Hi! I have to apologize a bit for the long delay; starting grad school and things like that have made me have to scramble to adjust to the new life. But a couple of people have asked me to finish up and wrap up this series, and I think I owe it to them then :) Welcome to the final chapter.
In the last post, we looked deeper into the Auto type, played around with instancing it as familiar typeclasses, saw it as a member of the powerful Category and Arrow typeclasses, and took advantage of this by composing Autos both manually and using proc/do notation, and were freed from the murk and mire of explicit recursion. We observed the special nature of this composition, and saw some neat properties, like local statefulness.
At this point I consider most of the important concepts about working with
Auto covered, but now, we are going to push this abstraction further, to the limits of real-world industrial usage. We’re going to be exploring mechanisms for adding effects and, making the plain ol’
Auto into something more rich and featureful. We’ll see how to express denotative and declarative compositions using recursively binded
Autos, and what that even means. It’ll be a trip down several avenues to motivate and see practical Auto usage.1 Basically, it’ll be a “final hurrah”.
A fair bit of warning — if the last post is not fresh in your mind, or you still have some holes, I recommend going back and reading through them again. This one is going to hit hard and fast :) (Also, it’s admittedly kind of long for a single post, but I didn’t want to break things up into two really short parts.)
As always, feel free to leave a comment if you have any questions, drop by freenode’s #haskell, or find me on twitter :)
All of the code in this post is available for download and to load up into ghci for playing along!
Recall our original definition of
Auto a b as a newtype wrapper over a function:
This can be read as saying, “feed the
a, and (purely) get a resulting
b, and a ‘next stepper’” — the
b is the result, and the
Auto a b contains the information on how to proceed from then on.
If you’ve been doing Haskell for any decent amount of time, you can probably guess what’s going to happen next!
Instead of “purely” creating a naked result and a “next step”…we’re going to return it in a context.
What, you say? What good does that do?
Well, what does returning things in a context ever let you do?
In Haskell, contexts like these are usually meant to be able to defer the process of “getting the value” until the end, after you’ve built up your contextual computation. This process can be complicated, or simple, or trivial.
For example, a function like:
means that it simply creates a
b from an
a. But a function like:
Means that, given an
a, you get a state machine that can create a
b using a stateful process, once given an initial state. The
b doesn’t “exist” yet; all you’ve given is instructions for creating that
b that is eventually created will in general depend on whatever initial
s you give the state machine.
A function like:
Means that, given an
a, you’re given a computer program that, when executed by a computer, will generate a
b doesn’t “exist” yet; depending on how the world is and how IO processes interact, how you are feeling that day…the
b generated will be different. The process of IO execution has the ability to choose the
So how about something like:
This means that, given
a, “running” the
Auto with an
a will give you a state machine that gives you, using a stateful process, both the result and the next step. The crazy thing is that now you are given the state machine the ability to decide the next
Auto, the next “step”.
means that your new
Auto-running function will give you a result and a “next step” that is going to be dependent on IO actions.
Let’s jump straight to abstracting over this and explore a new type, shall we?
We already explained earlier the new power of this type. Let’s see if we can write our favorite instances with it. First of all, what would a
Category instance even do?
Recall that the previous
Category instance “ticked” each
Auto one after the other and gave the final results, and then the “next Auto” was the compositions of the ticked autos.
In our new type, the “ticking” happens in a context. And we need to tick twice; and the second one is dependent on the result of the first. This means that your context has to be monadic in order to allow you to do this.
So we sequence two “ticks” inside the monadic context, and then return the result afterwards, with the new composed autos.
The neat thing is that Haskell’s built-in syntax for handling monadic sequencing is nice, so you might be surprised when you write the
Does it look familiar?
It should! Remember the logic from the
Auto Category instance?
It’s basically identical and exactly the same :O The only difference is that instead of
let, we have
= we have
<-, and instead of
in we have
The takeaway here is that when you have monadic functions, their sequencing and application and composition can really be abstracted away to look pretty much like application and composition of normal values. And Haskell is one of the few languages that gives you language features and a culture to be able to fully realize the symmetry and similarities.
Check out the
Arrow instances, too — they’re exactly the same!
-- source: https://github.com/mstksg/inCode/tree/master/code-samples/machines/Auto2.hs#L20-L47 instance Functor (Auto r) where fmap f a = ACons $ \x -> let (y, a') = runAuto a x in (f y, fmap f a') instance Arrow Auto where arr f = ACons $ \x -> (f x, arr f) first a = ACons $ \(x, z) -> let (y, a') = runAuto a x in ((y, z), first a') second a = ACons $ \(z, x) -> let (y, a') = runAuto a x in ((z, y), second a') a1 *** a2 = ACons $ \(x1, x2) -> let (y1, a1') = runAuto a1 x1 (y2, a2') = runAuto a2 x2 in ((y1, y2), a1' *** a2') a1 &&& a2 = ACons $ \x -> let (y1, a1') = runAuto a1 x (y2, a2') = runAuto a2 x in ((y1, y2), a1' &&& a2')
-- source: https://github.com/mstksg/inCode/tree/master/code-samples/machines/Auto3.hs#L50-L77 instance Monad m => Functor (AutoM m r) where fmap f a = AConsM $ \x -> do (y, a') <- runAutoM a x return (f y, fmap f a') instance Monad m => Arrow (AutoM m) where arr f = AConsM $ \x -> return (f x, arr f) first a = AConsM $ \(x, z) -> do (y, a') <- runAutoM a x return ((y, z), first a') second a = AConsM $ \(z, x) -> do (y, a') <- runAutoM a x return ((z, y), second a') a1 *** a2 = AConsM $ \(x1, x2) -> do (y1, a1') <- runAutoM a1 x1 (y2, a2') <- runAutoM a2 x2 return ((y1, y2), a1' *** a2') a1 &&& a2 = AConsM $ \x -> do (y1, a1') <- runAutoM a1 x (y2, a2') <- runAutoM a2 x return ((y1, y2), a1' &&& a2')
(I’ve left the rest of the instances from the previous part as an exercise; the solutions are available in the downloadable.)
