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<pre><a name="line-1"></a><span class='hs-comment'>{- |
<a name="line-2"></a>Module : Control.Monad.Cont.Class
<a name="line-3"></a>Copyright : (c) The University of Glasgow 2001,
<a name="line-4"></a> (c) Jeff Newbern 2003-2007,
<a name="line-5"></a> (c) Andriy Palamarchuk 2007
<a name="line-6"></a>License : BSD-style (see the file LICENSE)
<a name="line-7"></a>
<a name="line-8"></a>Maintainer : libraries@haskell.org
<a name="line-9"></a>Stability : experimental
<a name="line-10"></a>Portability : portable
<a name="line-11"></a>
<a name="line-12"></a>[Computation type:] Computations which can be interrupted and resumed.
<a name="line-13"></a>
<a name="line-14"></a>[Binding strategy:] Binding a function to a monadic value creates
<a name="line-15"></a>a new continuation which uses the function as the continuation of the monadic
<a name="line-16"></a>computation.
<a name="line-17"></a>
<a name="line-18"></a>[Useful for:] Complex control structures, error handling,
<a name="line-19"></a>and creating co-routines.
<a name="line-20"></a>
<a name="line-21"></a>[Zero and plus:] None.
<a name="line-22"></a>
<a name="line-23"></a>[Example type:] @'Cont' r a@
<a name="line-24"></a>
<a name="line-25"></a>The Continuation monad represents computations in continuation-passing style
<a name="line-26"></a>(CPS).
<a name="line-27"></a>In continuation-passing style function result is not returned,
<a name="line-28"></a>but instead is passed to another function,
<a name="line-29"></a>received as a parameter (continuation).
<a name="line-30"></a>Computations are built up from sequences
<a name="line-31"></a>of nested continuations, terminated by a final continuation (often @id@)
<a name="line-32"></a>which produces the final result.
<a name="line-33"></a>Since continuations are functions which represent the future of a computation,
<a name="line-34"></a>manipulation of the continuation functions can achieve complex manipulations
<a name="line-35"></a>of the future of the computation,
<a name="line-36"></a>such as interrupting a computation in the middle, aborting a portion
<a name="line-37"></a>of a computation, restarting a computation, and interleaving execution of
<a name="line-38"></a>computations.
<a name="line-39"></a>The Continuation monad adapts CPS to the structure of a monad.
<a name="line-40"></a>
<a name="line-41"></a>Before using the Continuation monad, be sure that you have
<a name="line-42"></a>a firm understanding of continuation-passing style
<a name="line-43"></a>and that continuations represent the best solution to your particular
<a name="line-44"></a>design problem.
<a name="line-45"></a>Many algorithms which require continuations in other languages do not require
<a name="line-46"></a>them in Haskell, due to Haskell's lazy semantics.
<a name="line-47"></a>Abuse of the Continuation monad can produce code that is impossible
<a name="line-48"></a>to understand and maintain.
