The "99 Bottles of Beers" of Type Systems

"Hello World" is a good first example program because it is small, but also because it encourages the reader to get into the driver's seat and take control of the program. Copy-paste the "Hello World" listing from a website, and you're just blindly following instructions. Change it to print "Hello Mom", and you're boldly taking your first step towards the unknown, into a world where it is now you who is giving the instructions.

New programmers need to take that step, because programming anything non-trivial requires taking your own decisions about how things are implemented. If your boss was taking all the decisions for you, you wouldn't be a programmer, you'd be a typist.

The "Hello World" of Type Systems

Once you become an experienced programmer, "Hello World" examples are still useful as an introduction to new languages and new systems. Once you have a working base, you can experiment by making small changes and verifying whether they work as you expect, or if you need to read more tutorials.

For type systems, I guess a "Hello World" program would be a small datatype/structure/class containing a few simple fields. The standard here isn't as well-established as with "Hello World", but describing a person is quite common:

data Person = Person
  { name :: String
  , age :: Int
  }

I've used Haskell here, but regardless of the language in which I would have written this, you could still easily infer how to add a field for the person's height.

99 Bottles of Beer

A little down the road, another introductory program is "99 bottles of beer on a wall". This one teaches budding programmers another important lesson: it's possible to write a program which prints out more text than what you've written in its source code. More specifically, the program shows how to use a variable to abstract over the part of the text which varies from one iteration to the other, and how to use a loop to determine how many iterations to make and which value the variable should take in each one.

For type systems, a "99 bottles of beer" program would teach the same lesson: it's possible to write a program which uses larger types than those you've written in the source code. This is rarely needed, but it's possible! Even in a large, complicated application, you might have a manager of pools of worker threads processing lists of person values, but Manager (Pool (WorkerThread (List Person))) is still a fixed type which you write down explicitly in your program. It's as if you had abstracted out the number of beers to print, but then wrote explicit calls with n = 99, n = 98 and so on, instead of using a loop to generate the calls at runtime. Our "99 bottles of beer" example should generate types at runtime.

The "99 Bottles of Beer" of Type Systems

The simplest such example I could think of is as follows:

1. Parse a non-negative integer n from standard input or from a command-line argument.
2. If n is 0, print 42.
3. Otherwise, print the pair (x,x), where x is the text which would have been printed if n was one unit smaller. For example, the output for n = 3 should be "(((42,42),(42,42)),((42,42),(42,42)))".

With the important restriction that the pair (x, x) must first be constructed before being printed, and its representation must not have the same type as x.

An incorrect solution

The reason the restriction is important is that otherwise, it would be possible to implement the program using a single type, that of integer trees:

-- *not* a valid solution
data Tree a = Leaf a | Branch (Tree a) (Tree a)

showTree :: Show a => Tree a -> String
showTree (Leaf x)       = show x
showTree (Branch t1 t2) = printf "(%s,%s)" (showTree t1)
                                           (showTree t2)

printTree :: Tree Int -> Int -> IO ()
printTree v 0 = putStrLn (showTree v)
printTree v n = printTree (Branch v v) (n-1)

main :: IO ()
main = readLn >>= printTree (Leaf 42)

That program does not demonstrate that it's possible to write a program which uses larger types than those you've written in the source code.

Haskell solution

Instead of using the same type Tree Int at every iteration, we want to construct a sequence of larger and larger types:

0. Int
1. (Int,Int)
2. ((Int,Int),(Int,Int))
3. (((Int,Int),(Int,Int)),((Int,Int),(Int,Int)))
4. ...

In Haskell, this can be achieved via polymorphic recursion, meaning that we recur at a different type than the one which the current call is being instantiated at. For example, the call printTree 42 1 instantiates the type variable a = Int, while the recursive call printTree (42,42) 0 instantiates the type variable a = (Int,Int).

printTree :: Show a => a -> Int -> IO ()
printTree v 0 = print v
printTree v n = printTree (v,v) (n-1)

main :: IO ()
main = readLn >>= printTree 42

Polymorphic recursion is often used to recur on a smaller type, but since in this function it is the Int argument which is getting smaller, we can recur on a larger type without risking an infinite loop.

C++ solution

Speaking of infinite loops, C++ uses compile-time templates to handle polymorphic recursion, and this implementation strategy causes the compiler to instantiate more and more templates when we recur on a larger type. Eventually, gcc gives up with "template instantiation depth exceeds maximum of 900".

We can work around the problem by specializing the template at one of the types encountered before that limit, and printing an error message instead of recurring further.

#include <stdio.h>

template<typename A>
struct Pair {
  A fst;
  A snd;
  
  Pair(A fst, A snd)
  : fst(fst), snd(snd)
  {}
};

void print(int n) {
  printf("%d", n);
}

template<typename A>
void print(Pair<A> pair) {
  printf("(");
  print(pair.fst);
  printf(",");
  print(pair.snd);
  printf(")");
}

template<typename A>
void println(A value) {
  print(value);
  printf("\n");
}


template<typename A>
struct PrintTree {
  static void call(int depth, A value) {
    if (depth == 0) {
      println(value);
    } else {
      PrintTree<Pair<A> >::call(depth - 1, Pair<A>(value, value));
    }
  }
};

template<>
struct PrintTree<
  Pair<Pair<Pair<Pair<Pair<Pair<Pair<Pair<int>
> > > > > > > >
{
  static void call(int, Pair<
                          Pair<Pair<Pair<Pair<Pair<Pair<Pair<int>
                        > > > > > > >
  ) {
    fprintf(stderr, "maximum depth exceeded.\n");
  }
};

int main() {
  int depth;
  scanf("%d", &depth);
  
  PrintTree<int>::call(depth, 42);
  
  return 0;
}

Java solution

Other implementation strategies, such as Java's type erasure, need no such artificial bounds.

class Pair<A> {
  private A fst;
  private A snd;
  
  public Pair(A fst, A snd) {
    this.fst = fst;
    this.snd = snd;
  }
  
  public String toString() {
    return "(" + fst.toString() + "," + snd.toString() + ")";
  }
}

public class Main {
  public static <A> void printTree(int depth, A value) {
    if (depth == 0) {
      System.out.println(value.toString());
    } else {
      printTree(depth - 1, new Pair<A>(value, value));
    }
  }
  
  public static void main(String[] args) {
    Integer n = Integer.valueOf(args[0]);
    Integer m = 42;
    printTree(n, m);
  }
}

Conclusion

Many programming languages have the ability to work with larger types than those which are known at compile time, but for some reason, the feature is rarely used.

Perhaps one of the reasons is that the feature is rarely covered in tutorials. I have presented a small example demonstrating the feature, and I have demonstrated that the example isn't specific to one particular type system by implementing it in a few different languages. If you're writing a tutorial for a language and you have already covered "Hello World", "99 bottles of beer" and the "Hello World" of type systems, please consider also covering the "99 bottles of beer" of type systems.

Although, if I want this example to catch on, I should probably give it a better name. Maybe "Complete trees whose leaves are 42", or simply "Complete 42" for short?

How to package up binaries for distribution

This weekend, I wrote a game (in Haskell of course!) for Ludum Dare, an event in which you have 48h or 72h to create a game matching an imposed theme. It was really challenging!

Once the event was over, it was time to package my game in a form which others could play. Since the packaging procedure wasn't obvious, I'm documenting it here for future reference. The procedure isn't specific to Haskell, but I'll mention that linking Haskell programs statically, as advised around the web, didn't work for me on any platform.

Windows

While your program is running, use Process Explorer to list the .dll files your program is currently using (There is also Dependency Walker, but on my program it missed glut32.dll). Copy those DLLs to the same folder as your executable, zip the folder, and ship it.

OS X

Use otool -L to list the .dylib files on which your executable depends, and copy them to the same folder as your executable (or a libs subfolder). Use install_name_tool to change all the dylib paths embedded in your executable to @executable_path/foo.dylib (or @executable_path/libs/foo.dylib). Zip the folder, and ship it.

Linux

Use ldd to list the .so files on which your executable depends, and copy all of them except libc.so.X to the same folder as your executable (or a libs subfolder). Add ld-options: -Wl,-rpath -Wl,$ORIGIN (or ld-options: -Wl,-rpath -Wl,$ORIGIN/libs) to your cabal file, pass those flags directly to gcc, or use chrpath to change the existing RPATH if there is one. Zip the folder, and ship it.

Understanding "Strongly-typed Bound", part 1

First, giving credits where credit is due. The Bound library is written by Edward Kmett, and so is the strongly-typed variant I want to explore in this series. I learned about the strongly-typed version via a comment by geezusfreeek, in response to a question by _skp.

I have a lot to say about this script, and since the first thing I want to say about it involves writing down some typing rules, I thought I'd write them on the whiteboard and publish a video! Please let me know what you think of this new format.

