Archive for the ‘programming’ Category

Monoidal instances for pipes

February 4, 2012

In this post, I’m going to introduce a new class of combinators for pipes, with an interesting categorical interpretation. I will be using the pipe implementation of my previous post.

> {-# LANGUAGE MultiParamTypeClasses #-}
> {-# LANGUAGE FlexibleInstances #-}
> {-# LANGUAGE TypeFamilies #-}
> {-# LANGUAGE GeneralizedNewtypeDeriving #-}
> module Blog.Pipes.MonoidalInstances where
> 
> import Blog.Pipes.Guarded hiding (groupBy)
> import qualified Control.Arrow as A
> import Control.Category
> import Control.Categorical.Bifunctor
> import Control.Category.Associative
> import Control.Category.Braided
> import Control.Category.Monoidal
> import Control.Monad (forever)
> import Control.Monad.Free
> import Data.Maybe
> import Data.Void
> import Prelude hiding ((.), id, filter, until)

When pipes were first released, some people noticed the lack of an Arrow instance. In fact, it is not hard to show that, even identifying pipes modulo some sort of observational equality, there is no Arrow instance that satisfies the arrow laws.

The problem, of course, is with first, because we already have a simple implementation of arr. If we try to implement first we immediately discover that there's a problem with the Yield case:

first (Yield x c) = yield (x, ???) >> first c

Since ??? can be of any type, the only possible value is bottom, which of course we don't want to introduce. Alternative definitions of first that alter the structure of a yielding pipe are not possible if we want to satisfy the law:

first p >+> pipe fst == pipe fst >+> p

Concretely, the problem is that the cartesian product in the type of first forces a sort of "synchronization point" that doesn't necessarily exist. This is better understood if we look at the type of (***), of which first can be thought of as a special case:

(***) :: Arrow k => k a b -> k a' b' -> k (a, a') (b, b')

first = (*** id)

If the two input pipes yield at different times, there is no way to faithfully match their yielded values into a pair. There are hacks around that, but they don't behave well compositionally, and exhibit either arbitrarily large space leaks or data loss.

This has been addressed before: stream processors, like those of the Fudgets library, being very similar to Pipes, have the same problem, and some resolutions have been proposed, although not entirely satisfactory.

Arrows as monoidal categories

It is well known within the Haskell community that Arrows correspond to so called Freyd categories, i.e. premonoidal categories with some extra structures.

Using the Monoidal class by Edward Kmett (now in the categories package on Hackage), we can try to make this idea precise.

Unfortunately, we have to use a newtype to avoid overlapping instances in the case of the Hask category:

> newtype ACat a b c = ACat { unACat :: a b c }
>   deriving (Category, A.Arrow)

First, cartesian products are a bifunctor in the category determined by an Arrow.

> instance A.Arrow a => PFunctor (,) (ACat a) (ACat a) where
>   first = ACat . A.first . unACat
> instance A.Arrow a => QFunctor (,) (ACat a) (ACat a) where
>   second = ACat . A.second . unACat
> instance A.Arrow a
>       => Bifunctor (,) (ACat a) (ACat a) (ACat a) where
>   bimap (ACat f) (ACat g) = ACat $ f A.*** g

Now we can say that products are associative, using the associativity of products in Hask:

> instance A.Arrow a => Associative (ACat a) (,) where
>   associate = ACat $ A.arr associate
> instance A.Arrow a => Disassociative (ACat a) (,) where
>   disassociate = ACat $ A.arr disassociate

Where we use the Disassociative instance to express the inverse of the associator. And finally, the Monoidal instance:

> type instance Id (ACat a) (,) = ()
> instance A.Arrow a => Monoidal (ACat a) (,) where
>   idl = ACat $ A.arr idl
>   idr = ACat $ A.arr idr
> instance A.Arrow a => Comonoidal (ACat a) (,) where
>   coidl = ACat $ A.arr coidl
>   coidr = ACat $ A.arr coidr

Where, again, the duals are actually inverses. Also, products are symmetric:

> instance A.Arrow a => Braided (ACat a) (,) where
>   braid = ACat $ A.arr braid
> instance A.Arrow a => Symmetric (ACat a) (,)

As you see, everything is trivially induced by the cartesian structure on Hask, since A.arr gives us an identity-on-objects functor. Note, however, that the Bifunctor instance is legitimate only if we assume a strong commutativity law for arrows:

first f >>> second g == second g >>> first f

which we will, for the sake of simplicity.

Replacing products with arbitrary monoidal structures

Once we express the Arrow concept in terms of monoidal categories, it is easy to generalize it to arbitrary monoidal structures on Hask.

In particular, coproducts work particularly well in the category of pipes:

> instance Monad m
>       => PFunctor Either (PipeC m r) (PipeC m r) where
>   first = PipeC . firstP . unPipeC
> 
> firstP :: Monad m => Pipe a b m r
>        -> Pipe (Either a c) (Either b c) m r
> firstP (Pure r) = return r
> firstP (Free (M m)) = lift m >>= firstP

Yielding a sum is now easy: just yield on the left component.

> firstP (Free (Yield x c)) = yield (Left x) >> firstP c

Awaiting is a little bit more involved, but still easy enough: receive left and null values normally, and act like an identity on the right.