Neat, huh? Instead of having to learn over again the logic of
ArrowPlus, etc., you can directly use the intuition that you gained from the past part and apply it to here, if you abstract away function application and composition to application and composition in a context.
Our previous instances were then just a “specialized” version of
AutoM, one where we used naked application and composition,2
If you look at the instances we wrote out, you might see that for some of them,
Monad is a bit overkill. For example, for the
is just fine. We only need
Functor to make
AutoM m r a
Functor. Cool, right?
If you try, how much can we “generalize” our other instances to? Which ones can be generalized to
Functor, which ones
Applicative…and which ones can’t?
By the way, it might be worth noting that our original
Auto type is identical to
AutoM Identity — all of the instances do the exact same thing.
Putting it to use
Now let’s try using these!
First some utility functions just for playing around:
autoM, which upgrades an
Auto a b to an
AutoM m a b for any
arrM, which is like
arr, but instead of turning an
a -> b into an
Auto a b, it turns an
a -> m b into an
AutoM m a b:
-- source: https://github.com/mstksg/inCode/tree/master/code-samples/machines/Auto3.hs#L97-L107 autoM :: Monad m => Auto a b -> AutoM m a b autoM a = AConsM $ \x -> let (y, a') = runAuto a x in return (y, autoM a') arrM :: Monad m => (a -> m b) -> AutoM m a b arrM f = AConsM $ \x -> do y <- f x return (y, arrM f)
We will need to of course re-write our trusty
testAuto functions from the first entry, which is again a direct translation of the original ones:
-- source: https://github.com/mstksg/inCode/tree/master/code-samples/machines/Auto3.hs#L31-L39 testAutoM :: Monad m => AutoM m a b -> [a] -> m ([b], AutoM m a b) testAutoM a  = return (, a) testAutoM a (x:xs) = do (y , a' ) <- runAutoM a x (ys, a'') <- testAutoM a' xs return (y:ys, a'') testAutoM_ :: Monad m => AutoM m a b -> [a] -> m [b] testAutoM_ a = liftM fst . testAutoM a
First, let’s test
arrM putStrLn is, like
arr show, just an
Auto with no internal state. It outputs
() for every single input string, except, in the process of getting the “next Auto” (and producing the
()), it emits a side-effect — in our case, printing the string.
We can sort of abuse this to get an
Auto with “two input streams”: one from the normal input, and the other from
monoidAccum (or, an
AutoM version) to accumulate a string. At every step, it adds
inpStr to the running accumulated string.
inpStr is the result of repeating the the string that
getLine returns replicated
n times —
n being the official “input” to the
AutoM when we eventually run it.
Here, we used
IO to get a “side channel input”. The main input is the number of times we repeat the string, and the side input is what we get from sequencing the
You can also use this to “tack on” effects into your pipeline.
logging a will “run”
a with the input like normal (no side-channel inputs), but then also log the results line-by-line to log.txt.
(By the way, as a side note,
logging :: Auto a b -> AutoM IO a b here can be looked at as an “
Auto transformer”. It takes a normal
Auto and transforms it into an otherwise identical
Auto, yet which logs its results as it ticks on.)
At this point, hopefully you are either excited about the possibilities that monadic
Auto composition/ticking offers, or are horribly revolted at how we mixed IO and unconstrained effects and “implicit side channel” inputs. Or both!
After all, if all we were doing in
replicateGets was having two inputs, we could have just used:
And have the user “get” the string before they run the
And hey, if all we were doing in
logging was having an extra logging channel, we could have just manually logged all of the outputs as they popped out.
All valid suggestions. Separate the pure from the impure. We went out of our way to avoid global states and side-effects, so why bother to bring it all back?
Superficially, it might seem like just moving the burden from one place to the other. Instead of having the user having to worry about getting the string, or writing the log, the
Auto can just handle it itself internally without the “running” code having to worry.
The real, deep advantage in
AutoM, however, is — like in
Auto — its (literal) composability.
replicateGets' was not our “final
Auto” that we run…imagine it was in fact an
Auto used in a composition inside the definition of an
Auto used several times inside a composition inside the definition of another
Auto. All of a sudden, having to “manually thread” the extra channel of input in is a real nightmare. In addition, you can’t even statically guarantee that the
replicateGets eventually was the same
String that the user originally passed in. When composing/calling it, who knows if the Auto that composes
replicateGets' passes in the same initially gotten String?4
Imagine that the
Auto whose results we wanted to log actually was not the final output of the entire
Auto we run (maybe we want to log a small internal portion of a big
Auto). Again, now you have to manually thread the output. And if you’re logging several things through several layers — it gets ugly very fast.
And now, all of your other
Autos in the composition get to (and have to) see the values of the log! So much for “locally stateful”!
As you can see, there is a trade-off in either decision we make. But these monadic compositions really just give us another tool in our toolset that we can (judiciously) use.