Prisms lead to typeclasses for subtractive types

In my last post, I identified some issues with subtractive types, namely that math identities such as ∀ a. a + -a = 0, once they are translated into Haskell, would not be valid for all a. More precisely, the existence of a function of type

cancelSub :: forall a. (a :+: Negative a) -> Zero

would make it easy to implement a contradiction, regardless of the way in which we represent Negative a:

type a :+: b = Either a b
type Zero = Void

contradiction :: Void
contradiction = cancelSub (Left ())

I concluded by blaming the unconstrained forall. That is, I was hoping that the identity could be saved by finding some typeclass C such that C a => (a :+: Negative a) -> Void would be inhabited, or something along those lines. But what should C look like?

Prisms

Earlier today, I was re-listening to Edward Kmett on Lenses, in the first Haskell Cast episode. While explaining Prisms at 38:30, Kmett explained that a Lens' s a splits an s into a product consisting of an a and of something else, and that correspondingly, a Prism' s a splits an s into a sum consisting of an a and of something else. It occurred to me that the first "something else" should be s :/: a, while the second "something else" should be s :-: a.

Since Prism' s a is only inhabited for some combinations of s and a but not others, I thought a good choice for my C typeclass might be a proof that there exists a prism from s to a.

cancelSub :: HasPrism a (Negative a)
          => (a :+: Negative a) -> Void

That is, instead of restricting which types can be negated, let's restrict which types are allowed to appear together on the left- and right-hand sides of a subtractive type.

Four typeclasses

All right, so what should the HasPrism typeclass look like? In the podcast, Kmett explains that we can "construct the whole thing out of the target of a prism", and that we can pattern-match on the whole thing to see if it contains the target. In other words:

class HasPrism s a where
    construct :: a -> s
    match :: s -> Maybe a

This Maybe a discards the case I am interested in, the s :-: a. Let's ask the typeclass to provide a representation for this type, so we can give a more precise type to match.

class HasPrism s a where
    type s :-: a
    constructLeft :: a -> s
    constructRight :: (s :-: a) -> s
    match :: s -> Either a (s :-: a)

Our typeclass now has three methods, for converting back and forth between s and its two alternatives. We can combine those three methods into a single bijection, and with this final transformation, we obtain a form which is easily transferable to the other inverse types:

class Subtractable a b where
    type a :-: b
    asSub :: Iso a ((a :-: b) :+: b)

class Divisible a b where
    type a :/: b
    asDiv :: Iso a ((a :/: b) :*: b)

class Naperian b a where
    type Log b a
    asLog :: Iso a (Log b a -> b)

class Rootable n a where
    type Root n a
    asRoot :: Iso a (n -> Root n a)

Routing around the contradiction

The real test for these new definitions is whether they allow us to define constructive versions of the math identities for subtraction, division, logarithms and roots. Once annotated with the proper type class constraint, does cancelSub still lead to a contradiction? If not, can it be implemented?

It can!

type Negative a = Zero :-: a

cancelSub :: forall a. Subtractable Zero a
          => Iso (a :+: Negative a) Zero
         -- a :+: Negative a
cancelSub = swap
         -- Negative a :+: a
        >>> inverse iso
         -- Zero
  where
    iso :: Iso Zero (Negative a :+: a)
    iso = asSub

The math version of the constrained type is still ∀ a. a + -a = 0, but with a new proviso "whenever -a exists". It's still the same identity, it's just that with real numbers, -a always exists, so the proviso does not usually need to be spelled out.

In the world of types, Negative a does not always exist. If fact, there's only one possible instance of the form Subtractable Zero a:

instance Subtractable Zero Zero where
    type Zero :-: Zero = Zero
    
    asSub :: Iso Zero ((Zero :-: Zero) :+: Zero)
    asSub = Iso Right (either id id)

In other words, in the world of types, the proviso "whenever -a exists" simply means "when a = 0".

Other identities

I wish I could say that all the other identities become straightforward to implement once we add the appropriate typeclass constraints, but alas, this is not the case. I plan to discuss the remaining issues in a subsequent post.

For now, I am content to celebrate the fact that at least one contradiction has been slain :)

Edward Kmett likes my library :)

I have be re-listening to old episodes of the Haskell Cast, and it turns out I missed something really, shall we say, relevant to my interests.



In the very first episode, Edward Kmett talks about lens and a few of his other libraries. Then, near the end, he is asked about interesting Haskell stuff aside from his libraries. His answer, at 59:45:

"There was a really cool Commutativity monad [...] that really struck me as an interesting approach to things, I thought it was particularly neat toy."

— Edward Kmett

Yay, that's my library! And here are Wren's blog posts he mentions, about generalizing my approach.

Issues with subtractive types

I tried to expand on my earlier attempt at understanding root types, but I hit several obstacles. Here is a summary of those obstacles, in case future-me or someone else feels like trying again.

Goals

The conversation began with Naperian types, and the way in which they satisfy the same laws as mathematical logarithms, except at the type level. For example, the law log_b xy = log_b x + log_b y would translate to a function of type

logProduct :: Log b (a1, a2) -> Either (Log b a1) (Log b a2)

where, as usual, pairs represent multiplication and Either represents a sum. We would also expect a function with the converse type, and we would expect the two functions to form an isomorphism.

My goal would be to find implementations for Log and the other inverse types such that the corresponding isomorphisms exist and are useful. As the rest of the post will demonstrate, "exists" is already quite a strong requirement.

I should mention before moving on that yes, I am familiar with "The Two Dualities of Computation: Negative and Fractional Types", and I am intentionally using a different approach. Their non-deterministic invertible language in quite interesting, but ultimately too weird for me.

I would prefer to find a way to implement negative and fractional stuff as Haskell datatypes, or failing that, to understand why it can't be done. Today's post is about the latter: if negative and fractional types can exist in Haskell, then identities such as 0 ↔ x + -x wouldn't be valid for all types x, like they are in that paper.

Naïve definitions

Since subtraction is the inverse of addition, I tried to define a subtractive type and the other inverses types in term of the types we already have.

type a :+: b = Either a b
type a :*: b = (a, b)

data ab :-: b where
  MkSub :: a -> (a :+: b) :-: b

data ab :/: b where
  MkDiv :: a -> (a :*: b) :/: b

data Log b a where
  MkLog :: p -> Log b (p -> b)

data Root n a where
  MkRoot :: b -> Root n (n -> b)

One obvious problem with those definitions is that they don't support any of the math identities, except for the ones used in the definitions themselves: x + y - y = x, etc. For most types a, the type a :-: b isn't even inhabited, so while we might be able to implement absurd-style isomorphisms, the result would be completely useless.

Isomorphisms to the rescue

In the reddit comment linked at the top of this post, I worked around the problem via the hypothesis that maybe we shouldn't expect math identities to correspond to type isomorphisms so directly. Instead, I postulated an extra operator which would lift an isomorphism between a and a' to an isomorphism between a :-: b and a' :-: b, and similarly for the right-hand side and for the other type operators. It worked well enough, but since this transformation isn't constructive, we still don't get useful isomorphic functions at the end.

So, can we make the transformation constructive? Something like this:

data a :-: b where
  MkSub :: r -> Iso a (r :+: b) -> a :-: b

data a :/: b where
  MkDiv :: r -> Iso a (r :*: b) -> a :/: b

data Log b a where
  MkLog :: p -> Iso a (p -> b) -> Log b a

data Root n a where
  MkRoot :: b -> Iso a (n -> b) -> Root n a

By using id as the Iso, we can construct the same inhabitants as with the previous definitions. In addition, we can now constructively lift an isomorphism on the left- or right-hand side to an isomorphism on the whole type. The code for doing this is a bit longer than I'd like, but the idea is that since isomorphisms can already be lifted to either side of a (:+:), (:*:), or (->), we should therefore be able to concatenate an isomorphism for the left- or right-hand side with the existing Iso. For example:

liftRightAdd :: forall a b b'
              . Iso b b'
             -> Iso (a :+: b) (a :+: b')
liftRightAdd isoB = ...

liftRightSub :: forall a b b'
              . Iso b b'
             -> Iso (a :-: b) (a :-: b')
liftRightSub isoB = Iso fwdS bwdS
  where
    fwdS :: a :-: b -> a :-: b'
    fwdS (MkSub r iso) = MkSub r (liftIso iso)
      where
        liftIso :: Iso a (r :+: b) -> Iso a (r :+: b')
        liftIso iso = liftRightAdd isoB . iso
    
    bwdS :: a :-: b' -> a :-: b
    bwdS (MkSub r iso) = MkSub r (liftIso iso)
      where
        liftIso :: Iso a (r :+: b') -> Iso a (r :+: b)
        liftIso iso = liftRightAdd (inverse isoB) . iso)

With those tools, we should be able to take a non-constructive transformation like the one I wrote in my reddit comment:

log_b(a1) + log_b(a2) ~ Either (Log b a1) (Log b a2)
                     ≈≈ Either (Log b (p1 -> b))
                               (Log b (p2 -> b))
                      ≈ Either p1 p2
                      ≈ Log b (Either p1 p2 -> b)
                     ≈≈ Log b (p1 -> b, p2 -> b)
                     ≈≈ Log b (a1, a2)
                      ~ log_b(a1*a2)

And translate it into a constructive version:

addLogs :: forall a1 a2 b. Iso (Log b a1 :+: Log b a2)
                               (Log b (a1 :*: a2))
          -- Log b (a1 :*: a2)
addLogs = liftRightLog (liftBothMul (inverse asExp1)
                                    (inverse asExp2))
          -- Log b ((p1 -> b) :*: (p2 -> b))
        . liftRightLog expSum
          -- Log b (p1 :+: p2 -> b)
        . inverse logExp
          -- p1 :+: p2
        . liftBothAdd logExp logExp
          -- Log b (p1 -> b) :+: Log b (p2 -> b)
        . liftBothAdd (liftRightLog asExp1)
                      (liftRightLog asExp2)
          -- Log b a1 :+: Log b a2
  where
    asExp1 :: Iso a1 (P1 -> b)
    asExp2 :: Iso a2 (P2 -> b)

expSum :: Iso (a1 :+: a2 -> b)
              ((a1 -> b) :*: (a2 -> b))
logExp:: Iso (Log b (p -> b))
             p

Success? Not so fast.