> firstP (Free (Await k)) = go
>         where
>           go = tryAwait
>            >>= maybe (firstP $ k Nothing)
>                      (either (firstP . k . Just)
>                              (\x -> yield (Right x) >> go))

And of course we have an analogous instance on the right:

> instance Monad m
>       => QFunctor Either (PipeC m r) (PipeC m r) where
>   second = PipeC . secondP . unPipeC
> 
> secondP :: Monad m => Pipe a b m r
>         -> Pipe (Either c a) (Either c b) m r
> secondP (Pure r) = return r
> secondP (Free (M m)) = lift m >>= secondP
> secondP (Free (Yield x c)) = yield (Right x) >> secondP c
> secondP (Free (Await k)) = go
>         where
>           go = tryAwait
>            >>= maybe (secondP $ k Nothing)
>                      (either (\x -> yield (Left x) >> go)
>                              (secondP . k . Just))

And a bifunctor instance obtained by composing first and second in arbitrary order:

> instance Monad m
>       => Bifunctor Either (PipeC m r)
>                    (PipeC m r) (PipeC m r) where
>   bimap f g = first f >>> second g

At this point we can go ahead and define the remaining instances in terms of the identity-on-objects functor given by pipe:

> instance Monad m => Associative (PipeC m r) Either where
>   associate = PipeC $ pipe associate
> instance Monad m => Disassociative (PipeC m r) Either where
>   disassociate = PipeC $ pipe disassociate
> 
> type instance Id (PipeC m r) Either = Void
> instance Monad m => Monoidal (PipeC m r) Either where
>   idl = PipeC $ pipe idl
>   idr = PipeC $ pipe idr
> instance Monad m => Comonoidal (PipeC m r) Either where
>   coidl = PipeC $ pipe coidl
>   coidr = PipeC $ pipe coidr
> 
> instance Monad m => Braided (PipeC m r) Either where
>   braid = PipeC $ pipe braid
> instance Monad m => Symmetric (PipeC m r) Either

Multiplicative structures

There is still a little bit of extra structure that we might want to exploit. Since PipeC m r is a monoidal category, it induces a (pointwise) monoidal structure on its endofunctor category, so we can speak of monoid objects there. In particular, if the identity functor is a monoid, it means that we can define a "uniform" monoid structure for all the objects of our category, given in terms of natural transformations (i.e. polymorphic functions).

We can represent this specialized monoid structure with a type class (using kind polymorphism and appropriately generalized category-related type classes, it should be possible to unify this class with Monoid and even Monad, similarly to how it's done here):

> class Monoidal k p => Multiplicative k p where
>   unit :: k (Id k p) a
>   mult :: k (p a a) a

Dually, we can have a sort of uniform coalgebra:

> class Comonoidal k p => Comultiplicative k p where
>   counit :: k a (Id k p)
>   comult :: k a (p a a)

The laws for those type classes are just the usual laws for a monoid in a (not necessarily strict) monoidal category:

first unit . mult == idl
second unit . mult == idr
mult . first mult == mult . second mult . associate

first counit . comult == coidl
second counit . comult == coidr
first diag . diag == disassociate . second diag . diag

Now, products have a comultiplicative structure on Hask (as in every category with finite products), given by the terminal object and diagonal natural transformation:

> instance Comultiplicative (->) (,) where
>   counit = const ()
>   comult x = (x, x)

while coproducts have a multiplicative structure:

> instance Multiplicative (->) Either where
>   unit = absurd
>   mult = either id id

that we can readily transport to PipeC m r using pipe:

> instance Monad m => Multiplicative (PipeC m r) Either where
>   unit = PipeC $ pipe absurd
>   mult = PipeC $ pipe mult

Somewhat surprisingly, pipes also have a comultiplicative structure of their own:

> instance Monad m => Comultiplicative (PipeC m r) Either where
>   counit = PipeC discard
>   comult = PipeC . forever $ do
>     x <- await
>     yield (Left x)
>     yield (Right x)

Heterogeneous metaprogramming

All the combinators we defined can actually be used in practice, and the division in type classes certainly sheds some light on their structure and properties, but there's actually something deeper going on here.

The fact that the standard Arrow class uses (,) as monoidal structure is not coincidental: Hask is a cartesian closed category, so to embed Haskell's simply typed λ-calculus into some other category structure, we need at the very least a way to transport cartesian products, i.e. a premonoidal functor [1].

However, as long as our monoidal structure is comultiplicative and symmetric, we can always recover a first-order fragment of λ-calculus inside the "guest" category, and we don't even need an identity-on-objects functor [2].

The idea is that we can use the monoidal structure of the guest category to represent contexts, where weakening is given by counit, contraction by comult, and exchange by swap.

There is an experimental GHC branch with a preprocessor which is able to translate expressions written in an arbitrary guest language into Haskell, given instances of appropriate type classes , which correspond exactly to the ones we have defined above.

Examples

This exposition was pretty abstract, so we end with some examples.

We first need to define a few wrappers for our monoidal combinators, so we don't have to deal with the PipeC newtype:

> split :: Monad m => Pipe a (Either a a) m r
> split = unPipeC comult
> 
> join :: Monad m => Pipe (Either a a) a m r
> join = unPipeC mult
> 
> (*+*) :: Monad m => Pipe a b m r -> Pipe a' b' m r
>       -> Pipe (Either a a') (Either b b') m r
> f *+* g = unPipeC $ bimap (PipeC f) (PipeC g)
> 
> discardL :: Monad m => Pipe (Either Void a) a m r
> discardL = unPipeC idl
> 
> discardR :: Monad m => Pipe (Either a Void) a m r
> discardR = unPipeC idr

Now let's write a tee combinator, similar to the tee command for shell pipes:

> tee :: Monad m => Pipe a Void m r -> Pipe a a m r
> tee p = split >+> firstP p >+> discardL
> 
> printer :: Show a => Pipe a Void IO r
> printer = forever $ await >>= lift . print
> 
> ex6 :: IO ()
> ex6 = do
>   (sourceList [1..5] >+>
>     tee printer >+>
>     (fold (+) 0 >>= yield) $$
>     printer)
>   return ()
> {- ex6 == mapM_ print [1,2,3,4,5,15] -}

