Friday, August 29, 2014

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.

Monday, August 18, 2014

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.

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 logb xy = logb x + logb 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
    fwdS :: a :-: b -> a :-: b'
    fwdS (MkSub r iso) = MkSub r (liftIso iso)
        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)
        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
    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))

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 blogb 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)
    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.

Saturday, August 16, 2014

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))
            (pure (Circle 10))
            (\_ -> 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))
            (pure (Circle 10))
            (\_ -> 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))
            (pure (Circle 10))
            (\_ -> 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))
    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)
    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
    go t = x' `seq` (xs', accumE x' e')
        (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
    go t = (xs', accumE x' e')
        (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))
            (pure (Circle 10))
            (\_ -> 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))
                (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.
 :: 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 ()

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.
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.