Impossible isomorphisms

In order to execute the above constructive proof, we must of course implement all the smaller isomorphisms on which it is based. Two of them, asExp1 and asExp2, seem pretty silly: can we really expect any type a1 to be isomorphic to a function of the form p1 -> b, for any type b of our choosing?

I had originally postulated that I could do this because in my original definition for Log b a1, log types were only inhabited when a1 had the required form p1 -> b. With the new Iso-based definition, I'm no longer sure I can do this, and even with the old definition, it was only ever justified to transform Log b a1 into Log b (p1 -> b), not to transform a naked a1 into p1 -> b.

However, if we simply pick p1 = Log b a1, then the math identity b^{log_b x} = x justifies the principle. Can we write a constructive version of this identity?

One direction is easy:

mkExpLog :: a -> (Log b a -> b)
mkExpLog x (MkLog p (Iso fwd _)) = fwd x p

But the other direction direction is impossible. It would allow us to implement unsafeCoerce!

unExpLog :: (Log b a -> b) -> a

unsafeCoerce' :: b -> a
unsafeCoerce' = unExpLog . const

Similar issues occur with identities from the other inverse types. With a constructive version of the identity x - x = 0, for example, we can construct an inhabitant for the empty type:

unSubSelf :: a :-: a -> Void

subSelf :: [()] :-: [()]
subSelf = MkSub () (Iso fwdL bwdL)
  where
    fwdL :: [()] -> () :+: [()]
    fwdL []      = Left ()
    fwdL (():xs) = Right xs
    
    bwdL :: () :+: [()] -> [()]
    bwdL (Left  ()) = []
    bwdL (Right xs) = ():xs

bottom :: Void
bottom = unSubSelf subSelf

The fact that a recursive type is used in this counter-example hints at an explanation of the issue. The identity x - x = 0 is certainly true in math, for any number x. So at least for types with exactly x inhabitants, we would expect the isomorphism to hold. But in this case, the type [()] has infinitely-many inhabitants, and as we know from math, ∞ - ∞ is not zero, it's an indeterminate form.

Here is another implementation of the empty type, based on the identities x / x = 1 and x / y = x * (1/y):

mkDivSelf :: () -> a :/: a
mkTimesReciprocal :: a :/: b
                  -> a :*: (() :/: b)

divZero :: Void :/: Void
divZero = mkDivSelf ()

bottom' :: Void
bottom' = fst (mkTimesReciprocal divZero)

I haven't managed to derive a contradiction from any of the root identities, but they seem just as impossible to implement.

Restricted isomorphisms

Okay, so the indeterminate forms ∞ - ∞ and 0 / 0 both led to contradictions. In math, we work around the problem by saying that the division identities are only valid when the denominator is non-zero, and we don't even bother mentioning infinity because it's usually not a member of our universe of discourse, the real numbers for example.

In the world of types, instead of forbidding particular elements such as zero, it's much more common to require a witness proving that the types under consideration are valid for the current operation. In Agda, this would be a predicate on Set, while in Haskell, it would be a typeclass constraint. So, if future me is reading this and wants to continue exploring the world of inverse types, I leave him with the following recommendation: try looking for typeclass constraints under which the identities don't lead to contradictions, or even better, under which they can actually be implemented.

Homemade FRP: mystery leak

The mystery leak is back, and this time it's personal: the bug is clearly in my code somewhere.

My first hypothesis is that since I am emulating reactive-banana's API, and I now have the same kind of leak as I had with the real reactive-banana, maybe it's the same bug with the same solution? Nope, adding a <* stepper undefined events for each kind of event does not plug the leak.

My next attempt is to simplify: what is the simplest program I can write which reproduces the leak?

main :: IO ()
main = play (InWindow "Nice Window" (200, 200) (800, 200))
            white
            30
            (pure (Circle 10))
            currentValue
            (\_ -> handleEventB ())
            (\_ -> handleEventB ())

Obtained using the "-hy" flag; see Real World Haskell's chapter on profiling.
The rest of the profile information is useless: it says all the space is used by "MAIN".

That's as simple as main could get while still using gloss and my homemade FRP implementation. I already know that a pure (Circle 10) behaviour doesn't leak with gloss and the real reactive-banana, so clearly, the bug must be in my homemade version.

In order to simplify further, let's bring in the FRP primitives.

data Behavior t a = Behavior
  { currentValue :: a
  , runBehavior :: t -> Behavior t a
  }

pure :: a -> Behavior t a
pure x = Behavior x (\_ -> pure x)

handleEventB :: t -> Behavior t a -> Behavior t a
handleEventB = flip runBehavior

main :: IO ()
main = play (InWindow "Nice Window" (200, 200) (800, 200))
            white
            30
            (pure (Circle 10))
            currentValue
            (\_ -> handleEventB ())
            (\_ -> handleEventB ())

Still leaks; as expected, since I didn't really change anything yet.

I'm not using Behavior's ability to accept input events, so I can simplify further:

data Behavior t a = Behavior
  { currentValue :: a
  , runBehavior :: Behavior t a
  }

pure :: a -> Behavior t a
pure x = Behavior x (pure x)

main :: IO ()
main = play (InWindow "Nice Window" (200, 200) (800, 200))
            white
            30
            (pure (Circle 10))
            currentValue
            (\_ -> runBehavior)
            (\_ -> runBehavior)

I don't know why the colours have changed, but clearly, it's still leaking.
(spoiler alert: the colour change will turn out to be important)

Usually, space leaks are due to insufficient strictness. I waste a bit of time adding bangs and seqs here and there, but of course, the application is already strict: in order to display each Picture, gloss is forced to fully-evaluate each step.

Still, just to validate that gloss really is doing the simple iterative loop I think it is doing, I replace play with my own loop.

main :: IO ()
main = loop (pure (Circle 10))
  where
    loop b = do
        print (currentValue b)
        loop (runBehavior b)

Compared with the previous graph, it looks like the memory is now high all the time,
but look at the Y axis: the memory usage is now constant, and much smaller than before.

Interesting, the leak is gone! Could the leak be in gloss after all? To make absolutely sure, I go back to my full, unsimplified program, and I replace its top-level by a similar loop.

main :: IO ()
main = loop (reactiveMain floats inputs)
  where
    floats = inputEvents
    inputs = never
    
    loop b = do
        print (currentValue b)
        loop (handleEventB 1.0 b)

This time, the colour change is expected, because the program is completely different.
But did you expect the leak to be back?

Nope, it's not gloss. Or at least, it's not just gloss: I observe a leak when I use a gloss loop with a simple update function (G and ¬U), I also observe a leak when use a simple loop with a complex update function (¬G and U), but I don't see a leak when I use the simple loop with the simple update function (¬G and ¬U). Therefore, there is probably a leak in gloss (G), and a second leak in the complex update function (U). That's why the graph colours changed: the different programs are illustrating different leaks.

I begin with the complex update function; since I wrote all the code, I should be able to understand the problem better.

The update function leak

The profile data is very different this time, and also very useful. It's no longer Behavior which uses up all the space: it's lists, and the profiling report (obtained with "-p") even tells us where to look:

COST CENTRE     MODULE    %time %alloc

main.loop       Main       78.0   75.8
accumE.go.(...) PureFrp     5.9    8.2
filterE.go      PureFrp     2.1    4.3
fmap            PureFrp     2.0    2.6
accumE.go       PureFrp     1.5    2.7
mappend         PureFrp     1.3    1.3
accumE.go.xs    PureFrp     1.1    1.5

accumE is indeed manipulating lists: it uses scanl to produce a list of intermediate values, obtained by accumulating all the events occurring this frame. Let's add some strictness to that list:

accumE :: a -> Event t (a -> a) -> Event t a
accumE x e = Event go
  where
    go t = x' `seq` (xs', accumE x' e')
      where
        (fs, e') = runEvent e t
        xs = scanl (flip ($)) x fs
        x' = last xs
        xs' = tail xs  -- skip the initial unmodified x

Leak fixed. Next!