Another interesting exercise is reimplementing the groupBy combinator of the previous post:

> groupBy :: Monad m => (a -> a -> Bool) -> Pipe a [a] m r
> groupBy p =
>    -- split the stream in two
>    split >+>
> 
>    -- yield Nothing whenever (not (p x y))
>    -- for consecutive x y
>   ((consec >+>
>     filter (not . uncurry p) >+>
>     pipe (const Nothing)) *+*
>   
>   -- at the same time, let everything pass through
>   pipe Just) >+>
> 
>   -- now rejoin the two streams
>   join >+>
>   
>   -- then accumulate results until a Nothing is hit
>   forever (until isNothing >+>
>            pipe fromJust >+>
>            (consume >>= yield))
> 
> -- yield consecutive pairs of values
> consec :: Monad m => Pipe a (a, a) m r
> consec = await >>= go
>   where
>     go x = await >>= \y -> yield (x, y) >> go y
> 
> ex7 :: IO ()
> ex7 = do (sourceList [1,1,2,2,2,3,4,4]
>           >+> groupBy (==)
>           >+> pipe head
>            $$ printer)
>          return ()
> {- ex7 == mapM_ print [1,2,3,4] -}

References

[1] J. Power and E. Robinson, “Premonoidal categories and notions of computation,” Mathematical. Structures in Comp. Sci., vols. 7, pp. 453-468, 1997.

[2] A. Megacz, “Multi-Level Languages are Generalized Arrows,” arXiv:1007.2885, 2010.

An introduction to guarded pipes

February 2, 2012

Pipes are a very simple but powerful abstraction which can be used to implement stream-based IO, in a very similar fashion to iteratees and friends, or conduits. In this post, I introduce guarded pipes: a slight generalization of pipes which makes it possible to implement a larger class of combinators.

> {-# LANGUAGE NoMonomorphismRestriction #-}
> module Blog.Pipes.Guarded where
> 
> import Control.Category
> import Control.Monad.Free
> import Control.Monad.Identity
> import Data.Maybe
> import Data.Void
> import Prelude hiding (id, (.), until, filter)

The idea behind pipes is straightfoward: fix a base monad m, then construct the free monad over a specific PipeF functor:

> data PipeF a b m x = M (m x)
>                    | Yield b x
>                    | Await (Maybe a -> x)
> 
> instance Monad m => Functor (PipeF a b m) where
>   fmap f (M m) = M $ liftM f m
>   fmap f (Yield x c) = Yield x (f c)
>   fmap f (Await k) = Await (f . k)
> 
> type Pipe a b m r = Free (PipeF a b m) r

Generally speaking, a free monad can be thought of as an embedded language in CPS style: every summand of the base functor (PipeF in this case), is a primitive operation, while the x parameter represents the continuation at each step.

In the case of pipes, M corresponds to an effect in the base monad, Yield produces an output value, and Await blocks until it receives an input value, then passes it to its continuation. You can see that the Await continuation takes a Maybe a type: this is the only thing that distinguishes guarded pipes from regular pipes (as implemented in the pipes package on Hackage). The idea is that Await will receive Nothing whenever the pipe runs out of input values. That will give it a chance to do some cleanup or yield extra outputs. Any additional Await after that point will terminate the pipe immediately.

We can write a simplistic list-based (strict) interpreter formalizing the semantics I just described:

> evalPipe :: Monad m => Pipe a b m r -> [a] -> m [b]
> evalPipe p xs = go False xs [] p

The boolean parameter is going to be set to True as soon as we execute an Await with an empty input list.

A Pure value means that the pipe has terminated spontaneously, so we return the accumulated output list:

>   where
>     go _ _ ys (Pure r) = return (reverse ys)

Execute inner monadic effects:

>     go t xs ys (Free (M m)) = m >>= go t xs ys

Save yielded values into the accumulator:

>     go t xs ys (Free (Yield y c)) = go t xs (y : ys) c

If we still have values in the input list, feed one to the continuation of an Await statement.

>     go t (x:xs) ys (Free (Await k)) = go t xs ys $ k (Just x)

If we run out of inputs, pass Nothing to the Await continuation…

>     go False [] ys (Free (Await k)) = go True [] ys (k Nothing)

… but only the first time. If the pipe awaits again, terminate it.

>     go True [] ys (Free (Await _)) = return (reverse ys)

To simplify the implementation of actual pipes, we define the following basic combinators:

> tryAwait :: Monad m => Pipe a b m (Maybe a)
> tryAwait = wrap $ Await return
> 
> yield :: Monad m => b -> Pipe a b m ()
> yield x = wrap $ Yield x (return ())
> 
> lift :: Monad m => m r -> Pipe a b m r
> lift = wrap . M . liftM return

and a couple of secondary combinators, very useful in practice. First, a pipe that consumes all input and never produces output:

> discard :: Monad m => Pipe a b m r
> discard = forever tryAwait

then a simplified await primitive, that dies as soon as we stop feeding values to it.

> await :: Monad m => Pipe a b m a
> await = tryAwait >>= maybe discard return

now we can write a very simple pipe that sums consecutive pairs of numbers:

> sumPairs :: (Monad m, Num a) => Pipe a a m ()
> sumPairs = forever $ do
>   x <- await
>   y <- await
>   yield $ x + y

we get:

> ex1 :: [Int]
> ex1 = runIdentity $ evalPipe sumPairs [1,2,3,4]
> {- ex1 == [3, 7] -}

Composing pipes

The usefulness of pipes, however, is not limited to being able to express list transformations as monadic computations using the await and yield primitives. In fact, it turns out that two pipes can be composed sequentially to create a new pipe.