It’s fun to imagine what sort of implications the different popular monads in Haskell can provide.
Writer gives you a running log that all
Autos can append to, for example.
Reader gives you every composed
Auto the ability to access a shared global environment…and has an advantage over manual “passing in” of parameters because every composed
Auto is guaranteed to “see” the same global environment per tick.
State gives every composed
Auto the ability to access and modify a globally shared state. We talk a lot about every
Auto having their own local, internal state; usually, it is impossible for two composed
Autos to directly access each other’s state (except by communicating through output and input). With
State, we now give the opportunity for every
Auto to share and modify a collective and global state, which they can use to determine how to proceed, etc.
Good? Bad? Uncontrollable, unpredictable? Perhaps. You now bring in all of the problems of shared state and reasoning with shared mutable state…and avoiding these problems was one of the things that originally motivated the usage of
Auto in the first place! But, we can make sound and judicious decisions without resorting to “never do this” dogma.5 Remember, these are just tools we can possibly explore. Whether or not they work in the real world — or whether or not they are self-defeating — is a complex story!
Here is a toy state example to demonstrate different autos talking to each other; here, the state is a measure of “fuel”; we can take any
Auto a b and give it a “cost” using the
limit function defined here. Here, every
Auto consumes fuel from the same pool, given at the initial
-- source: https://github.com/mstksg/inCode/tree/master/code-samples/machines/Auto3.hs#L139-L175 limit :: Int -> Auto a b -> AutoM (State Int) a (Maybe b) limit cost a = proc x -> do fuel <- arrM (\_ -> get) -< () if fuel >= cost then do arrM (\_ -> modify (subtract cost)) -< () y <- autoM a -< x id -< Just y else id -< Nothing sumSqDiff :: AutoM (State Int) Int Int sumSqDiff = proc x -> do sums <- fromMaybe 0 <$> limit 3 summer -< x sumSqs <- fromMaybe 0 <$> limit 1 summer -< x^2 id -< sumSqs - sums stuff :: AutoM (State Int) Int (Maybe Int, Maybe Int, Int) stuff = proc x -> do doubled <- limit 1 id -< x * 2 tripled <- if even x then limit 2 id -< x * 3 else id -< Just (x * 3) sumSqD <- sumSqDiff -< x id -< (doubled, tripled, sumSqD)
-- a State machine returning the result and the next Auto ghci> let stuffState = runAutoM stuff 4 -- a State machine returning the result ghci> let stuffState_ = fst <$> stuffState ghci> :t stuffState_ stuffState_ :: State Int (Maybe Int, Maybe Int, Int) -- start with 10 fuel ghci> runState stuffState_ 10 ((Just 8, Just 12, 12), 3) -- end up with 3 fuel left -- start with 2 fuel ghci> runState stuffState_ 2 ((Just 8, Nothing, 16), 0) -- poop out halfway
You can see that an initial round with an even number should cost you seven fuel…if you can get to the end. In the case where we only started with two fuel, we only were able to get to the “doubled” part before running out of fuel.
Let’s see what happens if we run it several times:
ghci> let stuffStateMany = testAutoM_ stuff [3..6] ghci> :t stuffStateMany stuffStateMany :: State Int [(Maybe Int, Maybe Int, Int)] ghci> runState stuffStateMany 9 ( [ (Just 6 , Just 9 , 6 ) , (Just 8 , Just 12, 25) , (Nothing, Just 15, 0 ) , (Nothing, Nothing, 0 ) ] , 0 )
So starting with nine fuel, we seem to run out halfway through the second step. The third field should be the sum of the squares so far, minus the sum so far…at
25, it’s probably just the sum of the squares so far. So it couldn’t even subtract out the sum so far. Note that the
Just 15 on the third step goes through because for odd inputs (5, in this case), the second field doesn’t require any fuel.
Anyways, imagine having to thread this global state through by hand. Try it. It’d be a disaster! Everything would have to take an extra parameter and get and extra parameter…it really is quite a headache. Imagine the source for
stuff being written out in
Auto with manual state threading.
But hey, if your program needs global state, then it’s probably a good sign that you might have had a design flaw somewhere along the way, right?
Here we use
Reader to basically give a “second argument” to an
Auto when we eventually run it, but we use the fact that every composed
Auto gets the exact same input to great effect:
-- source: https://github.com/mstksg/inCode/tree/master/code-samples/machines/Auto3.hs#L199-L234 integral :: Double -> AutoM (Reader Double) Double Double integral x0 = AConsM $ \dx -> do dt <- ask let x1 = x0 + dx * dt return (x1, integral x1) derivative :: AutoM (Reader Double) Double (Maybe Double) derivative = AConsM $ \x -> return (Nothing, derivative' x) where -- x0 is the "previous input" derivative' x0 = AConsM $ \x1 -> do let dx = x1 - x0 dt <- ask return (Just (dx/dt), derivative' x1) fancyCalculus :: AutoM (Reader Double) Double (Double, Double) fancyCalculus = proc x -> do deriv <- fromMaybe 0 <$> derivative -< x deriv2 <- fromMaybe 0 <$> derivative -< deriv intdev <- integral 0 -< deriv id -< (deriv2, intdev)
Now, we are treating our input stream as time-varying values, and the “Reader environment” contains the “time passed since the last tick” — The time step or sampling rate, so to speak, of the input stream. We have two stateful
Autos (“locally stateful”, internal state) that compute the time integral and time derivative of the input stream of numbers…but in order to do so, it needs the time step. We get it using
ask. (Note that the time step doesn’t have to be the same between every different tick …
derivative should work just fine with a new timestep every tick.) (Also note that
Nothing on its first step, because there is not yet any meaningful derivative on the first input)
fancyCalculus, we calculate the integral, the derivative, the second derivative, and the integral of the derivative, and return the second derivative and the integral of the derivative.