One leak down, one to go. Before we move on, though, I'd like to understand why strictness is needed in this particular function.

Let's see, each time a Float event comes in, reactiveMain is called and its current value is fully evaluated. Somewhere inside reactiveMain, there is a call to accumE, and without the extra strictness annotation, we know that accumeE's current value (the xs list and the x' it contains) doesn't get evaluated. Why not?

There is more than one use of accumE in reactiveMain, but the one in firstEvent looks particularly fishy:

firstEvent :: Event t a -> Event t a
firstEvent = fmap snd
           . filterE ((== 1) . fst)
           . numberEvents

numberEvents uses accumE to pair each event with an incrementing number. But for all events except the first one, we only evaluate the numeric half of the pair; could it be that the right-hand side is accumulating thunks?

Given the fact that a single seq was sufficient to solve the problem, that seems unlikely. After all, seq only forces x' to WHNF; depending on the implementation of numberEvents, that might either mean evaluating the outer pair or the incrementing number, but definitely not the inner members of the pair.

And indeed, if we revert the seq fix and remove firstEvent from the equation, the leak returns:

firstEvent is not the problem here.

There is another occurrence of numberEvents inside clickEvents. This location is interesting because I have noticed that clicking on a button causes a lot of extraneous memory to be released.

clickEvents :: Event t (Char, Int)
clickEvents = fmap swap
            $ numberEvents
            $ clickLabels

To explain the memory release, Float and click events must somehow be evaluated differently: one accumulates thunks, while the other evaluates them. And there is indeed a big difference: clickEvents ignores all the Float events, and only reacts to clicks. So how is ignoring events leading to thunk accumulation?

Well, ignoring events simply means filtering them out of the list of input events, which means that most of the time, clickEvents is going to be receiving an empty list as input and producing an empty list as output. This empty list gets fully-evaluated on every frame, in order to confirm that there is nothing to update on the screen, but that doesn't mean that all the thunks get forced: x', for example, is not a member of this empty list.

Let's look at the code for accumE again:

accumE :: a -> Event t (a -> a) -> Event t a
accumE x e = Event go
  where
    go t = (xs', accumE x' e')
      where
        (fs, e') = runEvent e t
        xs = scanl (flip ($)) x fs
        x' = last xs 
        xs' = tail xs  -- skip the initial unmodified x

scanl always returns a non-empty list starting with x, which is why we can call partial functions like last and tail without fear. Also, since x is passed through without being examined, any thunks accumulated to compute x will be passed through without being evaluated. In this case, it's the calls to last which will accumulate.

The reason the memory gets released when a button is clicked is that x is also involved in the computation of the elements of the output list, so as soon as this output list becomes non-empty and its elements get fully-evaluated, so does x. With the new strictness annotation, just evaluating the output list to WHNF is now sufficient to evaluate the next x to WHNF, thereby fixing the leak.

The gloss leak

Let's look into the second leak now, the one which occurs when I use gloss with a simple update function.

data Behavior t a = Behavior
  { currentValue :: a
  , runBehavior :: Behavior t a
  }

pure :: a -> Behavior t a
pure x = Behavior x (pure x)

main :: IO ()
main = play (InWindow "Nice Window" (200, 200) (800, 200))
            white
            30
            (pure (Circle 10))
            currentValue
            (\_ -> runBehavior)
            (\_ -> runBehavior)

If it's really a bug in gloss, then I should be able to reproduce it without any FRP-related stuff at all. Simplified as it is, a Behaviour t a is isomorphic to [a], and pure x isomorphic to repeat x. Can gloss handle infinite lists?

main :: IO ()
main = simulate (InWindow "Nice Window" (200, 200) (800, 200))
                white
                30
                [0..]
                (renderInt . head)
                (\_ _ -> tail)

Nope, it can't.

Okay, so this is an extremely simple gloss program exhibiting a clear memory leak, so even if I can't find the leak myself, I should be able to send this example program to the gloss developers as a bug report. Still, where does the profiler say that the issue is?

COST CENTRE   MODULE                                 %time %alloc

MAIN          MAIN                                    89.2  71.8
main          Main                                    6.7    0.0
CAF           Graphics.Rendering.(...).Functions      1.4    0.0
CAF           GHC.IO.Encoding                         1.2    0.0
callbackIdle  Graphics.Gloss.Internals.(...).Idle     0.6   13.7
drawComponent Graphics.Gloss.Internals.(...).Picture  0.3    9.5
fromList      Main                                    0.2    4.8

It looks like callbackIdle would be a good place to start. Looking at the code, this cost center is actually a function named callback_simulate_idle. Here is a slightly simplified version:

-- | Called when we're finished drawing
-- and it's time to do some computation.
callback_simulate_idle
 :: IORef SM.State  -- ^ the simulation state
 -> IORef world   -- ^ the current world
 -> world   -- ^ the initial world
 -> (Float -> world -> IO world) -- ^ advance the world
 -> IO ()
 
callback_simulate_idle simSR worldSR worldStart worldAdvance
 = {-# SCC "callbackIdle" #-}
   do simS <- readIORef simSR
 let result | SM.stateReset simS
            = simulate_reset simSR worldSR worldStart

            | SM.stateRun   simS
            = simulate_run   simSR worldSR worldAdvance
 
            | SM.stateStep  simS
            = simulate_step  simSR worldSR worldAdvance
 
            | otherwise
            = \_ -> return ()
 
 result

The memory is probably being allocated every time worldAdvance is called, but it's another value which catches my eye: worldStart? Why would the idle function need to remember the starting state? I see that it's being used when we want to "reset". I didn't know gloss allowed you to reset to the start state.

This reset feature explains the leak: in order to reset to the start state, gloss need to keep a pointer to the head of the infinite list, which prevents elements from being garbage-collected after being displayed.

Well, this is problematic. How to fix the leak while retaining the ability to reset the state? If the start state was obtained by calling a function, then the infinite lists returned by different invocations of this function would not be shared, thereby allowing the elements to be garbage-collected. But that's not the API gloss uses: play receive the start state directly, not as a wrapping function returning the start state. And creating our own function returning this constant state wouldn't help, because the function would hold a pointer to the head of the list, which would again prevent the garbage collection.

Anyway, I'm quite curious about this reset feature by now, so I look in the code for the key combination which triggers it, to no avail. In fact... it doesn't seem like the reset feature can be triggered at all? It's dead code! Along with plenty of other code which also looks dead. If fact, of the above four cases, only the simulate_run branch is ever taken.

This is weird, but it's actually great news for the memory leak. After removing all the dead code, the reference to worldStart is no longer kept alive after the first step, and the leak goes away.

The sweet plateau of fixed space leaks.

Conclusion

It can be hard to reason about a bug, be it a space leak or otherwise, when your assumptions are flawed. In this case, I had assumed that there was only one leak, when in fact there were two.

The key to uncovering incorrect assumptions is to run lots of sanity checks. In this case, after I had concluded that the leak had to be in gloss, I tested this hypothesis by removing gloss from the unsimplified version of my program, expecting the leak to disappear. When it didn't, I knew that one of my assumptions had to be flawed.

When that happens, one tedious but very effective weapon is to simplify the program: in a smaller program, there are fewer places for incorrect assumptions to hide. It also simplifies chains of reasoning, by not having lots of elements through which to chain.

Homemade FRP: a study in following the types

I wonder how FRP is implemented? Since I only know about a small part of reactive-banana so far, this is the interface I am currently thinking of when I think of FRP:

data Event t a
data Behavior t a

instance Functor (Event t)
instance Functor (Behavior t)
instance Monoid (Event t)
instance Applicative (Behavior t)

filterE :: (a -> Bool) -> Event t a -> Event t a
accumE :: a -> Event t (a -> a) -> Event t a
stepper :: a -> Event t a -> Behavior t a

I have omitted never and union, because they are just mempty and mappend from Monoid. I have also omitted accumB, because my understanding is that accumB x e is just a more efficient version of stepper x (accumE x e), and I am not concerned at all about efficiency yet.

Interactivity

So, how do I implement this interface? Could I represent Event as a map from timestamp to values, for example? It sounds like this representation would admit an implementation for all the required functions, but that would not allow me to handle events as they come, like I can do with gloss-banana. I would have to open the window, and let the user click around without being able to update the display. After accumulating all the mouse events, I would gather them into one big map, an Event t InputEvent, from which I could construct an Event t Picture. At this point, I would know exactly what I should have displayed after each of the mouse clicks, but it would be too late for that. Clearly, I need to find a way to obtain the early outputs before I learn about the later inputs...

With a list instead of a map, I can think of at least two laziness-based approaches for this kind of problem, but I'm not ready for a clever implementation yet. Let's keep things simple!