> infixl 9 >+>
> (>+>) :: Monad m => Pipe a b m r -> Pipe b c m r -> Pipe a c m r
> (>+>) = go False False
>   where

When implementing evalPipe, we needed a boolean parameter to signal upstream input exhaustion. This time, we need two boolean parameters, one for the input of the upstream pipe, and one for its output, i.e. the input of the downstream pipe. First, if downstream does anything other than waiting, we just let the composite pipe execute the same action:

>     go _ _ p1 (Pure r) = return r
>     go t1 t2 p1 (Free (Yield x c)) = yield x >> go t1 t2 p1 c
>     go t1 t2 p1 (Free (M m)) = lift m >>= \p2 -> go t1 t2 p1 p2

then, if upstream is yielding and downstream is waiting, we can feed the yielded value to the downstream pipe and continue from there:

>     go t1 t2 (Free (Yield x c)) (Free (Await k)) =
>       go t1 t2 c $ k (Just x)

if downstream is waiting and upstream is running a monadic computation, just let upstream run and keep downstream waiting:

>     go t1 t2 (Free (M m)) p2@(Free (Await _)) =
>       lift m >>= \p1 -> go t1 t2 p1 p2

if upstream terminates while downstream is waiting, finalize downstream:

>     go t1 False p1@(Pure _) (Free (Await k)) =
>       go t1 True p1 (k Nothing)

but if downstream awaits again, terminate the whole composite pipe:

>     go _ True (Pure r) (Free (Await _)) = return r

now, if both pipes are waiting, we keep the second pipe waiting and we feed whatever input we get to the first pipe. If the input is Nothing, we set the first boolean flag, so that next time the first pipe awaits, we can finalize the downstream pipe.

>     go False t2 (Free (Await k)) p2@(Free (Await _)) =
>       tryAwait >>= \x -> go (isNothing x) t2 (k x) p2
>     go True False p1@(Free (Await _)) (Free (Await k)) =
>       go True True p1 (k Nothing)
>     go True True p1@(Free (Await _)) p2@(Free (Await _)) =
>       tryAwait >>= \_ -> {- unreachable -} go True True p1 p2

This composition can be shown to be associative (in a rather strong sense), with identity given by:

> idP :: Monad m => Pipe a a m r
> idP = forever $ await >>= yield

So we can define a Category instance:

> newtype PipeC m r a b = PipeC { unPipeC :: Pipe a b m r }
> 
> instance Monad m => Category (PipeC m r) where
>   id = PipeC idP
>   (PipeC p2) . (PipeC p1) = PipeC $ p1 >+> p2

Running pipes

A runnable pipe, also called Pipeline, is a pipe that doesn’t yield any value and doesn’t wait for any input. We can formalize this in the types as follows:

> type Pipeline m r = Pipe () Void m r

Disregarding bottom, calling await on such a pipe does not return any useful value, and yielding is impossible. Another way to think of Pipeline is as an arrow (in PipeC) from the terminal object to the initial object of Hask1.

Running a pipeline is straightforward:

> runPipe :: Monad m => Pipeline m r -> m r
> runPipe (Pure r) = return r
> runPipe (Free (M m)) = m >>= runPipe
> runPipe (Free (Await k)) = runPipe $ k (Just ())
> runPipe (Free (Yield x c)) = absurd x

where the impossibility of the last case is guaranteed by the types, unless of course the pipe introduced a bottom value at some point.

The three primitive operations tryAwait, yield and lift, together with pipe composition and the runPipe function above, are basically all we need to define most pipes and pipe combinators. For example, the simple pipe interpreter evalPipe can be easily rewritten in terms of these primitives:

> evalPipe' :: Monad m => Pipe a b m r -> [a] -> m [b]
> evalPipe' p xs = runPipe $
>   (mapM_ yield xs >> return []) >+>
>   (p >> discard) >+>
>   collect id
>   where
>     collect xs =
>       tryAwait >>= maybe (return $ xs [])
>                          (\x -> collect (xs . (x:)))

Note that we use the discard pipe to turn the original pipe into an infinite one, so that the final return value will be taken from the final pipe.

Extra combinators

The rich structure on pipes (category and monad) makes it really easy to define new higher-level combinators. For example, here are implementations of some of the combinators in Data.Conduit.List, translated to pipes:

> sourceList = mapM_ yield
> sourceNull = return ()
> fold f z = go z
>   where
>     go x = tryAwait >>= maybe (return x) (go . f x)
> consume = fold (\xs x -> xs . (x:)) id >>= \xs -> return (xs [])
> sinkNull = discard
> take n = (isolate n >> return []) >+> consume
> drop n = replicateM n await >> idP
> pipe f = forever $ await >>= yield . f -- called map in conduit
> concatMap f = forever $ await >>= mapM_ yield . f
> until p = go
>   where
>     go = await >>= \x -> if p x then return () else yield x >> go
> groupBy (~=) = p >+>
>   forever (until isNothing >+>
>            pipe fromJust >+>
>            (consume >>= yield))
>   where 
>     -- the pipe p yields Nothing whenever the current item y
>     -- and the previous one x do not satisfy x ~= y, and behaves
>     -- like idP otherwise
>     p = await >>= \x -> yield (Just x) >> go x
>     go x = do
>       y <- await
>       unless (x ~= y) $ yield Nothing
>       yield $ Just y
>       go y
> isolate n = replicateM_ n $ await >>= yield
> filter p = forever $ until (not . p)

To work with the equivalent of sinks, it is useful to define a source to sink composition operator:

> infixr 2 $$
> ($$) :: Monad m => Pipe () a m r' -> Pipe a Void m r -> m (Maybe r)
> p1 $$ p2 = runPipe $ (p1 >> return Nothing) >+> liftM Just p2

which ignores the source return type, and just returns the sink return value, or Nothing if the source happens to terminate first. So we have, for example:

