Difference between revisions of "Contstuff"
m (→Introduction: Fixed wiki markup typo.) 
(Added a lot of stuff.) 

Line 3:  Line 3:  
The [http://hackage.haskell.org/package/contstuff 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''. 
The [http://hackage.haskell.org/package/contstuff 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''. 

−  == 
+  == Basics == 
+  === ContT === 

The <hask>ContT</hask> monad transformer is the simplest of all CPSbased 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: 
The <hask>ContT</hask> monad transformer is the simplest of all CPSbased 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: 

Line 13:  Line 13:  
* etc. 
* etc. 

−  All these features are effects of <hask>ContT</hask>. If you don't use them, then <hask>ContT</hask> behaves like the identity monad. A computation of type <hask>ContT r m a</hask> is a CPS computation with an intermediate result of type <hask>a</hask> and a final result of type <hask>r</hask>. The <hask>r</hask> type can be polymorphic most of the time. You only need to specify it, if you use some of the CPS effects like <hask>abort</hask>. 
+  All these features are effects of <hask>ContT</hask>. If you don't use them, then <hask>ContT</hask> behaves like the identity monad. A computation of type <hask>ContT r m a</hask> is a CPS computation with an intermediate result of type <hask>a</hask> and a final result of type <hask>r</hask>. The <hask>r</hask> type can be polymorphic most of the time. You only need to specify it, if you use some of the CPS effects like <hask>abort</hask>. 
+  
+  To run a <hask>ContT</hask> computation you can use <hask>runContT</hask> or the convenience function <hask>evalContT</hask>: 

+  
+  <haskell> 

+  runContT :: (a > m r) > ContT r m a > m r 

+  evalContT :: Applicative m => ContT r m r > m r 

+  </haskell> 

+  
+  The <hask>runContT</hask> function takes a final continuation transforming the last intermediate result into a final result. The <hask>evalContT</hask> function simply passes <hask>pure</hask> as the final continuation. 

+  
+  === Abortion === 

+  
+  Let's have a look at a small example: 

<haskell> 
<haskell> 

Line 30:  Line 30:  
Each <hask>ContT</hask> 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 <hask>abort</hask> function does that. 
Each <hask>ContT</hask> 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 <hask>abort</hask> function does that. 

−  To run a <hask>ContT</hask> computation you can use <hask>runContT</hask> or the convenience function <hask>evalContT</hask>: 

+  === Resumption and branches === 

+  
+  You can capture the current continuation using the common <hask>callCC</hask> function. If you just need branches, there are two handy functions for this: 

<haskell> 
<haskell> 

−  +  labelCC :: a > ContT r m (a, Label (ContT r m) a) 

−  +  goto :: Label (ContT r m) a > a > ContT r m b 

</haskell> 
</haskell> 

−  The <hask>runContT</hask> function takes a final continuation transforming the last intermediate result into a final result. The <hask>evalContT</hask> function simply passes <hask>pure</hask> as the final continuation. 

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

+  
+  <haskell> 

+  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) 

+  </haskell> 

+  
+  The <hask>labelCC</hask> function establishes a label to jump to by capturing its own continuation. It returns both its argument and a label. The <hask>goto</hask> function takes a label and a new argument. The effect is jumping to the corresponding label, but returning the new argument. So when <hask>labelCC</hask> is reached the <hask>i</hask> variable becomes 0. Later <hask>goto</hask> jumps back to the same point, but gives <hask>i</hask> a new value 1, as if <hask>labelCC</hask> 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 <hask>ContT</hask> computation. 

+  
+  === Lifting === 

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

+  
+  <haskell> 

+  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 

+  </haskell> 

+  
+  The <hask>lift</hask> function promotes a computation of the underlying monad. The <hask>base</hask> function promotes a computation of the base monad. It is a generalization of <hask>liftIO</hask> from other monad transformer libraries. Finally there is <hask>io</hask>, which is simply an alias for <hask>base</hask>, but restricted to <hask>IO</hask>. 

+  
+  === Accumulating results === 

+  
+  <hask>ContT</hask> does not require the underlying functor to be a monad. Whenever the underlying functor is an <hask>Alternative</hask> functor, there is support for accumulating results using the <hask>(<>)</hask> combinator. In other words, if <hask>m</hask> is an <hask>Alternative</hask>, then <hask>ContT r m</hask> is, too. Here is an example: 

+  
+  <haskell> 

+  testComp3 :: Num a => ContT r [] (a, a) 

+  testComp3 = do 

+  x < pure 10 <> pure 20 

+  y < pure (x+1) <> pure (x1) 

+  return (x, y) 

+  </haskell> 

+  
+  The ''contstuff'' library implements a convenience function <hask>listA</hask>, which turns a list into an <hask>Alternative</hask> computation: 

+  
+  <haskell> 

+  listA :: (Alternative f) => [a] > f a 

+  </haskell> 

+  
+  Using this you can simplify <hask>testComp3</hask> to: 

+  
+  <haskell> 

+  testComp3' :: Num a => ContT r [] (a, a) 

+  testComp3' = do 

+  x < listA [10, 20] 

+  y < listA [x+1, x1] 

+  return (x, y) 

+  </haskell> 

+  
+  You can collapse branches using <hask>abort</hask>: 

+  
+  <haskell> 

+  testComp4 :: Num a => ContT (a, a) [] (a, a) 

+  testComp4 = do 

+  x < listA [10, 20] 

+  when (x == 10) (abort (10, 10)) 

+  y < listA [x+1, x1] 

+  return (x, y) 

+  </haskell> 
Revision as of 23:49, 20 September 2010
Contents
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.
Basics
ContT
The ContT
monad transformer is the simplest of all CPSbased 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.
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.
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.
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
.
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 (x1)
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, x1]
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, x1]
return (x, y)