I said that a map-based representation would not allow me to handle events as they come. So clearly, being able to handle events as they come should be part of my API. What would a function for handling a new event look like?

handleInputEvent :: InputEvent -> Event t a -> IO (Maybe a)

I give the next InputEvent, the Event decides what it wants to do with it, and optionally triggers an event of type a based on it. But it's weird to have such a concrete type as InputEvent; surely reactive-banana isn't aware of the specifics of the event types supported by gloss. Event should have a type parameter indicating which type of input events it expects. Oh! Maybe that's what the t is for?

handleEvent :: t -> Event t a -> IO (Maybe a)

Much better. You might wonder why I am returning an IO action instead of a pure Maybe a: it's because of accumE. Accumulating the values of the events as they come requires storing an intermediate state somewhere, and a pure function would not be able to store its new state anywhere.

Well, I don't want to be stuck inside IO yet, so let's thread this state explicitly:

handleEvent :: t -> Event t a -> (Maybe a, Event t a)

I give the next t-valued input event, the Event decides what it wants to do with it, and optionally triggers an event of type a. The Event might have changed its internal state somehow, so it also returns a modified copy of itself, one which is ready to receive the next event.

Event

I now have a much clearer idea of the way in which I am going to use (or "eliminate", in type-theory-speak) values of type Event, but I'm no closer to figuring out how to represent those Event values. Or am I? When a type has a single (or a most general) eliminator, we can just use that eliminator as a representation:

data Event t a = Event
  { runEvent :: t -> (Maybe a, Event t a)
  } deriving Functor

handleEvent = flip runEvent

Is this representation good enough? Can the rest of the API be implemented on top of it? As I get stuck trying to implement union, I remember that one input event can cause more than one output event to be emitted at the same time. I make a small adjustment:

data Event t a = Event
  { runEvent :: t -> ([a], Event t a)
  } deriving Functor

With this representation, I can easily derive an implementation for the first few combinators, simply by following the types. A good sign!

instance Monoid (Event t a)
  where
    mempty = Event (const ([], mempty))
    e1 `mappend` e2 = Event go
      where
        go t = (xs1 ++ xs2, e1' <> e2')
          where
            (xs1, e1') = runEvent e1 t
            (xs2, e2') = runEvent e2 t

filterE :: (a -> Bool) -> Event t a -> Event t a
filterE p e = Event go
  where
    go t = (filter p xs, filterE p e')
      where
        (xs, e') = runEvent e t

Interlude: on the technique known as "follow the types"

Following the types is a technique which allows a function to be implemented very quickly, by taking decisions based on the types of the values involved instead of their meaning. Since it happens so quickly, it's a process which is a bit difficult to describe, as stopping in the middle of it in order to take notes will break the magic. Here is an attempt at reconstructing the magic after the fact.

First, let's dispel a possible misconception: I'm not a human version of djinn. That is, when I "follow the types", I do not blindly pick a random expression of the expected type, hoping it will be correct. Instead, I use the types as a shortcut when there is only one obvious way to go forward, and otherwise I pick an expression which fits the immediate context.

For example, in the code for filterE, I begin with the Event constructor, I pass it an intermediate function receiving a value of type t, and I eliminate the e argument using its only eliminator and the only value of type t I have on hand. I did not waste any time on those uninteresting details, because there are simply no alternatives to consider.

The call to filter p xs, on the other hand, is deliberate: a simple xs would have had the same type, but would have been incorrect. Yet the reason I used filter was not because I stopped to think about correctness; that would have involved reasoning based on the meaning of the values. Instead, I knew from the start that I wanted to filter out values of type a. It is the context of implementing an event-filtering function which guided me through the non-forced parts of the implementation: as I was following the types, I was on the lookout for values of type a or [a], ready to filter them on sight. This kind of context is also the reason why I used [] in the definition of mempty, versus (++) in the definition of mappend.

A very different example of context is that of the recursive call to filterE p e', where e and e' would have also type-checked. This part of the implementation is in a recursive position for Event t a, just like the tail is a recursive position for lists. In such a recursive context, I naturally picked a recursive expression, of the proper type of course, and using smaller arguments where possible.

Simplification

After a type-based implementation, I like to take a moment to go beyond the types and examine the meaning of all the values involved. Reading the code I just blindly wrote for never/mempty, the Event in which no event ever occurs, I see that the first input event is ignored, and that no output events are produced in response. Afterwards, recursively, no event ever occurs, and that's exactly what I want.

This reviewing phase is also a good opportunity for simplifications, such as replacing (\_ -> ...) with const or rearranging the pieces in a pointfree style. One simplification opportunity I notice is that [] is the mempty for lists, and (mempty, mempty) is the mempty for pairs, and const mempty is the mempty for functions. This, and a similar chain for mappend, allows the Monoid implementation to be simplified greatly:

instance Monoid (Event t a)
  where
    mempty = Event mempty
    Event e1 `mappend` Event e2 = Event (e1 `mappend` e2)

In this version, all the details are implicit, so much so that it's hard to follow the meaning of the values. But it's also a much more elegant implementation, and I've just read and understood the expanded-out version anyway, so I'm confident that the meaning is what I expect.

Behavior

The workflow for Behavior and its combinators is very similar to Event, so let's go through it quickly.

How do I represent/eliminate a Behavior? Probably the same way I eliminate events, except that behaviours have a value at every point in time, so I should receive exactly one a instead of a (potentially-empty) list of them.

data Behavior t a = Behavior
  { runBehavior :: t -> (a, Behavior t a)
  } deriving Functor

Trying to implement stepper, I realize that a behaviour is also supposed to hold a value before the very first event, and also between events. I need a second eliminator:

currentValue :: Behavior t a -> a

Now I have two eliminators. What would be a most general eliminator, from which the two others could be implemented?

generalEliminator :: Behavior t a -> (a, t -> Behavior t a)

I no longer need an a on the right-hand side of the (t -> ...), because currentValue can extract this a from the returned behaviour. I transform this most general eliminator into a representation for Behavior, giving convenient names to the pair's two components:

data Behavior t a = Behavior
  { currentValue :: a
  , runBehavior :: t -> Behavior t a
  } deriving Functor

Following the types is very straightforward this time.

instance Applicative (Behavior t)
  where
    pure x = Behavior x fx
      where
        fx _ = pure x
    Behavior f ff <*> Behavior x fx = Behavior (f x) fy
      where
        fy t = ff t <*> fx t

As before, this can be simplified: (\_ -> x) is the pure x for functions, and thus fx could be written as pure (pure x). Similarly, fy could be written by nesting two (<*>), whatever that means. This hints at a variant implementation of Behavior based on Compose, which will hide the details of the nesting:

data Behavior t a = Behavior
  { currentValue :: a
  , runBehavior :: Compose ((->) t) (Behavior t) a
  } deriving Functor

instance Applicative (Behavior t)
  where
    pure x = Behavior x (pure x)
    Behavior f cf <*> Behavior x cx = Behavior (f x) (cf <*> cx)

I'm not convinced that this variant is actually better, because the other combinators don't benefit from Compose: they just peel off the Compose layer, do their work, and put it back on. Speaking of the other combinators, I've skipped two of them in my exposition. I left them for the end, because...

When following the types leads nowhere

The last two combinators don't fit the mantra of following the types very well. Here is part of accumE:

accumE :: a -> Event t (a -> a) -> Event t a
accumE x e = Event go
  where
    go t = (_xs, ...)
      where
        (fs, e') = runEvent e t

The next step is to find an expression of type [a] for _xs. Unused values include fs, of type [a -> a], and x, of type a. One obvious and type-correct way to combine those two values is to apply all the functions in the list to x:

        _xs = fmap ($ x) fs

But we're in an accumulator context, so I'm expecting something like a fold instead. When something doesn't seem right, it's best to stop following the types and switch to "figure out what I'm trying to do" mode.

In this case, what I am trying to do is to thread the x through all the functions, returning all the intermediate values. It turns out there is a standard library function for that, scanl:

accumE :: a -> Event t (a -> a) -> Event t a
accumE x e = Event go
  where
    go t = (xs', accumE x' e')
      where
        (fs, e') = runEvent e t
        xs = scanl (flip ($)) x fs
        x' = last xs
        xs' = tail xs  -- skip the initial unmodified x

If I was more familiar with scanl, I could have used it without thinking, remaining in follow-the-types mode. But then I wouldn't have thought to skip scanl's initial copy of x, I might have recurred with x instead of last xs, and everything would have been completely wrong.

My implementation for the last combinator is also quite wrong:

stepper :: a -> Event t a -> Behavior t a
stepper x e = Behavior x (Compose go)
  where
    go t = stepper x e'
      where
        (xs, e') = runEvent e t

I did not use xs, and as a result, I recur on the wrong x. Here is what I should have written instead:

stepper :: a -> Event t a -> Behavior t a
stepper x e = Behavior x (Compose go)
  where
    go t = stepper (last (x:xs)) e'
      where
        (xs, e') = runEvent e t

The only trick here is that I prepend x to xs in order to ensure that the argument to last is non-empty.

Conclusion

Following the types didn't work for every single combinator, but when it did work, it quickly produced a correct implementation on the first attempt.