> ex2 :: Maybe [Int]
> ex2 = runIdentity $ sourceList [1..10] >+> isolate 4 $$ consume
> {- ex2 == Just [1,2,3,4] -}
> 
> ex3 :: Maybe [Int]
> ex3 = runIdentity $ sourceList [1..10] $$ discard
> {- ex3 == Nothing -}
> 
> ex4 :: Maybe Int
> ex4 = runIdentity $ sourceList [1,1,2,2,2,3,4,4]
>                 >+> groupBy (==)
>                 >+> pipe head
>                  $$ fold (+) 0
> {- ex4 == Just 10 -}
> 
> ex5 :: Maybe [Int]
> ex5 = runIdentity $ sourceList [1..10]
>                 >+> filter (\x -> x `mod` 3 == 0)
>                  $$ consume
> {- ex5 == Just [3, 6, 9] -}

Pipes in practice

You can find an implementation of guarded pipes in my fork of pipes. There is also a pipes-extra repository where you can find some pipes to deal with chunked ByteStreams and utilities to convert conduits to pipes.

I hope to be able to merge this into the original pipes package once the guarded pipe concept has proven its worth. Without the tryAwait primitive, combinators like fold and consume cannot be implemented, nor even a simple stateful pipe like one to split a chunked input into lines. So I think there are enough benefits to justify a little extra complexity in the definition of composition.


  1. In reality, Hask doesn’t have an initial object, and the terminal object is actually Void, because of non-strict semantics.

Reinversion of control with continuations

January 18, 2012

In my last post I mentioned how it is possible to achieve a form of "reinversion of control" by using (green) threads. Some commenters noted how this is effectively a solved problem, as demonstrated for example by Erlang, as well as the numerous variations on CSP currently gaining a lot of popularity.

I don’t disagree with that, but it’s just not the point of this series of posts. This is about understanding the computational structure of event-driven code, and see how it’s possible to transform it into a less awkward form without introducing concurrency (or at least not in the traditional sense of the term).

Using threads to solve what is essentially a control flow problem is cheating. And you pay in terms of increased complexity, and code which is harder to reason about, since you introduced a whole lot of interleaving opportunities and possible race conditions. Using a non-preemptive concurrency abstraction with manual yield directives (like my Python gist does) will solve that, but then you’d have to think of how to schedule your coroutines, so that is also not a complete solution.

Programmable semicolons

To find an alternative to the multitask-based approach, let’s focus on two particular lines of the last example:

    reply = start_request();
    get_data(reply)

where I added an explicit semicolon at the end of the first line. A semicolon is an important component of an imperative program, even though, syntactically, it is often omitted in languages like Python. It corresponds to the sequencing operator: execute the instruction on the left side, then pass the result to the right side and execute that.

If the instruction on the left side corresponds to an asynchronous operation, we want to alter the meaning of sequencing. Given a sequence of statements of the form

    x = A(); B(x)

we want to interpret that as: call A, then return control back to the main loop; when A is finished, bind its result to x, then run B.

So what we want is to be able to override the sequencing operator: we want programmable semicolons.

The continuation monad

Since it is often really useful to look at the types of functions to understand how exactly they fit together, we'll leave Python and start focusing on Haskell for our running example.

We can make a very important observation immediately by looking at the type of the callback registration function that our framework offers, and try to interpret it in the context of controlled side effects (i.e. the IO monad). For Qt, it could look something like:

1
    connect :: Object -> String -> (a -> IO ()) -> IO ()

to be used, for example, like this:

1
2
    connect httpReply "finished()" $ \_ -> do
        putStrLn "request finished"

so the first argument is the object, the second is the C++ signature of the signal, and the third is a callback that will be invoked by the framework whenever the specified signal is emitted. Now, we can get rid of all the noise of actually connecting to a signal, and define a type representing just the act of registering a callback.

1
    newtype Event a = Event { on :: (a -> IO ()) -> IO () }

Doesn't that look familiar? It is exactly the continuation monad transformer applied to the IO monad! The usual monad instance for ContT perfectly captures the semantics we are looking for:

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    instance Monad Event where
      return x = Event $ \k -> k x
      e >>= f = Event $ \k ->
        on e $ \x ->
          on (f x) k

The return function simply calls the callback immediately with the provided value, no actual connection is performed. The bind operator represents our custom semicolon: we connect to the first event, and when that fires, we take the value it yielded, apply it to f, and connect to the resulting event.

Now we can actually translate the Python code of the previous example to Haskell:

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    ex :: Event ()
    ex = forever $ do
      result <- untilRight . replicate 2 $ do
        reply <- startRequest
        either (return . Left) (liftM Right . getData) reply
      either handleError displayData result

    untilRight :: Monad m => [m (Either a b)] -> m (Either a b)
    untilRight [m] = m
    untilRight (m : ms) = m >>= either (const (untilRight ms)) (return . Right)

Again, this could be cleaned up by adding some error reporting functionality into the monad stack.

Implementing the missing functions in terms of connect is straightforward. For example, startRequest will look something like this:

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    startRequest :: Event (Either String Reply)
    startRequest = Event $ \k -> do
      reply <- AccessManager.get "http://example.net"
      connect reply "finished()" $ \_ -> k (Right reply)
      connect reply "error(QString)" $ \e -> k (Left e)

where I took the liberty of glossing over some irrelevant API details.

How do we run such a monad? Well, the standard runContT does the job:

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    runEvent :: Event () -> IO ()
    runEvent e = on $ \k -> return ()

so

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    runEvent ex

will run until the first connection, return control to the main loop, resume when an event occurs, and so on.