In order for us to even meaningfully say “the second derivative” or “the integral of the derivative”, the double derivative has to be calculated with the same time step, and the integral and the derivative have to be calculated with the same time step. If they are fed different time steps, then we aren’t really calculating a real second derivative or a real integral of a derivative anymore. We’re just calculating arbitrary numbers.
Anyways, if you have taken any introduction to calculus course, you’ll know that the integral of a derivative is the original function — so the integral of the derivative, if we pick the right
x0, should just be an “id” function:
Let’s try this out with some input streams where we know what the second derivative should be, too.
We’ll try it first with
x^2, where we know the second derivative will just be 2, the entire time:
ghci> let x2s = map (^2) [0,0.05..1] ghci> let x2Reader = testAutoM_ fancyCalculus x2s ghci> :t x2Reader x2Reader :: Reader Double [(Double, Double)] ghci> map fst (runReader x2Reader 0.05) [ ... 2.0, 2.0 ... ] -- with a couple of "stabilizing" first terms ghci> map snd (runReader x2Reader 0.05) [ 0.0, 0.0025, 0.01, 0.0225, 0.04 ...] ghci> x2s [ 0.0, 0.0025, 0.01, 0.0225, 0.04 ...]
Perfect! The second derivative we expected (all 2’s) showed up, and the integral of the derivative is pretty much exactly the original function.
For fun, try running it with a
sin function. The second derivative of
netage . sin. Does it end up as expected?
The alternative to using
Reader here would be to have each composed Auto be manually “passed” the
dt timestep. But then we really don’t have any “guarantees”, besides checking ourselves, that every
Auto down the road, down every composition, will have the same
dt. We can’t say that we really are calculating integrals or derivatives. And plus, it’s pretty messy when literally every one of your composed
We talked about a huge drawback of
State s — global mutable state is really something that we originally looked to
Auto to avoid in the first place. But some portions of logic are much more convenient to write with autos that all have access to a global state.
What if we wanted the best of both worlds? What would that look like?
In Haskell, one common technique we like to use, eloquently stated by Gabriel Gonzalez in his post the Functor design pattern, is to pick a “common denominator” type, and push all of our other types into it.
We have two fundamentally different options here. We can pick our “main type” to be
AutoM (State s) and have global state, and “push” all of our non-global-state Autos into it, or we can pick our “main type” to be
Auto, and “seal” our global-state-Autos into non-global-state ones.
For the former, we’d use
autoM’s whenever we want to bring our
AutoM (State s)…or we can always write
AutoM’s parameterized over
It is statically guaranteed that
summer cannot touch any global state.
For the latter option, we take
AutoM (State s)’s that operate on global state and then basically “seal off” their access to be just within their local worlds, as we turn them into
-- source: https://github.com/mstksg/inCode/tree/master/code-samples/machines/Auto3.hs#L182-L192 sealStateAuto :: AutoM (State s) a b -> s -> Auto a b sealStateAuto a s0 = ACons $ \x -> let ((y, a'), s1) = runState (runAutoM a x) s0 in (y, sealStateAuto a' s1) runStateAuto :: AutoM (State s) a b -> Auto (a, s) (b, s) runStateAuto a = ACons $ \(x, s) -> let ((y, a'), s') = runState (runAutoM a x) s in ((y, s'), runStateAuto a')
sealStateAuto does exactly this. Give it an initial state, and the
Auto will just continuously feed in its output state at every tick back in as the input state. Every
Auto inside now has access to a local state, untouchable from the outside.
runStateAuto is a way to do this were you can pass in a new initial state every time you “step” the
Auto, and observe how it changes — also another useful use case.
We can even pull this trick to turn any
AutoM (StateT s m) into an
AutoM m. See if you can write it :)
In both of these methods, what is the real win? The big deal is that you can now chose to “work only in the world of non-global-state”, combining non-global
Autos like we did in part 1 and part 2 to create non-global algorithms. And then you can also chose to “work in the world of global state”, combining global
Autos like we did in the previous section, where having
State s made everything more clear.
We’re allowed to live and compose (using
Category, proc notation, etc.) in whatever world we like — create as complex compositions as we could even imagine — and at the end of it all, we take the final complex product and “glue it on” to our big overall type so everything can work together.
This discussion is about
State, but the ramifications work with almost any
Auto or type of
Auto or underlying monad we talk about.
We can simulate an “immutable local environment”, for example:
Now you can use a
Reader — composed with “global environment” semantics — inside a normal
Auto! Just give it the new environment very step! (Can you write a
sealReaderAuto that just takes an initial
r and feeds it back in forever?)
Let’s move back to our normal
Auto for now, and imagine a very common use case that might come up.
What if you wanted two chained
Autos to “talk to each other” — for their inputs to depend on the other’s outputs?
Here’s a common example — in control theory, you often have to have adjust an input to a system to get it to “respond” to a certain desired output (a control).
One way is to start with a test input, at every step, observe the resulting response and adjust it up or down until we get the response we want. We call the difference between the response and the control the “error”.