Yup, it's working!
It even has the same leak-until-clicked behaviour as the original :)

When it didn't work, following the types did not lead to any wasted work. For ease of presentation, I have shown an incorrect type-based version followed by a corrected meaning-based version, but during my original implementation, there wasn't a sharp break between the two phases. Like I said, it all happens very quickly, so it's hard to describe exactly what is going on inside my head when I code. I think I always focus on the correctness of what I write. It's just that over time, I have learned that when the types which are in scope are sufficiently distinct, it's okay for "correct" to mean simply "type-correct". Whereas at other times, perhaps mere seconds later, it becomes important to understand what the values actually mean.

If you came here to learn how to follow the types yourself, and you're disappointed by my vague explanations, don't be. I did not set out to learn to follow the types; I just wrote lots of Haskell code, and my brain learned to focus on the types when appropriate. You'll get there, don't worry.

reactive-banana mystery leak

So I accidentally let my reactive-banana toy app opened all night, and in the morning the computer was horribly unresponsive until I killed it. After starting a new instance, I noticed that the CPU goes up from ~5% to 100%, during which the memory usage also grows uncontrollably. What is causing this?

The first thing I try is to lower the refresh rate at which gloss recomputes its frames, in case the problem is that the computer can't keep up with the work it has to do. The resource usage still increases, but much more slowly. Next, I try to isolate the commit in which the problem was introduced. Did the problem exist when I was using gloss, but not reactive-banana? No. Git-bisecting, I find that the problem was introduced by commit 66f4e7, "accumulating clicks":

@@ -27,13 +28,31 @@ reactiveMain :: forall t. Frameworks t
              => Event t Float
              -> Event t InputEvent
              -> Moment t (Behavior t Picture)
-reactiveMain floats _ = return pictures
+reactiveMain floats events = return pictures
   where
-    partialSums :: Behavior t Float
-    partialSums = accumB 0 (fmap (+) floats)
+    buttonClicks :: [Event t ()]
+    buttonClicks = map (flip buttonClick events) buttons
+    
+    countA,countB,countC :: Behavior t Int
+    [countA,countB,countC] = map countEventsB buttonClicks
+    
+    clickCounts :: Behavior t (Int,Int,Int)
+    clickCounts = liftA3 (,,) countA countB countC
     
     pictures :: Behavior t Picture
-    pictures = fmap renderFloat partialSums
+    pictures = fmap render clickCounts
+
+countEventsB :: Num a => Event t b -> Behavior t a
+countEventsB = accumB 0 . fmap (+) . fmap (const 1)
+
+buttons :: [Extent]
+buttons = [extentA, extentB, extentC]
+
+buttonClick :: Extent -> Event t InputEvent -> Event t ()
+buttonClick ex = fmap (const ()) . filterE isInside
+  where
+    isInside (EventKey (MouseButton LeftButton) Down _ p) = pointInExtent ex p
+    isInside _                                            = False
 
 renderFloat :: Float -> Picture
 renderFloat = uscale 0.2 . text . show

This is the commit in which I switched from displaying the accumulated time to displaying the number of accumulating clicks on three buttons.

After a little more experimentation, I notice that the CPU and memory get released whenever I click on one of the buttons. Weird! Which special thing happens when I click one of the buttons? Well, one thing is that the graphics change. What if I force the graphics to change all the time by displaying the running elapsed time? The problem disappears. That's good, but I don't want to display the running elapsed time. Can I instead compute the running time, but display a fixed string? Embarrassingly, the first time I tested this I forgot to force the computed value, so the problem reappeared. When I do force the value, the problem remains hidden.

Ok, so I don't understand why the CPU usage would go up, but the memory increase seems to be due to a laziness issue.

I wish I could use ghci to step through the code, but the app hangs when I do. Seems to be an issue with gloss, not reactive-banana. Maybe if I disable the timed updates by setting the refresh rate to zero? Still hangs, but the leak disappears. Which is not a surprise, really: the fact that the leak grew more slowly when the refresh rate was lower clearly indicates that the leak is caused by the Float events.

Or it is that clear? With the refresh rate set to zero, there are no Float events, and at first the leak doesn't seem present. But if I click outside the buttons or move the mouse a lot, this generates a bunch of ignored events, which causes the memory to rise. Since this is not how I have previously been testing the leaks, I re-perform the git-bisect with a refresh rate of zero, and discover that the problem was actually introduced by commit 1386bf, "accumulating time":

@@ -18,7 +21,19 @@ main :: IO ()
 main = playBanana (InWindow "Nice Window" (200, 200) (800, 200))
                   white
                   30
-                  (\floats _ -> return $ fmap renderFloat $ stepper 0 floats)
+                  reactiveMain
+
+reactiveMain :: forall t. Frameworks t
+             => Event t Float
+             -> Event t InputEvent
+             -> Moment t (Behavior t Picture)
+reactiveMain floats _ = return pictures
+  where
+    partialSums :: Behavior t Float
+    partialSums = accumB 0 (fmap (+) floats)
+    
+    pictures :: Behavior t Picture
+    pictures = fmap renderFloat partialSums

This is the commit in which I switched from displaying the Float values to displaying the sum of all Float values so far. Interestingly, I am not even using the mouse events in my FRP computation, so the problem can't possibly be that my code is doing something it shouldn't. Maybe there is something it should do which it isn't doing? Perhaps I should be actively consuming and ignoring the events, instead of simply not using them?

reactiveMain floats events = return pictures
  where
    pictures :: Behavior t Picture
    pictures = fmap renderFloat partialSums <* stepper undefined events

It works! In the most recent commit, which handles mouse events but filters out most time events, it is the floats which must be ignored. And in general, just to be safe, both kinds of events should be actively ignored:

pictures :: Behavior t Picture
pictures = fmap render buttonTexts
        <* stepper undefined floats
        <* stepper undefined events

Or, better yet, let's hide this inside gloss-banana, so that nobody has to think about this again.

reactive-banana anti-tutorial

This is not a tutorial. I have never used reactive-banana nor any other FRP library. And that's the point: in this post, once again¹, I will reveal my thought process as I learn this library.

Motivation

A recent tutorial called The introduction to Reactive Programming you've been missing made me realize that FRP is no longer a niche technique used by a subset of the functional programming community. Apparently, there's now a manifesto, a javascript library involving Microsoft, a conference... FRP, or at least RP, is clearly a big deal now.

As a member of the functional programming community, I feel like I ought to at least have an opinion on FRP. And of course, if I'm going to learn a reactive library, I will learn one of the Haskell variants, not a Javascript knockoff.

Why not <name-of-alternate-frp-library>?

Because I began by writing the graphics in gloss, whose play interface is based on pure functions. When I looked at sodium and elerea, I realized that I couldn't use them because they expect to be run from IO. When I googled for "gloss frp", I found a package named gloss-banana² which allows the simple update function (InputEvent → world → world) to be replaced by an unfamiliar-looking reactive-banana type. Just what I need to get started!

forall t. Frameworks t ⇒ Event t Float
                       → Event t InputEvent
                       → Moment t (Behavior t Picture)

I suspect that it's still IO under the hood, and that a similar sodium or elerea wrapper could easily be written, but I'm not yet in a position to write such a wrapper. So reactive-banana it is.

Versions

I have been told that the APIs change a lot from version to version, so for completeness, here are the versions I am using:

ghc-7.8.2
gloss-1.8.2.1
gloss-banana-0.1.0.0 (modified to accept base-4.7.0.0)
reactive-banana-0.7.1.3

edit: Indeed, reactive-banana 1.0 ended up making a significant change to its API, so if you want to follow along, I recommend using reactive-banana-0.9.0.0, the last version which uses the API I am playing with in this post. For what it's worth, I like the 0.9 API better.

Goal

I would like to duplicate the functionality implemented in the javascript tutorial: three labels and four buttons, one of which updates all the labels, while the remaining three update one label each. For simplicity, I'll put the labels on their corresponding buttons.

Since the code is not running under IO, I won't try to make those updates fetch usernames from github, even though I suspect that interfacing with external event sources such as network connections must be a big part of learning FRP. For now, I want to learn the basics, so each update will simply pull the next available entry from an infinite list of integers.

commit 39647e:
I couldn't find any way to center text in gloss :(

First steps: Why so polymorphic?

I need to produce a value of type forall t. Frameworks t ⇒ something something. That's weird, why would they need me to write polymorphic code? Is there more than one "framework" involved?

Looking at the documentation for Frameworks, I learn that the constraint "indicates that we can add input and output to an event network". But then, why are there no typeclass methods for adding those things? Maybe they are just hidden? Looking at the source, I see that no, it really is a typeclass with no methods. Weird.

