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Examining Hackage: operational

Posted on December 25, 2014
Tags: haskell

In this installment of “jozefg is confused by other people’s code” we turn to operational. This is a package that’s a little less known than I’d like. It provides a monad for transforming an ADT of instructions, a monad that can be used with do notation and separates out interpretation.

Most people familiar with free monads are wondering what the difference is between operational’s approach and using free monads. Going into this, I have no clue. Hopefully this will become clear later on.

Diving Into The Source

Let’s get started shall we

~$ cabal get operational

Happily enough, there’s just one (small) file so we’ll go through that.

To start with Control.Monad.Operational exports

    module Control.Monad.Operational (
        Program, singleton, ProgramView, view,
        ProgramT, ProgramViewT(..), viewT,
        ) where

Like with most “provides a single monad” packages, I’m most interested in how Program works. Looking at this, we see that it’s just a synonym

    type Program instr = ProgramT instr Identity

Just like the mtl, this is defined in terms of a transformer. So what’s this transformer?

    data ProgramT instr m a where
        Lift   :: m a -> ProgramT instr m a
        Bind   :: ProgramT instr m b -> (b -> ProgramT instr m a)
               -> ProgramT instr m a
        Instr  :: instr a -> ProgramT instr m a

So ProgramT is a GADT, this is actually important because Bind has an existential type variable: b. Otherwise this is really just a plain tree, I assume (>>=) = Bind and return = Lift . return in the monad instance for this. And finally we can see that instructions are also explicitly supported with Instr.

We can confirm that the Monad instance is as boring as we’d expect with

    instance Monad m => Monad (ProgramT instr m) where
        return = Lift . return
        (>>=)  = Bind

    instance MonadTrans (ProgramT instr) where
        lift   = Lift

    instance Monad m => Functor (ProgramT instr m) where
        fmap   = liftM

    instance Monad m => Applicative (ProgramT instr m) where
        pure   = return
        (<*>)  = ap

So clearly there’s no interesting computation happening here. Looking at the export list again, we see that there’s a helpful combinator singleton for building up these Program[T]s since they’re kept abstract.

    singleton :: instr a -> ProgramT instr m a
    singleton = Instr

Which once again is very boring.

So this is a lot like free monads it seems since neither one of these actually does much in its monad instance. Indeed the equivalent with free monads would be

    data Free f a = Pure a | Free (f (Free f a))
    instance Functor f => Monad (Free f) where
      return = Pure
      Pure a >>= f = f a
      (Free a) >>= f = Free (fmap (>>= f) a)

    singleton :: Functor f => f a -> Free f a
    singleton = Free . fmap Pure

The obvious differences is that

  1. Free requires a functor while Program doesn’t
  2. Frees monad instance automatically guarantees laws

2 is the bigger one for me. Free has a tighter set of constraints on its f so it can guarantee the monad laws. This is clearly false with Program since return a >>= f introduces an extra Bind instead of just giving f a.

This would explain why ProgramT is kept abstract, it’s hopelessly broken just to expose it in its raw form. Instead what we have to do is somehow partially normalize it before we present it to the user.

Indeed that’s exactly what ProgramViewT is representing. It’s a simpler data type

    data ProgramViewT instr m a where
        Return :: a -> ProgramViewT instr m a
        (:>>=) :: instr b
               -> (b -> ProgramT instr m a)
               -> ProgramViewT instr m a

This apparently “compiles” a Program so that everything is either binding an instruction or a pure value. What’s interesting is that this seems to get rid of all Lift’s as well.

How do we produce one of these? Well that seems to be viewT’s job.

    viewT :: Monad m => ProgramT instr m a -> m (ProgramViewT instr m a)
    viewT (Lift m)                = m >>= return . Return
    viewT ((Lift m)     `Bind` g) = m >>= viewT . g
    viewT ((m `Bind` g) `Bind` h) = viewT (m `Bind` (\x -> g x `Bind` h))
    viewT ((Instr i)    `Bind` g) = return (i :>>= g)
    viewT (Instr i)               = return (i :>>= return)

Note that this function returns an m (ProgramViewT instr m a), not just a plain ProgramViewT. This makes sense because we have to get rid of the lifts. What I think is particularly interesting here is that the 2nd and 3rd cases are just the monad laws!

The second one says binding to a computation is just applying the function to it in the obvious manner. The third re-associates bind in a way guaranteed by the monad laws.

This means that while ProgramT isn’t going to satisfy the monad laws, we can’t tell because all the things said to be equal by the monad laws will compile to the same view. Terribly clever stuff.

The rest of the module is mostly boring stuff like Monad* instances. The last interesting functions is interpretWithMonad

    interpretWithMonad :: forall instr m b.
        Monad m => (forall a. instr a -> m a) -> (Program instr b -> m b)
    interpretWithMonad f = eval . view
        eval :: forall a. ProgramView instr a -> m a
        eval (Return a) = return a
        eval (m :>>= k) = f m >>= interpretWithMonad f . k

This nicely highlights how you’re supposed to write an interpreter for a Program. eval handles the two cases of the view using the mapping to a monad we provided and view handles actually compiling the program into these two cases. All in all, not too shabby.

Surprise, There Were Docs The Whole Time!

Now I assume that most people didn’t actually download the source to operational, but you really should! Inside you’ll find a whole directory, doc. It contains a few markdown files with explanations and references to the appropriate papers as well as a couple examples of actually building things with operational.

Now that you understand how the current implementation works, you should be able to understand most of what is being said there.

Wrap Up

So operational illustrates a neat trick I rather like: using modularity to provide an O(1) implementation of >>= and hide its rule breaking with a view.

This package also drops the positivity requirement that Free implies with its functor constraint. Which I suppose means you could have

    data Foo a where
      Bar :: (a -> ...) -> Bar a

Which is potentially useful.

Last but not least, operational really exemplifies having a decent amount of documentation even though there’s only ~100 lines of code. I think the ratio of documentation : code is something like 3 : 1 which I really appreciate.

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