How do you think you would calculate the adjustment? Well…if the error is big, we probably want a big adjustment. And, the longer we are away from the error, we also might want to make a bigger adjustment accordingly, too.
In other words, we might want our adjustment to have a term proportional to the error, and a term that is the sum of all errors so far.
This system is known as PI, and is actually used in many industrial control systems today, for controlling things like lasers and other super important stuff. Congrats, you are now a control theorist!
Let’s see how we might write this using our
So this is an
Auto that takes in a
Double — the control — and outputs a
Double — the response. The goal is to get the response to “match” control, by running a value,
input, through a “black box system” (To simplify here, we’re only running
Here is the “logic”, or the relationships between the values:
- The error value
erris the difference between the control and the response.
- The sum of errors
errSumsis the cumulative sum of all of the error values so far.
- The input
inputis the cumulative sum of all of the correction terms: a multiple of
errand a multiple of
- The response
responseis the result of running the input through the black box system (here, just
- The output is the response!
Look at what we wrote. Isn’t it just…beautifully declarative? Elegant? All we stated were relationships between terms…we didn’t worry about state, loops, variables, iterations…there is no concept of “how to update”, everything is just “how things are”. It basically popped up exactly as how we “said” it. I don’t know about you, but this demonstration always leaves me amazed, and was one of the things that sold me on this abstraction in the first place.
But, do you see the problem? To calculate
err, we used
resp. But to get
resp, we need
We need to be able to define “recursive bindings”. Have Autos recursively depend on each other.
In another language, this would be hopeless. We’d have to have to resort to keeping explicit state and using a loop. However, with Haskell…and the world of laziness, recursive bindings, and tying knots…I think that we’re going to have a real win if we can make something like what we wrote work.
There is actually a construct in proc notation that lets you do just that. I’m going to cut to the chase and show you how it looks, and how you use it. I’ll explain the drawbacks and caveats. And then I’ll explain how it works in an aside — it’s slightly heavy, but some people like to understand.
Without further ado —
-- source: https://github.com/mstksg/inCode/tree/master/code-samples/machines/Auto3.hs#L266-L276 piTargeter :: Auto Double Double piTargeter = proc control -> do rec let err = control - response errSums <- summer -< err input <- laggingSummer -< 0.2 * err + 0.01 * errSums response <- blackBoxSystem -< input id -< response where blackBoxSystem = id -- to simplify things :)
The key here is the rec keyword. Basically, we require that we write an instance of
ArrowLoop for our
Auto…and now things can refer to each other, and it all works out like magic! Now our solution works…the feedback loop is closed with the usage of
rec. Now, our algorithm looks exactly like how we would “declare” the relationship of all the variables. We “declare” that
err is the difference between the control and the response. We “declare” that
errSums is the cumulative sum of the error values. We “declare” that our
input is the cumulative sum of all of the adjustment terms. And we “declare” that our response is just the result of feeding our input through our black box.
No loops. No iteration. No mutable variables. Just…a declaration of relationships.
Wait wait wait hold on…but how does this even work? Is this magic? Can we just throw anything into a recursive binding, and expect it to magically figure out what we mean?
Kinda, yes, no. This works based on Haskell’s laziness. It’s the reason something like
Infinite loop, right?
fix (1:) is basically an infinite lists of ones. But remember that
head only requires the first element to be evaluated:
So that’s the key. If what we want doesn’t require the entire result of the infinite loop…then we can safely reason about infinite recursion in haskell.
The MVP here really is this function that I sneakily introduced,
laggingSummer is like
summer, except all of the sums are delayed. Every step, it adds the input to the accumulator…but returns the accumulator before the addition. Sort of like
x++ instead of
++x in C. If the accumulator is at 10, and it receives a 2, it outputs 10, and updates the accumulator to 12. The key is that it doesn’t need the input to immediately return that step’s output.
The accumulator starts off at 0, and receives a 5…it then outputs 0 and updates the accumulator to 5. The accumulator then has 5 and receives a 6…it outputs 5 and then updates the accumulator to 11. Etc. The next step it would output 45 no matter what input it gets.
Look at the definition of
piTargeter again. How would it get its “first value”?
- The first output is just
- The first response is just the first
- The first
inputis just the result of
- The first result of
And that’s it! Loop closed! The first result is zero…no infinite recursion here.
Now that we know that the first result of
response is 0, we can also find the first values of
errSums: The first
err is the first control (input to the
Auto) minus 0 (the first response), and the first
errSums is a cumulative sum of
errs, so it too starts off as the first control minus zero.
So now, we have all of the first values of all of our Autos. Check! Now the next step is the same thing!
Recursive bindings have a lot of power in that they allow us to directly translate natural language and (cyclic) graph-like “relationships” (here, between the different values of a control system) and model them as relationships. Not as loops and updates and state modifications. But as relationships. Something we can declare, at a high level.
And that’s definitely something I would write home about.
The only caveat is, of course, that we have to make sure our loop can produce a “first value” without worrying about its input. Autos like
laggingSummer give this to us.
In the following aside, I detail the exact mechanics of how this works :)
Ah, so you’re curious? Or maybe you are just one of those people who really wants to know how things work?
rec keyword in proc/do blocks desugars to applications of a function called
The type signature seems a bit funny. Loop takes a morphism from
(a, c) to
(b, c) and turns it into a morphism from
b. But…how does it do that?
I’ll point you to a whole article about the
(->) instance of
ArrowLoop and how it is useful, if you’re interested. But we’re looking at
Auto for now.