<a name="line-49"></a>-}</span>
<a name="line-50"></a>
<a name="line-51"></a><span class='hs-keyword'>module</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span><span class='hs-varop'>.</span><span class='hs-conid'>Cont</span><span class='hs-varop'>.</span><span class='hs-conid'>Class</span> <span class='hs-layout'>(</span>
<a name="line-52"></a> <span class='hs-conid'>MonadCont</span><span class='hs-layout'>(</span><span class='hs-keyglyph'>..</span><span class='hs-layout'>)</span><span class='hs-layout'>,</span>
<a name="line-53"></a> <span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-54"></a>
<a name="line-55"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span><span class='hs-varop'>.</span><span class='hs-conid'>Trans</span><span class='hs-varop'>.</span><span class='hs-conid'>Cont</span> <span class='hs-layout'>(</span><span class='hs-conid'>ContT</span><span class='hs-layout'>)</span>
<a name="line-56"></a><span class='hs-keyword'>import</span> <span class='hs-keyword'>qualified</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span><span class='hs-varop'>.</span><span class='hs-conid'>Trans</span><span class='hs-varop'>.</span><span class='hs-conid'>Cont</span> <span class='hs-keyword'>as</span> <span class='hs-conid'>ContT</span>
<a name="line-57"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span><span class='hs-varop'>.</span><span class='hs-conid'>Trans</span><span class='hs-varop'>.</span><span class='hs-conid'>Error</span> <span class='hs-keyword'>as</span> <span class='hs-conid'>Error</span>
<a name="line-58"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span><span class='hs-varop'>.</span><span class='hs-conid'>Trans</span><span class='hs-varop'>.</span><span class='hs-conid'>Identity</span> <span class='hs-keyword'>as</span> <span class='hs-conid'>Identity</span>
<a name="line-59"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span><span class='hs-varop'>.</span><span class='hs-conid'>Trans</span><span class='hs-varop'>.</span><span class='hs-conid'>List</span> <span class='hs-keyword'>as</span> <span class='hs-conid'>List</span>
<a name="line-60"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span><span class='hs-varop'>.</span><span class='hs-conid'>Trans</span><span class='hs-varop'>.</span><span class='hs-conid'>Maybe</span> <span class='hs-keyword'>as</span> <span class='hs-conid'>Maybe</span>
<a name="line-61"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span><span class='hs-varop'>.</span><span class='hs-conid'>Trans</span><span class='hs-varop'>.</span><span class='hs-conid'>Reader</span> <span class='hs-keyword'>as</span> <span class='hs-conid'>Reader</span>
<a name="line-62"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span><span class='hs-varop'>.</span><span class='hs-conid'>Trans</span><span class='hs-varop'>.</span><span class='hs-conid'>RWS</span><span class='hs-varop'>.</span><span class='hs-conid'>Lazy</span> <span class='hs-keyword'>as</span> <span class='hs-conid'>LazyRWS</span>
<a name="line-63"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span><span class='hs-varop'>.</span><span class='hs-conid'>Trans</span><span class='hs-varop'>.</span><span class='hs-conid'>RWS</span><span class='hs-varop'>.</span><span class='hs-conid'>Strict</span> <span class='hs-keyword'>as</span> <span class='hs-conid'>StrictRWS</span>
<a name="line-64"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span><span class='hs-varop'>.</span><span class='hs-conid'>Trans</span><span class='hs-varop'>.</span><span class='hs-conid'>State</span><span class='hs-varop'>.</span><span class='hs-conid'>Lazy</span> <span class='hs-keyword'>as</span> <span class='hs-conid'>LazyState</span>
<a name="line-65"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span><span class='hs-varop'>.</span><span class='hs-conid'>Trans</span><span class='hs-varop'>.</span><span class='hs-conid'>State</span><span class='hs-varop'>.</span><span class='hs-conid'>Strict</span> <span class='hs-keyword'>as</span> <span class='hs-conid'>StrictState</span>
<a name="line-66"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span><span class='hs-varop'>.</span><span class='hs-conid'>Trans</span><span class='hs-varop'>.</span><span class='hs-conid'>Writer</span><span class='hs-varop'>.</span><span class='hs-conid'>Lazy</span> <span class='hs-keyword'>as</span> <span class='hs-conid'>LazyWriter</span>
<a name="line-67"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span><span class='hs-varop'>.</span><span class='hs-conid'>Trans</span><span class='hs-varop'>.</span><span class='hs-conid'>Writer</span><span class='hs-varop'>.</span><span class='hs-conid'>Strict</span> <span class='hs-keyword'>as</span> <span class='hs-conid'>StrictWriter</span>
<a name="line-68"></a>
<a name="line-69"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Control</span><span class='hs-varop'>.</span><span class='hs-conid'>Monad</span>
<a name="line-70"></a><span class='hs-keyword'>import</span> <span class='hs-conid'>Data</span><span class='hs-varop'>.</span><span class='hs-conid'>Monoid</span>
<a name="line-71"></a>
<a name="line-72"></a><a name="MonadCont"></a><span class='hs-keyword'>class</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monad</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>MonadCont</span> <span class='hs-varid'>m</span> <span class='hs-keyword'>where</span>
<a name="line-73"></a> <span class='hs-comment'>{- | @callCC@ (call-with-current-continuation)
<a name="line-74"></a> calls a function with the current continuation as its argument.