Conclusion

I love the simplicity and elegance of this approach, but unfortunately, it is far from a complete solution. So far we have only dealt with "one-shot" events, but what happens when an event fires multiple times? Also, as this is still very imperative in nature, can we do better? Is it possible to employ a more functional style, with emphasis on composability?

I'll leave the (necessarily partial) answers to those questions for a future post.

From event-driven programming to FRP

January 10, 2012

The problem

Most of modern programming is based on events. Event-driven frameworks are the proven and true abstraction to express any kind of asynchronous and interactive behavior, like in GUIs or client-server architectures.

The core idea is inversion of control: the main loop is run by the framework, users only have to register some form of “callbacks”, and the framework will take care of calling them at the appropriate times.

This solves many issues that a straightforward imperative/procedural approach would present, eliminates the need for any kind of polling, and creates all sorts of opportunities for general-purpose optimizations inside the framework, with no impact on the complexity of user code. All of this without introducing any concurrency.

There are drawbacks, however. Event-driven code is hideous to write in most languages, especially those lacking support for first class closures. More importantly, event-driven code is extremely hard to reason about. The very nature of this callback-based approach makes it impossible to use a functional style, and even the simplest of interactions requires some form of mutable state which has to be maintained across callback calls.

As a very simple example, suppose we want to perform a GET request and retrieve some data, handling any HTTP error that might occur. In a generic event-driven frameworkm, we would need to implement a simple state machine whose graph will look somewhat like this:

A simple state machine

Each state (except the initial one) corresponds to a callback. The transitions are determined by the framework. To avoid starting more than one request at a time, we will need to explicitly keep track of the current state.

Now let’s try to make a simple change to our program: suppose we want to retry requests when they fail, but not more than once. Now the state machine becomes more complicated, since we need to add extra nodes for the non-fatal error condition.

A slightly more complicated state machine

In our hypotetical event-driven code, we need to keep track of whether we already encountered an error, check this flag at each callback to perform the right action, and update it appropriately. Moreover, this time the code isn’t even shaped exactly like the state machine, because we reuse the same callback for multiple nodes. To test our code exhaustively, we need to trace every possible path through the graph and reproduce it.

Now assume we want to allow simultaneous requests… you get the idea. The code gets unwieldy pretty fast. Small changes in requirements have devastating consequences in terms of the state graph. In practice, what happens most of the times is that the state graph is kept implicit, which makes the code impossible to test reliably, and consequently impossible to modify.

Towards a solution

A very simple but effective solution can be found by observing that state graphs like those of the previous examples have a very clear operational interpretation in the context of the equivalent synchronous code.

A single forward transition from A to B can be simply modelled as the sequence A;B, i.e. execute A, then execute B. Extra outward transitions from a single node can be mapped to exceptions, while backward arrows can be thought of as looping constructs.

Our second state machine can then be translated to the following pseudopython:

while True:
    for i in xrange(2):
        error = None
        try:
            reply = start_request()
            data = get_data(reply)
            break
        except Exception as e:
            error = get_error(e)
    if error:
        handle_error(error)
    else:
        display_data(data)

This code is straightforward. It could be made cleaner by splitting it up in a couple of extra functions and removing local state, but that's beside the point. Note how easy it is now to generalize to an arbitrary number of retries.

So the key observation is that we can transform asynchronous code into synchronous-looking code, provided that we attach the correct semantics to sequencing of operations, exceptions and loops.

Now the question becomes: is it possible to do so?

We could turn functions like start_request and get_data into blocking operations that can throw. This will work locally, but it will break asynchronicity, so it's not an option.

One way to salvage this transformation is to run the code in its own thread. Asynchronous operations will block, but won't hang the main loop, and the rest of the program will continue execution.

However, we need to be careful with the kind of threads that we use. Since we don't need (and don't want!) to run multiple threads simultaneously, but we need to spawn a thread for each asynchronous operation, we have to make sure that the overhead is minimal, context switching is fast, and we're not paying the cost of scheduling and synchronization.

Here you can find a sketched solution along these lines that I wrote in python. It's based on the greenlet library, which provides cooperative multithreading.

In the next post I will talk about alternative solutions, as well as how to extend the idea further, and make event-driven code more declarative and less procedural.

Effective Qt in ruby (part 2)

September 24, 2010

In the first part of this series, I listed some of the reasons why you should consider writing your Qt/KDE applications in ruby. This post details some of the technical differences between writing Qt code in C++ and in ruby.

One of the first problems that pop up when starting a new Qt/KDE project in ruby is how to use it in such a way that your code doesn’t end up being completely unidiomatic. This can happen very easily if one tries to stick to the usual conventions that apply when writing Qt code in C++.

If you take any piece of C++ code using Qt, you can very trivially translate it into ruby. That works, and sometimes is useful, but writing code in this way completely misses the point of using a dynamic language. You might as well write directly in C++, and enjoy the improved performance.

So I believe it’s important to identify the baggage that Qt brings from its C++ roots, and eliminate it when using it from ruby. Here are some ideas to achieve that.

Use the ruby convention for method names

A minor point, but important for code readability.

Qt uses camel case for method names, while ruby methods are conventionally written with underscores. Mixing the two styles inevitably results in an unreadable mess, so the ruby convention should be used at all times.

Fortunately, QtRuby allows you to call C++ methods by spelling their name with underscores, so it’s quite easy to achieve a satisfactory level of consistency with minimum effort.

Never declare signals

The signal/slot mechanism is a very important Qt feature, because it allows to work around the static nature of C++ by allowing dynamic calls to methods. You won’t need that in ruby. For instance, you can use the standard observer library to fire events and set callbacks. It’s completely dynamic and there’s no need to define your signals beforehand.

