, chaining them together.
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(>>>) : Automaton a b -> Automaton b c -> Automaton a c
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f >>> g =
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Step (\a -> let (f', b) = step a f
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(g', c) = step b g
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in (f' >>> g', c))
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-- Compose two automatons, chaining them together.
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(<<<) : Automaton b c -> Automaton a b -> Automaton a c
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g <<< f = f >>> g
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-- Combine a list of automatons into a single automaton that produces a list.
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combine : [Automaton a b] -> Automaton a [b]
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combine autos =
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Step (\a -> let (autos', bs) = unzip (map (step a) autos)
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in (combine autos', bs))
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-- Create an automaton with no memory. It just applies the given function to
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-- every input.
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pure : (a -> b) -> Automaton a b
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pure f = Step (\x -> (pure f, f x))
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-- Create an automaton with state. Requires an initial state and a step
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-- function to step the state forward. For example, an automaton that counted
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-- how many steps it has taken would look like this:
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--
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-- count = Automaton a Int
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-- count = state 0 (\\_ c -> c+1)
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--
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-- It is a stateful automaton. The initial state is zero, and the step function
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-- increments the state on every step.
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state : b -> (a -> b -> b) -> Automaton a b
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state s f = Step (\x -> let s' = f x s
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in (state s' f, s'))
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-- Create an automaton with hidden state. Requires an initial state and a
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-- step function to step the state forward and produce an output.
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hiddenState : s -> (a -> s -> (s,b)) -> Automaton a b
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hiddenState s f = Step (\x -> let (s',out) = f x s
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in (hiddenState s' f, out))
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-- Count the number of steps taken.
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count : Automaton a Int
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count = state 0 (\_ c -> c + 1)
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type Queue t = ([t],[t])
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empty = ([],[])
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enqueue x (en,de) = (x::en, de)
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dequeue q = case q of
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([],[]) -> Nothing
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(en,[]) -> dequeue ([], reverse en)
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(en,hd::tl) -> Just (hd, (en,tl))
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-- Computes the running average of the last `n` inputs.
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average : Int -> Automaton Float Float
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average k =
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let step n (ns,len,sum) =
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if len == k then stepFull n (ns,len,sum)
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else ((enqueue n ns, len+1, sum+n), (sum+n) / (toFloat len+1))
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stepFull n (ns,len,sum) =
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case dequeue ns of
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Nothing -> ((ns,len,sum), 0)
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Just (m,ns') -> let sum' = sum + n - m
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in ((enqueue n ns', len, sum'), sum' / toFloat len)
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in hiddenState (empty,0,0) step
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{-- TODO(evancz): See the following papers for ideas on how to make this
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library faster and better:
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- Functional Reactive Programming, Continued
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- Causal commutative arrows and their optimization
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Speeding things up is a really low priority. Language features and
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libraries with nice APIs and are way more important!
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--}
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