<a name="line-75"></a> Provides an escape continuation mechanism for use with Continuation monads.
<a name="line-76"></a> Escape continuations allow to abort the current computation and return
<a name="line-77"></a> a value immediately.
<a name="line-78"></a> They achieve a similar effect to 'Control.Monad.Error.throwError'
<a name="line-79"></a> and 'Control.Monad.Error.catchError'
<a name="line-80"></a> within an 'Control.Monad.Error.Error' monad.
<a name="line-81"></a> Advantage of this function over calling @return@ is that it makes
<a name="line-82"></a> the continuation explicit,
<a name="line-83"></a> allowing more flexibility and better control
<a name="line-84"></a> (see examples in "Control.Monad.Cont").
<a name="line-85"></a>
<a name="line-86"></a> The standard idiom used with @callCC@ is to provide a lambda-expression
<a name="line-87"></a> to name the continuation. Then calling the named continuation anywhere
<a name="line-88"></a> within its scope will escape from the computation,
<a name="line-89"></a> even if it is many layers deep within nested computations.
<a name="line-90"></a> -}</span>
<a name="line-91"></a> <span class='hs-varid'>callCC</span> <span class='hs-keyglyph'>::</span> <span class='hs-layout'>(</span><span class='hs-layout'>(</span><span class='hs-varid'>a</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>b</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>-></span> <span class='hs-varid'>m</span> <span class='hs-varid'>a</span>
<a name="line-92"></a>
<a name="line-93"></a><span class='hs-keyword'>instance</span> <span class='hs-conid'>MonadCont</span> <span class='hs-layout'>(</span><span class='hs-conid'>ContT</span> <span class='hs-varid'>r</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-94"></a> <span class='hs-varid'>callCC</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>ContT</span><span class='hs-varop'>.</span><span class='hs-varid'>callCC</span>
<a name="line-95"></a>
<a name="line-96"></a><span class='hs-comment'>-- ---------------------------------------------------------------------------</span>
<a name="line-97"></a><span class='hs-comment'>-- Instances for other mtl transformers</span>
<a name="line-98"></a>
<a name="line-99"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>Error</span> <span class='hs-varid'>e</span><span class='hs-layout'>,</span> <span class='hs-conid'>MonadCont</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>MonadCont</span> <span class='hs-layout'>(</span><span class='hs-conid'>ErrorT</span> <span class='hs-varid'>e</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-100"></a> <span class='hs-varid'>callCC</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>Error</span><span class='hs-varop'>.</span><span class='hs-varid'>liftCallCC</span> <span class='hs-varid'>callCC</span>
<a name="line-101"></a>
<a name="line-102"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>MonadCont</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>MonadCont</span> <span class='hs-layout'>(</span><span class='hs-conid'>IdentityT</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-103"></a> <span class='hs-varid'>callCC</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>Identity</span><span class='hs-varop'>.</span><span class='hs-varid'>liftCallCC</span> <span class='hs-varid'>callCC</span>
<a name="line-104"></a>
<a name="line-105"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>MonadCont</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>MonadCont</span> <span class='hs-layout'>(</span><span class='hs-conid'>ListT</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-106"></a> <span class='hs-varid'>callCC</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>List</span><span class='hs-varop'>.</span><span class='hs-varid'>liftCallCC</span> <span class='hs-varid'>callCC</span>
<a name="line-107"></a>
<a name="line-108"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>MonadCont</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>MonadCont</span> <span class='hs-layout'>(</span><span class='hs-conid'>MaybeT</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-109"></a> <span class='hs-varid'>callCC</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>Maybe</span><span class='hs-varop'>.</span><span class='hs-varid'>liftCallCC</span> <span class='hs-varid'>callCC</span>
<a name="line-110"></a>
<a name="line-111"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>MonadCont</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>MonadCont</span> <span class='hs-layout'>(</span><span class='hs-conid'>ReaderT</span> <span class='hs-varid'>r</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-112"></a> <span class='hs-varid'>callCC</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>Reader</span><span class='hs-varop'>.</span><span class='hs-varid'>liftCallCC</span> <span class='hs-varid'>callCC</span>