Still, using polymorphism kind of makes intuitive sense for allowing extra stuff to be added. Consider the Arrow typeclass, for example: it has a method for adding extra (untouched) input and output along your existing transformation of type f a b (if you are unfamiliar with Arrow, think of it as a → b).

first :: f a b → f (a, s) (b, s)

If I was asked to construct an f Int String, and the code who is asking me for this had to embed my f Int String inside a bigger computation, it might very well need something like first to adapt my computation into theirs. If f was not an arrow, then I might have to produce a polymorphic value of type forall s. f (Int, s) (String, s) instead. I assume that gloss-banana requests a polymorphic function for similar reasons.

Compile, Run, and Quit

The next part of the type, Event t Float → Event t InputEvent → something, is straightforward: to assist me in constructing the something, I have access to the reactive-banana representation of typical gloss events: a Float for the passage of time, or an InputEvent representing a keypress or a mouse action. For each of those, gloss typically expects me to modify a "representation of the world" of my choice, and I also need to have a way to represent this world as a gloss Picture. This time, with gloss-banana, I don't get to choose an intermediate representation, I need to produce a something-something Picture directly (a Moment t (Behavior t Picture) to be exact).

At the moment, I just want something which compiles, so let's see if I can ignore the two Event inputs and produce a constant Picture, using something like return or pure. Is Moment or Behavior a Monad or Applicative?

Looking at the documentation, yes, Moment is a Monad and Behavior is an Applicative. Piece of cake, then...

main :: IO ()
main = playBanana (InWindow "Nice Window" (200, 200) (800, 200))
                  white
                  30
                  (\_ _ -> return $ pure $ circle 10)

commit f4c25a:
At least circles are centered!

Well, it worked, but unlike the play from gloss, playBanana doesn't quit when I press ESC. Does reactive-banana have a command to terminate a Moment or a Behavior? Usually a computation which consists of a single return ends immediately, but clearly reactive-banana's monadic composition does not simply consist in executing one computation after the other.

Since I can't find anything with a quit-sounding name in the documentation, my hypothesis is that reactive-banana computations are able to provide values forever, and that it is the user of that computation who decides when to stop asking it for more values. So let's look's look at said user, the source for playBanana:

playBanana display colour frequency mPicture = do
  ...
  playIO display colour frequency ()
    (\      _ → readIORef pictureref)
    (\ ev   _ → () <$ event ev)
    (\ time _ → () <$ tick time)

Okay, so it looks like there was another version of Gloss.play which I did not know about, playIO. I bet play delegates the bulk of the work to playIO, and hardcodes the logic of quitting, probably using exitSuccess. Let's look at the code for play and playIO...

play ... worldHandleEvent ...
 = playWithBackendIO defaultBackendState 
        display backColor simResolution
        worldStart 
        (return . worldToPicture)
        (\event world -> return $ worldHandleEvent event world)
        (\time  world -> return $ worldAdvance     time  world)

playIO = playWithBackendIO defaultBackendState

Now that's weird. play doesn't seem to hardcode ESC to quit, so it must be playWithBackendIO which hardcodes it. Both versions delegate to playWithBackendIO, so both versions should quit on ESC. But the FRP version doesn't. Why? playWithBackendIO is not documented, so there is no "source" button on which I can click to view its source, but its source is available regardless. Like play and playIO before it, playWithBackendIO delegates its work to yet another internal function, createWindow.

I feel like I went deep enough into this ESC rabbit hole already, and that I should focus on the FRP part. I'll simply write my own version of playBanana³, one which abruptly quits on ESC:

playBanana display colour frequency mPicture = do
  ...
  playIO display colour frequency ()
    (\      _ → readIORef pictureref)
    (\ ev   _ → quitOnEsc ev >> () <$ event ev)
    (\ time _ → () <$ tick time)
  where
    quitOnEsc (G.EventKey (G.SpecialKey G.KeyEsc)
                          G.Down _ _) = exitSuccess
    quitOnEsc _                       = return ()

Event to Moment

On the FRP side, I now have a simple computation which ignores all events and always produces the same Picture, a small circle. One of my inputs is an Event t Float; can I display that float? I can easily convert a Float to a Picture...

uscale :: Float -> Picture -> Picture
uscale v = scale v v

renderFloat :: Float -> Picture
renderFloat = uscale 0.2 . text . show

...but how do I lift this Float → Picture function to a Event t Float → Moment t (Behavior t Picture)? From a talk on sodium I saw a while ago, I remember that some FRP systems have two kinds of streams; events and some other kind. Event streams only have values when events occur, while the other kind of stream has a value at every point in time. Scrubbing through that video, I quickly find a slide which lists "Event" and "Behaviour", reminding me that the other kind of stream is called a "behaviour". I also remember that there is a primitive for converting an event stream into a behaviour: when events occur, they have the same value, and when they don't, the behaviour holds the value of the last event. Clearly this implies that we also needs an initial value, to be held before the first event occurs. I hoogle for a -> Event t a -> Behavior t a (it took me a few attempts to remember to add the t), and I obtain (two copies of?) a function called stepper with that exact signature.

main :: IO ()
main = playBanana (InWindow "Nice Window" (200, 200) (800, 200))
                  white
                  30
                  -- Couldn't match type ‘Float’ with ‘Picture’
                  (\floats _ -> return $ stepper 0 floats)

Oh, right, I forgot to call renderFloat. I don't actually have a Float on which to call it, I only have a something something Float. Which is fine, as I can probably fmap through the somethings to reach the Float. I can either fmap over the Behavior I just constructed or over the original Event, I don't think it matters.

main :: IO ()
main = playBanana (InWindow "Nice Window" (200, 200) (800, 200))
                  white
                  30
                  (\floats _ -> return $ fmap renderFloat
                                       $ stepper 0 floats)

Youhoo!

commit ee5dd7:
The float is supposed to be the number of seconds since the last frame,
which should be 1/30 since I asked to be updated 30 times per second.
I think it's a bug in gloss.
edit: silly me, it's not 3.3333..., it's 3.3333e-2.

Sum

Sooner or later, I will need to keep some state between frames. As a simple exercise, can I display the sum of the floats instead of the latest one?

Let's see, I start with a value, then each time I receive an event, I update this value. Does hoogle know of any function with the type s -> (a -> s -> s) -> Event t a -> Behavior t s? Nope. Maybe I can stay inside the world of events, and only convert to behaviour at the end. Does hoogle know of any function of type s -> (a -> s -> s) -> Event t a -> Event t s? Neither. I can't stay inside the world of behaviours: since they have values at all point in time, it wouldn't be clear when to apply the update function.

Let's see, what else could it be? Oh, I know! Instead of an event broadcasting dynamic values to be accumulated with a fixed update function, maybe it could be the functions which are dynamically broadcasted? As each one would arrive, they would be accumulated on top of a fixed initial value. Does hoogle know of any function of type s -> Event t (s -> s) -> Behavior t s? Yes it does! It's called accumB, and I can use it to compute a running sum.

main = playBanana (InWindow "Nice Window" (200, 200) (800, 200))
                  white
                  30
                  reactiveMain

reactiveMain :: forall t. Frameworks t
             => Event t Float
             -> Event t InputEvent
             -> Moment t (Behavior t Picture)
reactiveMain floats _ = return pictures
  where
    partialSums :: Behavior t Float
    partialSums = accumB 0 (fmap (+) floats)
    
    pictures :: Behavior t Picture
    pictures = fmap renderFloat partialSums

commit 1386bf:
It works! And as a bonus, the timing bug is now mysteriously fixed.

Buttons

So far so good! In the same vein, I should be able to accumulate button press events to increment digits on each of the buttons. But wait; each button is only concerned with a subset of the mouse click events. How do I filter an event stream? Another easy hoogle search says filterE should do the trick.

reactiveMain floats events = return pictures
  where
    buttonClicks :: [Event t ()]
    buttonClicks = map (flip buttonClick events) buttons
    
    countA,countB,countC :: Behavior t Int
    [countA,countB,countC] = map countEventsB buttonClicks
    
    clickCounts :: Behavior t (Int,Int,Int)
    clickCounts = liftA3 (,,) countA countB countC
    
    pictures :: Behavior t Picture
    pictures = fmap render clickCounts

countEventsB :: Num a => Event t b -> Behavior t a
countEventsB = accumB 0 . fmap (+) . fmap (const 1)

buttons :: [Extent]
buttons = [extentA, extentB, extentC]

buttonClick :: Extent -> Event t InputEvent -> Event t ()
buttonClick ex = fmap (const ()) . filterE isInside
  where
    isInside (EventKey (MouseButton LeftButton)
                       Down _ p) = pointInExtent ex p
    isInside _                   = False

To combine the three different behaviours into one in clickCounts, I guessed that Behavior was probably an Applicative. Events probably aren't; because of filtering, you wouldn't be able to match individual events from one stream with individual events from the other.

commit 66f4e7:
Click-click-click-click-click! Click-click! Click-click-click!

A shared source of numbers

Currently, the buttons all start at 0 and each click increments that individual button's number by one. Instead, I would like the three buttons to start at 1, 2 and 3, and for each click to set the clicked button to the next number in that sequence. The obvious imperative implementation, however, doesn't seem very FRP-ish. In particular, I don't think the event stream from one button is allowed to have any effect on the event stream of the other buttons: a stream can only be affected by its "upstream" streams, that is, the streams from which it is computed.