Never use slots

Slots are useless in ruby. QtRuby allows you to attach a block to a connect call, and that is what you should always be using. Never use the SLOT function with a C++ signature.

Avoid C++ signatures altogether

This seems impossible. It might be easy to use symbols (without using the SIGNAL "macro") to specify signals with no arguments, like

button.on(:clicked) { puts "hello world" }

but if a signal has arguments, and possibly overloads, specifying only its name doesn't seem to be enough to determine which particular overload we are interested in.

Indeed, it's not possible in general, but you can disambiguate using the block arity for most overloaded signals, and add type annotations in those rare cases where the arity is not enough.

Here is my on method, which accomplishes this:

def on(sig, types = nil, &blk)
  sig = Signal.create(sig, types)
  candidates = if is_a? Qt::Object
    signal_map[sig.symbol]
  end
  if candidates
    if types
      # find candidate with the correct argument types
      candidates = candidates.find_all{|s| s[1] == types }
    end
    if candidates.size > 1
      # find candidate with the correct arity
      arity = blk.arity
      if blk.arity == -1
        # take first
        candidates = [candidates.first]
      else
        candidates = candidates.find_all{|s| s[1].size == arity }
      end
    end
    if candidates.size > 1
      raise "Ambiguous overload for #{sig} with arity #{arity}"
    elsif candidates.empty?
      msg = if types
        "with types #{types.join(' ')}"
      else
        "with arity #{blk.arity}"
      end
      raise "No overload for #{sig} #{msg}"
    end
    sign = SIGNAL(candidates.first[0])
    connect(sign, &blk)
    SignalDisconnecter.new(self, sign)
  else
    observer = observe(sig.symbol, &blk)
    ObserverDisconnecter.new(self, observer)
  end
end

The Signal class maintains the signal name and (optional) specified types. The method lazily creates a signal map for each class, which maps symbols to C++ signatures, and proceeds to disambiguate among all the possibilities by using types, or just the block arity, when no explicit types are provided. If no signal is found, or if the ambiguity could not be resolved, an exception is thrown.

For example, the following line:

combobox.on(:current_index_changed, ["int"]) {|i| self.index = i }

is referring to currentIndexChanged(int) and not to the other possible signal currentIndexChanged(QString), because of the explicit type annotation.

The advantage of this trick is that I can write, for example:

model.on(:rows_about_to_be_inserted) do |p, i, j|
  # ...
end

without specifying any C++ signature, which in this case would be quite hefty:

rowsAboutToBeInserted(const QModelIndex& parent, int start, int end)

Conclusion

QtRuby is an exceptional library, but to use it effectively you need to let go of some of the established practices of Qt programming in C++, and embrace the greater dynamicity of ruby.

In the next article I'll show you how I tried to push this idea to the extreme with AutoGUI, a declarative GUI DSL built on top of QtRuby.

Effective Qt in ruby (part 1)

June 25, 2010

As some of you might know, I’m working on a generic board game platform called Kaya. Kaya is a Qt/KDE-based application to play chess, shogi and variants thereof, and easily extensible to all sorts of board games (Go is in the works, for example).

Kaya is written in ruby, and I have learned quite a few things about writing non-trivial GUI applications in ruby while working on it, so I decided to share some of my experience and code in the hope that it might be useful to others, and possibly inspire other Qt/KDE developers to try out ruby for their next project.

Advantages

Here is a list of what I think are the most important points that make programming GUIs in ruby so much more productive than in C++. I’ll leave out subjective arguments like “it’s more fun” or “it has a nicer syntax” because I think that the actual facts are more than compelling already.

Fast Prototyping

This is not specific to GUI programming. It is greatly aknowledged that ruby is orders of magnitude more convenient than C++ for throwing quick scripts together and in general for trying new ideas out. This turns out to be very important in a GUI context as well.

For example, it is very easy to write setup code for a single component of your application so that you can run it standalone and test it more efficiently. I have found myself resorting to this kind of trick very often, and it sometimes makes debugging a lot less painful. In C++, it would be unreasonable to write a new application skeleton (and alter your build scripts) just to test a new dialog you are developing.

Another example is “fancy logging“. If something breaks in the middle of complicated or highly interactive code, sometimes printing to the console or setting breakpoints doesn’t quite cut it. In these cases, it is a very good idea to hack together a simple but powerful visualization tool to show the internal state of the application in real time, and that is usually very helpful to identify the issue. Again, since this is basically throwaway code, ease of prototyping is very important.

Declarative GUI Definition

Ruby has very powerful metaprogramming facilities that make creating an embedded DSL extremely straightforward. I use a simple mechanism in Kaya that allows me to write things like:

    @gui = KDE::autogui(:engine_prefs,
                        :caption => KDE.i18n("Configure Engines")) do |g|
      g.layout(:type => :horizontal) do |l|
        l.list(:list)
        l.layout(:type => :vertical) do |buttons|
          buttons.button(:add_engine,
                         :text => KDE.i18nc("engine", "&New..."),
                         :icon => 'list-add')
          buttons.button(:edit_engine,
                         :text => KDE.i18nc("engine", "&Edit..."),
                         :icon => 'configure')
          buttons.button(:delete_engine,
                         :text => KDE.i18nc("engine", "&Delete"),
                         :icon => 'list-remove')
          buttons.stretch
        end
      end
    end
    setGUI(@gui)

I find it very convenient to define GUIs at this slightly higher level of abstraction, plus you have none of the boilerplate code typical of GUI construction in C++.

Defining GUIs in this way is so quick (and it's easy to run them to immediately see the results), that it makes tools like Qt Designer completely redundant, in my opinion, except possibly when GUI design and coding are done by different people.