<a name="line-113"></a>
<a name="line-114"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monoid</span> <span class='hs-varid'>w</span><span class='hs-layout'>,</span> <span class='hs-conid'>MonadCont</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>MonadCont</span> <span class='hs-layout'>(</span><span class='hs-conid'>LazyRWS</span><span class='hs-varop'>.</span><span class='hs-conid'>RWST</span> <span class='hs-varid'>r</span> <span class='hs-varid'>w</span> <span class='hs-varid'>s</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-115"></a> <span class='hs-varid'>callCC</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>LazyRWS</span><span class='hs-varop'>.</span><span class='hs-varid'>liftCallCC'</span> <span class='hs-varid'>callCC</span>
<a name="line-116"></a>
<a name="line-117"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monoid</span> <span class='hs-varid'>w</span><span class='hs-layout'>,</span> <span class='hs-conid'>MonadCont</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>MonadCont</span> <span class='hs-layout'>(</span><span class='hs-conid'>StrictRWS</span><span class='hs-varop'>.</span><span class='hs-conid'>RWST</span> <span class='hs-varid'>r</span> <span class='hs-varid'>w</span> <span class='hs-varid'>s</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-118"></a> <span class='hs-varid'>callCC</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>StrictRWS</span><span class='hs-varop'>.</span><span class='hs-varid'>liftCallCC'</span> <span class='hs-varid'>callCC</span>
<a name="line-119"></a>
<a name="line-120"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>MonadCont</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>MonadCont</span> <span class='hs-layout'>(</span><span class='hs-conid'>LazyState</span><span class='hs-varop'>.</span><span class='hs-conid'>StateT</span> <span class='hs-varid'>s</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-121"></a> <span class='hs-varid'>callCC</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>LazyState</span><span class='hs-varop'>.</span><span class='hs-varid'>liftCallCC'</span> <span class='hs-varid'>callCC</span>
<a name="line-122"></a>
<a name="line-123"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>MonadCont</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>MonadCont</span> <span class='hs-layout'>(</span><span class='hs-conid'>StrictState</span><span class='hs-varop'>.</span><span class='hs-conid'>StateT</span> <span class='hs-varid'>s</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-124"></a> <span class='hs-varid'>callCC</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>StrictState</span><span class='hs-varop'>.</span><span class='hs-varid'>liftCallCC'</span> <span class='hs-varid'>callCC</span>
<a name="line-125"></a>
<a name="line-126"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monoid</span> <span class='hs-varid'>w</span><span class='hs-layout'>,</span> <span class='hs-conid'>MonadCont</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>MonadCont</span> <span class='hs-layout'>(</span><span class='hs-conid'>LazyWriter</span><span class='hs-varop'>.</span><span class='hs-conid'>WriterT</span> <span class='hs-varid'>w</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-127"></a> <span class='hs-varid'>callCC</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>LazyWriter</span><span class='hs-varop'>.</span><span class='hs-varid'>liftCallCC</span> <span class='hs-varid'>callCC</span>
<a name="line-128"></a>
<a name="line-129"></a><span class='hs-keyword'>instance</span> <span class='hs-layout'>(</span><span class='hs-conid'>Monoid</span> <span class='hs-varid'>w</span><span class='hs-layout'>,</span> <span class='hs-conid'>MonadCont</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyglyph'>=></span> <span class='hs-conid'>MonadCont</span> <span class='hs-layout'>(</span><span class='hs-conid'>StrictWriter</span><span class='hs-varop'>.</span><span class='hs-conid'>WriterT</span> <span class='hs-varid'>w</span> <span class='hs-varid'>m</span><span class='hs-layout'>)</span> <span class='hs-keyword'>where</span>
<a name="line-130"></a> <span class='hs-varid'>callCC</span> <span class='hs-keyglyph'>=</span> <span class='hs-conid'>StrictWriter</span><span class='hs-varop'>.</span><span class='hs-varid'>liftCallCC</span> <span class='hs-varid'>callCC</span>
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