So I think I need something upstream which would handle the shared state, and each of the three buttons should derive their own event streams from that. The most obvious representation for an upstream thing from which the three kinds of events could be derived would be... a stream containing all three kinds of events.

I already have a list of click streams, obtained by filtering the raw mouse events from three separate regions. To obtain a single stream containing the events from all three regions, I could filter the raw events in a different way, but can I instead merge the three event streams I already have? Hoogle doesn't have a function of type Event t a -> Event t b -> Event t (Either a b), and I don't have any other idea of the form such a combinator could look like. Looking at the documentation for Event, I quickly find union, which needs both sides to hold events of the same type. And actually, it looks even more convenient that way!

labelledClicks :: [Event t Char]
labelledClicks = zipWith (fmap . const) ['a'..] buttonClicks

clickLabels :: Event t Char
clickLabels = foldr union never labelledClicks

Now I need each of those events to increment a shared state. The accumB we used earlier generated a Behavior; from the name, I guess that there is also a variant named accumE which generates an Event? Bingo.

clickEvents :: Event t (Char, Int)
clickEvents = accumE (undefined, 0) (fmap mkEvent clickLabels)

mkEvent :: Char -> (Char, Int) -> (Char, Int)
mkEvent label (_, n) = (label, n+1)

Since I want to keep the Char from the original event stream, the accumulated value must contain a Char, which means I must also provide a Char for the initial state. I never use this part of the initial state, so I can use undefined.

Now that I have a single stream containing the annotated events from all three buttons, I can split them back into three separate streams via filters which happen to be mutually-exclusive. Previously, we used filterE, while this time... we must still use filterE, because strangely enough, there is no filterB.

countA,countB,countC :: Behavior t Int
[countA,countB,countC] = map countN "abc"

countN :: Char -> Behavior t Int
countN label = stepper 0
             $ fmap snd
             $ filterE ((== label) . fst) clickEvents

commit 76b052:
The buttons start at (0,0,0) instead of (1,2,3),
but otherwise increment with a shared state, as desired.

The refresh button

When the refresh button is clicked, the three other buttons should behave as if they were clicked one after the other. Faking those clicks sounds easy enough: since the real stream of clicks is just a stream of () values, I should be able to add a few extra () values to represent the fake clicks. In particular, I can simply union the stream of real clicks with the stream of refresh clicks.

refreshClicks :: Event t ()
refreshClicks = buttonClick extentR events

fakeButtonClicks :: [Event t ()]
fakeButtonClicks = map (union refreshClicks) buttonClicks

labelledClicks :: [Event t Char]
labelledClicks = zipWith (fmap . const) ['a'..] fakeButtonClicks

commit d98691:
The buttons are clicked one after the other, like I wanted.
But how did reactive-banana know to click them from top to bottom?

It worked! But... why did it? The fake clicks from the refresh button are sent to the other three buttons at the same time. How come the "callbacks" were called from top to bottom? What if I wanted the buttons to be fake-clicked in the opposite order?

Let's see. Conceptually, at least (I don't know how reactive-banana is implemented), there are no callbacks. The clicks and the fake clicks from all three buttons are merged into a single stream, which uses accumE to number the events from 1 to infinity. This numbering must necessarily visit all the events in a specific order; I didn't specify this order, but it's pretty obvious that it's a chronological order. Since the events from the refresh button happen at the same time, there is an ambiguity here. In which order does accumE visit events which occur at the same time?

Reading the documentation for union again, I see that "in case of simultaneous occurrences, the left argument comes first". In clickLabels (defined earlier), I didn't really pay attention to the order in which I passed the buttons, but indeed, I am folding the click streams from top to bottom. If I wanted the fake clicks to occur from bottom to top, I could simply reverse the input list:

clickLabels :: Event t Char
clickLabels = foldr union never (reverse labelledClicks)

commit 5881ae:
Refreshing in the reverse order, because I can!
(one screenshot ago, I could not)

One more event at the beginning

One last detail is that the buttons still start at (0,0,0). Clearly, I need to add an extra refresh event at the beginning. But how? Up to now, every single event has been derived from some external event. I don't even know if events have timestamps. How do I specify "the beginning"?

Besides, the beginning of what? If I construct a stream and I ask reactive-banana to execute it somehow, and then at some later point I ask reactive-banana to execute it again, will this count as a second "beginning"? If so, that would be a bit weird, but the more I think about it, it would be even weirder if it didn't. To remember which streams already had their "beginning" event, the streams would have to keep their state between executions. But maintaining a state implies monadic stream-creation constructs, which is not the interface I see.

Speaking of monads, we still haven't used the Moment monad. With a name like this, maybe there is a monadic command for creating time-related events such as "at the beginning" or "every 5 seconds"? Hoogling for Moment t (Event t ()), I find a function with the very relevant-sounding name now:

now :: AnyMoment f a -> forall t. Moment t (f t a)

What a strange type. Why is the forall on the right? I don't even think it makes a difference, it would have been equivalent to put it on the left or to omit it altogether.

now has no documentation, but Moment does. Kinda. It says that it is "not [a] very interesting" monad. Very well, what about AnyMoment, is that an "interesting" value? Oh, I see now! It's the type of anyMoment which explains everything:

anyMoment :: (forall t. Moment t (f t a)) -> AnyMoment f a

I have already discussed the fact that our code needs to be polymorphic in t. The type AnyMoment simply encodes this requirement, and the strangely-named now converts back from AnyMoment f a to forall t. Moment t (f t a). Hence the strangely-placed forall.

In any case, now is not the event stream I am looking for...

I browse the documentation for reactive-banana, to no avail. I notice a handy shortcut for folding streams, but nothing about creating new non-derived events.

clickLabels :: Event t Char
clickLabels = unions labelledClicks

I guess I will have to roll my own. Remember the list of float events indicating the passage of time? I should be able to make the first such event stand in for the beginning of time.

accumZip :: a -> (a -> a) -> Event t b -> Event t (a, b)
accumZip zero suc = accumE (zero, undefined)
                  . fmap go
  where
    go y (x,_) = (suc x, y)

-- (1, x1), (2, x2), ...
numberEvents :: Event t a -> Event t (Int, a)
numberEvents = accumZip 0 (+1)

firstEvent :: Event t a -> Event t a
firstEvent = fmap snd
           . filterE ((== 1) . fst)
           . numberEvents

beginning :: Event t ()
beginning = voidE (firstEvent floats)

The code for this part ended up being longer than I expected: I originally wanted to use a simple accumulator which would discard everything after the first result, but such an accumulator is not as simple as const Nothing or something like that. It needs to have two distinct non-value states: the initial state, prior to encountering the first value, and the final state, from the second value onward. If we were to go back to the initial state instead of going to a distinct final state, the third value would be treated like the first value instead of being discarded.

Anyway, I now have an event from which to trigger the initial refresh.

addFakeClicks :: Event t () -> Event t ()
addFakeClicks realClicks = unions [beginning, refreshClicks, realClicks]

fakeButtonClicks :: [Event t ()]
fakeButtonClicks = map addFakeClicks buttonClicks

And with this, I am done!

commit 965803:
You know, technically, I could have obtained all of those
screenshots from the dumb version with one counter per button.
But I wouldn't do this to you :)

Conclusion

Okay, so after my first toy FRP app, what is my opinion of FRP? Well, I must say that I was quite impressed by how often iterations worked just fine the first time I ran them. This is, of course, a generic benefit of using Haskell, but I had not yet had this experience while building a GUI application. This made me realize that even though I am using a purely functional programming language, when it comes to GUI applications, I was still using antiquated imperative methods! Where else do I repeat this error?

More formally, I would say that the advantages and disadvantages seem to be the same as those of pure functional programming: without mutable variables, all the data dependencies must be made explicit, sometimes painfully so. For large applications, where the interaction between all the mutable variables and the callbacks can quickly become difficult to visualize, I can see how FRP could be very effective at reducing the complexity.

For my application, I ended up merging all the interesting events into a single stream, applying a master accumulator in order to produce all the output values, and distributing those to the appropriate GUI elements via filters. Of course, my app was a toy; but I wonder how common this architecture is when using FRP? In a larger application, would it be feasible and/or desirable to gather the events from all the widgets in the app, so they can be handled by some pure state machine in the middle; much like IO code is often a thin wrapper around a pure core?

Anyway, I feel like I have only scratched the surface of what FRP has to offer, so please take my thoughts on the subject with a grain of salt.



[1] Last time was surprisingly popular, so I thought I should do more.
[2] The library doesn't compile with ghc 7.8, but that's only because its bounds are too strict. Simply download the source with cabal get gloss-banana, remove the version constraints on base, go back to your project and add your custom version of gloss-banana via cabal sandbox add-source ../path/to/gloss-banana.
[3] I made the change to my local copy of gloss-banana, so you won't see it on the app's github repo.