We can write an
ArrowLoop instance for
So what does this mean? When will we be able to “get a
We will be able to get a
y in the case that the
Auto can just “pop out” your
y without ever evaluating its arguments…or only using
The evaluation of
a' is then deferred until later…and through this, everything kinda makes sense. The loop is closed. See the article linked above for more information on how
loop really works.
The actual desugaring of a
rec block is a little tricky, but we can trust that if we have a properly defined
loop (that typechecks and has the circular dependencies that loop demands), then
ArrowLoop will do what it is supposed to do.
In any case, we can actually understand how to work with rec blocks pretty well — as long as we can have an
Auto in the pipeline that can pop something out immediately ignoring its input, then we can rest assured that our knot will be closed.
By the way, this trick works with
ArrowM too — provided that the
Monad is an instance of
MonadFix, which is basically a generalization of the recursive
let bindings we used above:
This is going to be our last “modification” to the
Auto type — one more common
Auto variation/trick that is used in real life usages of
It might some times be convenient to imagine the results of the
Autos coming in contexts — for example,
How can we interpret/use this? In many domains, this is used to model “on/off” behavior of
Auto is “on” if the output is
Just, and “off” if the output is
We can imagine “baking this in” to our Auto type:
Where the semantics of composition are: if you get a
Nothing as an input, just don’t tick anything and pop out a
Nothing; if you get a
Just x as an input run the auto on the
The other instances are on the file linked above, but I won’t post them here, so you can write them as an exercise. Have fun on the
This aside contains category-theoretic justification for what we just did. You can feel free to skip it if you aren’t really too familiar with the bare basics of Category Theory (What an endofunctor is, for example)… but, if you are, this might be a fun perspective :)
What we’ve really done here is taken a category with objects as Haskell types and morphisms are
Auto a b, and turned it into a category with objects as Haskell types and whose morphisms are
Auto a (m b), where
m is a Monad.
The act of forming this second category from the first is called forming the Kleisli category on a category. We took
Auto and are now looking at the Kleisli category on
Auto formed by
By the way, a “Monad” here is actually different from the normal
Monad typeclass found in standard Haskell. A Monad is an endofunctor on a category with two associated natural transformations — unit and join.
Because we’re not dealing with the typical Haskell category anymore (on
(->)), we have to rethink what we actually “have”.
For any Haskell Monad, we get for free our natural transformations:
But what we don’t get, necessarily, the endofunctor. An endofunctor must map both objects and morphisms. A type constructor like
Maybe can map objects fine — we have the same objects in
Auto as we do in
(->) (haskell types). But we also need the ability to map morphisms:
So, if this function exists for a type constructor, following the usual
fmap laws, then that type is an endofunctor in our
Auto category. And if it’s also a Monad in
(->), then it’s also then a Monad in
We can write such an
And, it is a fact that if we have a Monad, we can write the composition of its Kleisli category for free:
In fact, for
f ~ Maybe, this definition is identical to the one for the
Category instance we wrote above for
And, if the
FunctorA is a real functor and the
Monad is a real monad, then we have for free the associativity of this super-fish operator:
Category theory is neat!
By the way, definitely not all endofunctors on
(->) are endofunctors on
Auto. We see that
Maybe is one. Can you think of any others? Any others where we could write an instance of
FunctorA that follows the laws? Think about it, and post some in the comments!
One immediate example is
Either e, which is used for great effect in many FRP libraries! It’s “inhibit, with a value”. As an exercise, see if you can write its
FunctorA instance, or re-write the
AutoOn in this section to work with
Either e (you might need to impose a typeclass constraint on the
e) instead of
I’m not going to spend too much time on this, other than saying that it is useful to imagine how it might be useful to have an “off” Auto “shut down” every next Auto in the chain.
One neat thing is that
AutoOn admits a handy
a1 <|> a2 will create a new
AutoOn that feeds in its input to both
a2, and the result is the first
-- source: https://github.com/mstksg/inCode/tree/master/code-samples/machines/AutoOn.hs#L80-L86 instance Alternative (AutoOn a) where empty = AConsOn $ \_ -> (Nothing, empty) -- (<|>) :: AutoOn a b -> AutoOn a b -> AutoOn a b a1 <|> a2 = AConsOn $ \x -> let (y1, a1') = runAutoOn a1 x (y2, a2') = runAutoOn a2 x in (y1 <|> y2, a1' <|> a2')
Unexpectedly, we also get the handy
empty, which is a “always off”
AutoOn. Feed anything through
empty and it’ll produce a
Nothing no matter what. You can use this to provide an “always fail”, “short-circuit here” kind of composition, like
Nothing in the
You also get this an interesting and useful concept called “switching” that comes from this; the ability to switch from running one Auto or the other by looking if the result is on or off — here is a common switch that behaves like the first
AutoOn until it is off, and then behaves like the second forever after:
Let’s test this out; first, some helper functions (the same ones we wrote for
-- source: https://github.com/mstksg/inCode/tree/master/code-samples/machines/AutoOn.hs#L91-L107 autoOn :: Auto a b -> AutoOn a b autoOn a = AConsOn $ \x -> let (y, a') = runAuto a x in (Just y, autoOn a') arrOn :: (a -> Maybe b) -> AutoOn a b arrOn f = AConsOn $ \x -> (f x, arrOn f) fromAutoOn :: AutoOn a b -> Auto a (Maybe b) fromAutoOn a = ACons $ \x -> let (y, a') = runAutoOn a x in (y, fromAutoOn a')
autoOn turns an
Auto a b into an
AutoOn a b, where the result is always
arrOn is like
arrM…it takes an
a -> Maybe b and turns it into an
AutoOn a b.
