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Contstuff

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Contents

1 Introduction

The contstuff library implements a number of monad transformers and monads, which make heavy use of continuation passing style (CPS). This makes them both fast and flexible. Please note that this is neither a CPS tutorial nor a monad transformer tutorial. You should understand these concepts, before attempting to use contstuff.

2 Basics

2.1 ContT

The ContT monad transformer is the simplest of all CPS-based monads. It essentially gives you access to the current continuation, which means that it lets you label certain points of execution and reuse these points later in interesting ways. With ContT you get an elegant encoding of computations, which support:

  • abortion (premature termination),
  • resumption (start a computation at a certain spot),
  • branches (aka goto),
  • result accumulation,
  • etc.
All these features are effects of ContT. If you don't use them, then ContT behaves like the identity monad. A computation of type
ContT r m a
is a CPS computation with an intermediate result of type
a
and a final result of type
r
. The
r
type can be polymorphic most of the time. You only need to specify it, if you use some of the CPS effects like
abort
. To run a ContT computation you can use
runContT
or the convenience function
evalContT
: 
runContT  :: (a -> m r) -> ContT r m a -> m r
evalContT :: Applicative m => ContT r m r -> m r
The
runContT
function takes a final continuation transforming the last intermediate result into a final result. The
evalContT
function simply passes
pure
as the final continuation.

2.2 Abortion

Let's have a look at a small example: 

testComp1 :: ContT () IO ()
testComp1 =
  forever $ do
    txt <- io getLine
    case txt of
      "info" -> io $ putStrLn "This is a test computation."
      "quit" -> abort ()
      _      -> return ()
This example demonstrates the most basic feature of ContT. First of all, ContT is a monad transformer, so you can for example lift IO actions to a CPS computation. The
io
function is a handy tool, which corresponds to
liftIO
from other transformer libraries and to
inBase
from monadLib, but is restricted to the IO monad. You can also use the more generic
base
function, which promotes a base monad computation to ContT. Each ContT subcomputation receives a continuation, which is a function, to which the subcomputation is supposed to pass the result. However, the subcomputation may choose not to call the continuation at all, in which case the entire computation finishes with a final result. The
abort
function does that.

2.3 Resumption and branches

You can capture the current continuation using the common
callCC
function. If you just need branches, there are two handy functions for this: 
labelCC :: a -> ContT r m (a, Label (ContT r m) a)
goto    :: Label (ContT r m) a -> a -> ContT r m b

These slightly complicated looking functions are actually very simple to use: 

testComp2 :: ContT r IO ()
testComp2 = do
  (i, again) <- labelCC 0
  io (print i)
  when (i < 10) $ goto again (i+1)
  io (putStrLn $ "Final result: " ++ show i)
The
labelCC
function establishes a label to jump to by capturing its own continuation. It returns both its argument and a label. The
goto
function takes a label and a new argument. The effect is jumping to the corresponding label, but returning the new argument. So when
labelCC
is reached the
i
variable becomes 0. Later
goto
jumps back to the same point, but gives
i
a new value 1, as if
labelCC
were originally called with 1 as the argument.

Labels are first class values in contstuff. This means you can carry them around. They are only limited in that they can't be carried outside of a ContT computation.

2.4 Lifting

As noted earlier there are three lifting functions, which you can use to access monads in lower layers of the transformer stack: 

lift :: (Transformer t, Monad m) => m a -> t m a
base :: (LiftBase m a) => Base m a -> m a
io   :: (Base m a ~ IO a, LiftBase m a) => Base m a -> m a
The
lift
function promotes a computation of the underlying monad. The
base
function promotes a computation of the base monad. It is a generalization of
liftIO
from other monad transformer libraries. Finally there is
io
, which is simply an alias for
base
, but restricted to IO.

2.5 Accumulating results

ContT does not require the underlying functor to be a monad. Whenever the underlying functor is an
Alternative
functor, there is support for accumulating results using the
(<|>)
combinator. In other words, if
m
is an
Alternative
, then
ContT r m
is, too. Here is an example: 
testComp3 :: Num a => ContT r [] (a, a)
testComp3 = do
  x <- pure 10 <|> pure 20
  y <- pure (x+1) <|> pure (x-1)
  return (x, y)
The contstuff library implements a convenience function
listA
, which turns a list into an
Alternative
computation: 
listA :: (Alternative f) => [a] -> f a
Using this you can simplify
testComp3
to: 
testComp3' :: Num a => ContT r [] (a, a)
testComp3' = do
  x <- listA [10, 20]
  y <- listA [x+1, x-1]
  return (x, y)
You can collapse branches using
abort
: 
testComp4 :: Num a => ContT (a, a) [] (a, a)
testComp4 = do
  x <- listA [10, 20]
  when (x == 10) (abort (10, 10))
  y <- listA [x+1, x-1]
  return (x, y)
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