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Contstuff

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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 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
. 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. 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.