fromAutoOn turns an
AutoOn a b into a normal
Auto a (Maybe b), just so that we can leverage our existing test functions on normal
Let’s play around with some test
-- source: https://github.com/mstksg/inCode/tree/master/code-samples/machines/AutoOn.hs#L131-L152 onFor :: Int -> AutoOn a a onFor n = proc x -> do i <- autoOn summer -< 1 if i <= n then id -< x -- succeed else empty -< x -- fail -- alternatively, using explit recursion: -- onFor 0 = empty -- onFor n = AConsOn $ \x -> (Just x, onFor' (n-1)) filterA :: (a -> Bool) -> AutoOn a a filterA p = arrOn (\x -> x <$ guard (p x)) untilA :: (a -> Bool) -> AutoOn a a untilA p = proc x -> do stopped <- autoOn (autoFold (||) False) -< p x if stopped then empty -< x -- fail else id -< x -- succeed -- alternatively, using explicit recursion: -- untilA p = AConsOn $ \x -> -- if p x -- then (Just x , untilA p) -- else (Nothing, empty )
One immediate usage is that we can use these to “short circuit” our proc blocks, just like with monadic
Maybe and do blocks:
-- source: https://github.com/mstksg/inCode/tree/master/code-samples/machines/AutoOn.hs#L163-L173 shortCircuit1 :: AutoOn Int Int shortCircuit1 = proc x -> do filterA even -< x onFor 3 -< () id -< x * 10 shortCircuit2 :: AutoOn Int Int shortCircuit2 = proc x -> do onFor 3 -< () filterA even -< x id -< x * 10
If either the
filterA or the
onFor are off, then the whole thing is off. How do you think the two differ?
ghci> testAuto (fromAutoOn shortCircuit1) [1..12] [ Nothing, Just 20, Nothing, Just 40, Nothing, Just 60 , Nothing, Nothing, Nothing, Nothing, Nothing, Nothing] ghci> testAuto (fromAutoOn shortCircuit2) [1..12] [ Nothing, Just 20, Nothing, Nothing, Nothing, Nothing , Nothing, Nothing, Nothing, Nothing, Nothing, Nothing]
shortCircuit1, as soon as the
filterA fails, it jumps straight to the end, short-circuiting; it doesn’t bother “ticking along” the
onFor and updating its state!
The arguably more interesting usage, and the one that is more used in real life7, is the powerful usage of the switching combinator
(-->) in order to be able to combine multiple
Auto’s that simulate “stages”…an
Auto can “do what it wants”, and then choose to “hand it off” when it is ready.
-- source: https://github.com/mstksg/inCode/tree/master/code-samples/machines/AutoOn.hs#L178-L183 stages :: AutoOn Int Int stages = stage1 --> stage2 --> stage3 --> stages where stage1 = onFor 2 . arr negate stage2 = untilA (> 15) . autoOn summer stage3 = onFor 3 . (pure 100 . filterA even <|> pure 200)
Note that the stages continually “loop around”, as our recursive definition seems to imply. Neat!
You might note that sometimes, to model on/off behavior, it might be nice to really be able to “keep on counting” even when receiving a
Nothing in a composition. For example, you might want both versions of
shortCircuit to be the same — let
onFor still “keep on counting” even when it has been inhibited upstream.
If this is the behavior you want to model (and this is actually the behavior modeled in some FRP libraries), then the type above isn’t powerful enough; you’ll have to go deeper:
So now, you can write something like
onFor, which keeps on “ticking on” even if it receives a
Nothing from upstream:
You can of course translate all of your
AutoOns into this new type:
Or you can use the smart constructor method detailed immediately following.
Working all together
Of course, we can always literally throw everything we can add together into our
(Again, instances are in the source file, but not here in the post directly)
Here is a same of a big conglomerate type where we throw in a bunch of things.
The benefit? Well, we could work and compose “normal”
Autos, selecting for features that we only need to work with. And then, when we need to, we can just “convert it up” to our “lowest common denominator” type.
This is the common theme, the “functor design pattern”. Pick your common unifying type, and just pop everything into it. You can compose, etc. with the semantics of the other type when convenient, and then have all the parts work together in the end.
This pattern is awesome, if only we didn’t have so many types to convert in between manually.