Using Closures for Slots

Take a look at this simple example, and try to imagine how many lines of code would be needed to implement it in C++:

win.display.text = "0"
inc = 1
Qt::Timer.every(1000) do
  win.display.text = (win.display.text.to_i + inc).to_s
end
win.button.on(:clicked) { inc = -inc }

Here win.display is a QTextEdit and win.button is a QPushButton. What the example does is count up seconds in the QTextEdit, and toggle between that and counting down when the button is pressed.

Pretty trivial, granted, but in C++ you would need to define two slots to do that, plus keep track of the direction in which you are counting by using a member variable. In ruby you can just use local variables without littering the parent scope.

Toolkit Independence

If you write your application in C++, once you've decided that you are going to use KDElibs (or Qt), that decision is pretty much set in stone. You can't even switch from KDE to pure Qt very easily, and it's so hard to support both environments in a single code base, to make it almost completely not worth the effort.

Of course, this might not seem that big of an issue, given that KDE is now a lot more portable, but not many people are running KDE SC on Windows or Mac at the moment, and if you want to reach as many users as possible, having a Qt-only version of your application is going to help a lot.

So Kaya can currently run on Qt-only as well as KDE. When in KDE mode, all the usual KDE goodies are running under the hood: KPushButtons, KDialog, K-everything, and of course KXMLGUI and all the good stuff that comes with it, but it can switch to plain Qt classes and a simplistic replacement for the XML GUI if you don't have KDElibs installed.

Easier Automatic Testing

Dynamicity, mock objects, plus the whole culture of test-driven development that characterizes the ruby ecosystem make it a lot easier to devise automated tests for your application. Effective GUI testing is still basically an open problem in software engineering, so don't expect it to be a piece of cake, but in my experience, you can definitely reach a considerably higher test coverage with ruby than with C++.

Trivial Extensibility Through Plugins

KDE really shines when it comes to making applications easily extensible via scripts or plugins, but it can't compare with ruby. The distinction between plugins and user scripts is now nonexistent, and loading a plugin is just a matter of calling the ruby load function.

Creating a sensibly extensible application still requires careful planning, of course, but it's a lot easier when you can directly expose application functionality to plugins, without creating tons and tons of interfaces for even the most trivial uses.

In Kaya, the use of a dynamically typed language turned out to be key: the flexibility required to make its plugins easy enough to write without knowing much about the application internals is pretty much impossible to achieve in a statically typed context.

Disadvantages

Of course, like everything in software engineering, choosing ruby over C++ for GUI development involves a number of tradeoffs.

Performance

Everyone knows that ruby is slow. Painfully slow sometimes. That seems to be improving with 1.9, but ruby isn't going to get faster than C anytime soon.

Now, the good thing is that its poor performance is almost always completely irrelevant. A typical GUI application is nowhere near being CPU bound, and the few computationally intensive parts are usually in the GUI library anyway. That said, if you have performance critical sections in your application, you are better off writing them in C/C++ and accessing them from your ruby code using the ruby extension API.

For example, Kaya includes a small C++ extension to supplement the missing blur functionality in Qt < 4.6. Writing it and integrating it was trivial.

Testing is Essential

Automated testing is very important for software written in any language, but for dynamic languages you simply can't do without it. Untested code will inevitably contain silly typos and type errors which will cause it to crash in your user's faces, or in the best case will make your testing sessions painful and slow.

So you don't have any choice but to add lots and lots of unit tests, integration tests and the like. Units tests for pure GUI code are not really effective, useful or easy to write, but you can settle on smoke tests that will cover the most common mistakes and be pretty confident that no silly errors will pop up that a compiler would catch.

Conclusion

I hope this gives a nice overview of the benefits you can get by switching from C++ to a dynamic language for your KDE (or general GUI) development. I used ruby as the main example, because that's what I have experience with, but most of the points probably apply to dynamic languages in general (python, perl, clojure, etc).

In the following posts, I will dig a little bit more into Kaya's code to show the kinds of tricks that I employed to better exploit the advantages that I discussed, and offer even more ways to get the most out of ruby for GUI programming. Stay tuned!

Rewriting from scratch

April 27, 2008

I have recently come across this article (and those linked by it), while I was considering a rewrite for my own pet project.

While I tend to think that rewriting from scratch is usually a bad idea, if we are talking about open source projects, there’s another element you should take into account: how pleasurable it is to work on it. If you end up with a big and hairy code base you cannot touch without suddenly feeling all sad and pessimistic about the fate of the world, you should realize that something has gone deeply wrong: you are supposed to work on your project for fun (yeah, to scratch an itch, maybe, too…), and when the fun stops, most of the reasons to continue disappear. You have no (paying) customers, you have no deadlines, you will be able (or unable, depending on what you do in the rest of your time) to pay your bills anyway.

At this point, either you throw everything into the garbage bin – which may be sad, disappointing and frustrating, both for you and your users – or you take whatever lessons you may have learned in the process, and use your extra wisdom to start from scratch a hopefully better and more mature design and implementation that won’t have those shortcomings that brought you to this miserable situation.

It will probably have other defects and issues, though, more subtle and harder to spot in an upfront design. You have to accept it. And you have to accept the fact that you may need to start from scratch once again in the future.

Here is a simple form you can use to announce your decision to start from scratch. Just copy-paste it into your blog, filling in the required fields. It may save you precious time that you will be able to spend on your n-th rewrite :)

I'll rewrite my pet project from scratch because:

[ ] It's not modular enough
[ ] The original design didn't take __________________ into consideration
[ ] It's too slow
[ ] There is too much legacy code which I can't seem to get rid of
[ ] I want to write it in ____________________

----

[ ] And this time I'll use TDD/BDD

[ ] And this time it will be done right!

Signed (the developer)

_____________________________

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