Well, we’re in luck. There’s actually a great trick, that makes all of this even more streamlined: we can replace the “normal constructors” like
AConsOn, with smart constructors
aConsOn, that work exactly the same way:
-- source: https://github.com/mstksg/inCode/tree/master/code-samples/machines/AutoX.hs#L85-L100 aCons :: Monad m => (a -> (b, AutoX m a b)) -> AutoX m a b aCons a = AConsX $ \x -> let (y, aX) = a x in return (Just y, aX) aConsM :: Monad m => (a -> m (b, AutoX m a b)) -> AutoX m a b aConsM a = AConsX $ \x -> do (y, aX) <- a x return (Just y, aX) aConsOn :: Monad m => (a -> (Maybe b, AutoX m a b)) -> AutoX m a b aConsOn a = AConsX $ \x -> let (y, aX) = a x in return (y, aX)
Compare these definitions of
untilA from their “specific type” “real constructor” versions to their
AutoX-generic “smart constructor” versions:
-- source: https://github.com/mstksg/inCode/tree/master/code-samples/machines/Auto.hs#L67-L73 summer :: Num a => Auto a a summer = sumFrom 0 where sumFrom :: Num a => a -> Auto a a sumFrom n = ACons $ \input -> let s = n + input in ( s , sumFrom s ) -- source: https://github.com/mstksg/inCode/tree/master/code-samples/machines/Auto3.hs#L104-L107 arrM :: Monad m => (a -> m b) -> AutoM m a b arrM f = AConsM $ \x -> do y <- f x return (y, arrM f) -- source: https://github.com/mstksg/inCode/tree/master/code-samples/machines/AutoOn.hs#L154-L158 untilA' :: (a -> Bool) -> AutoOn a a untilA' p = AConsOn $ \x -> if p x then (Just x , untilA p) else (Nothing, empty )
-- source: https://github.com/mstksg/inCode/tree/master/code-samples/machines/AutoX.hs#L106-L128 summer :: (Monad m, Num a) => AutoX m a a summer = sumFrom 0 where sumFrom n = aCons $ \input -> let s = n + input in ( s , sumFrom s ) arrM :: Monad m => (a -> m b) -> AutoX m a b arrM f = aConsM $ \x -> do y <- f x return (y, arrM f) untilA :: Monad m => (a -> Bool) -> AutoX m a a untilA p = aConsOn $ \x -> if p x then (Just x , untilA p) else (Nothing, empty )
They are literally exactly the same…we just change the constructor to the smart constructor!
You might also note that we can express a “pure, non-Monadic”
AutoX by making the type polymorphic over all monads:
Auto with a type like this says, “I cannot perform any effects during stepping” — and we know that
summer definitely does not.
summer is statically guaranteed not to affect any state or IO, and it’s reflected in its type.
The takeaway? You don’t even have to mungle around multiple types to make this strategy work — just make all your
Autos from the start using these smart constructors, and they all compose together! One type from the start — we just expose different constructors to expose the different “subtypes of power” we want to offer.
Now it’s all just to chose your “greatest common denominator”. If you don’t want inhibition-based semantics, just only use
AutoM, for example!
By the way, here’s a “smart constructor” for
That was a doozy, wasn’t it? For those of you who have been waiting, thank you for being patient. I hope most if not all of you are still with me.
Hopefully after going through all of these examples, you can take away some things:
From the previous parts, you’ve recognized the power of local statefulness and the declarative style offered by proc notation.
From here, you’ve seen that the Auto type can be equipped in many ways to give it many features which have practical applications in the real world.
You’ve learned how to handle those features and use and manage them together in sane ways, and their limitations.
You also know that you can really “program”, “compose”, or “think” in any sort of Auto or composition semantics that you want, for any small part of the problem. And then at the end, just push them all into your greatest common denominator type. So, you aren’t afraid to play with different effect types even in the same program!
You’ve seen the power of recursive bindings to make complete the promise of declarative programming — being able to extend the realm of what we can express “declaratively”, and what we can denote.
You are ready to really understand anything you encounter involving
So, what’s next?
Download the files of this post, play along with the examples in this post, CTRL+F this page for “exercise” to find exercises, and try writing your own examples!
Feel ready to be able to have a grasp of the situation you see
Autoin the real world, such as in the popular FRP library netwire!
Apply it to the real world and your real world problems!
Well, a bit of self-promotion, my upcoming library auto is basically supposed to be almost all of these concepts (except for implicit on/off behavior) implemented as a finely tuned and optimized performant library, attached with semantic tools for working with real-world problems with these concepts of local statefulness, composition, and declarative style. You can really apply what you learned here to start building projects right away!
Well, sorta. Unfortunately, as of Feburary 2015, it is not yet ready for real usage, and the API is still being finalized. But now that this post is finished, I will be posting more examples and hype posts in the upcoming weeks and months leading up to its official release.
I am open to pull requests and help on the final stages of documentation :)
If you’re interested, or are curious, stop by #haskell-auto on freenode or send me a message!
Look forward to an actual series on Arrowized FRP, coming up soon! We’ll be using the concepts in this series to implement FRP.
Some of you might recall an earlier plan for this post that would include FRP. Unfortunately, I’ve refactored FRP into a completely new topic, because I’ve realized that the two aren’t exactly as related as I had led you all to believe. Still, most if not all of these techniques here are used in actual arrowized FRP libraries today. So, look out for that one soon!↩︎
I’m going to go out on a limb here and say that, where Haskell lets you abstract over functions and function composition with
Category, Haskell lets you abstract over values and function application with
This function really could be avoided if we had written all of our
AutoM’s parameterized over all
min the first place — that is, written our
Auto a b’s as the equally powerful
Monad m => AutoM m a b. But we’re just going to run with
Autofor the rest of this series to make things a bit less confusing.↩︎
By the way, you might notice this pattern as something that seems more fit for
IO. We’ll look at that later!↩︎
Which really isn’t the point of these posts, anyway!↩︎
Another exercise you can do if you wanted is to write the exact same instances, but for
newtype AutoOn a b = AutoOn (Auto a (Maybe b)):)↩︎
Admittedly, implicit short-circuiting with
Autos is actually often times a lot more of a headache than it’s worth; note that “switching” still works if you have a normal
Auto a (Maybe b); this is the approach that many libraries like auto take — write switching combinators on normal
Auto a (Maybe b)’s instead.↩︎