Ticket #1169: Control-Monad-Cont-Class.html

File Control-Monad-Cont-Class.html, 10.0 KB (added by guest, 6 years ago)

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7>Control.Monad.Cont.Class</TITLE
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246
247<SCRIPT SRC="haddock.js" TYPE="text/javascript"
248></SCRIPT
249></HEAD
250><BODY
251><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
252><TR
253><TD CLASS="topbar"
254><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
255><TR
256><TD
257><IMG SRC="haskell_icon.gif" WIDTH="16" HEIGHT="16" ALT=" "
258></TD
259><TD CLASS="title"
260></TD
261><TD CLASS="topbut"
262><A HREF="index.html"
263>Contents</A
264></TD
265><TD CLASS="topbut"
266><A HREF="doc-index.html"
267>Index</A
268></TD
269></TR
270></TABLE
271></TD
272></TR
273><TR
274><TD CLASS="modulebar"
275><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
276><TR
277><TD
278><FONT SIZE="6"
279>Control.Monad.Cont.Class</FONT
280></TD
281><TD ALIGN="right"
282><TABLE CLASS="narrow" CELLSPACING="0" CELLPADDING="0"
283><TR
284><TD CLASS="infohead"
285>Portability</TD
286><TD CLASS="infoval"
287>non-portable (multi-parameter type classes)</TD
288></TR
289><TR
290><TD CLASS="infohead"
291>Stability</TD
292><TD CLASS="infoval"
293>experimental</TD
294></TR
295><TR
296><TD CLASS="infohead"
297>Maintainer</TD
298><TD CLASS="infoval"
299>libraries@haskell.org</TD
300></TR
301></TABLE
302></TD
303></TR
304></TABLE
305></TD
306></TR
307><TR
308><TD CLASS="s15"
309></TD
310></TR
311><TR
312><TD
313><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
314></TABLE
315></TD
316></TR
317><TR
318><TD CLASS="s15"
319></TD
320></TR
321><TR
322><TD CLASS="section1"
323>Description</TD
324></TR
325><TR
326><TD CLASS="doc"
327><DL
328><DT
329>Computation type:</DT
330><DD
331> Computations which can be interrupted and resumed.
332</DD
333><DT
334>Binding strategy:</DT
335><DD
336> Binding a function to a monadic value creates
337a new continuation which uses the function as the continuation of the monadic
338computation.
339</DD
340><DT
341>Useful for:</DT
342><DD
343> Complex control structures, error handling,
344and creating co-routines.
345</DD
346><DT
347>Zero and plus:</DT
348><DD
349> None.
350</DD
351><DT
352>Example type:</DT
353><DD
354> <TT
355>Cont r a</TT
356>
357</DD
358></DL
359><P
360>The Continuation monad represents computations in continuation-passing style
361(CPS).
362In continuation-passing style function result is not returned,
363but instead is passed to another function,
364received as a parameter (continuation).
365Computations are built up from sequences
366of nested continuations, terminated by a final continuation (often <TT
367>id</TT
368>)
369which produces the final result.
370Since continuations are functions which represent the future of a computation,
371manipulation of the continuation functions can achieve complex manipulations
372of the future of the computation,
373such as interrupting a computation in the middle, aborting a portion
374of a computation, restarting a computation, and interleaving execution of
375computations.
376The Continuation monad adapts CPS to the structure of a monad.
377</P
378><P
379>Before using the Continuation monad, be sure that you have
380a firm understanding of continuation-passing style
381and that continuations represent the best solution to your particular
382design problem.
383Many algorithms which require continuations in other languages do not require
384them in Haskell, due to Haskell's lazy semantics.
385Abuse of the Continuation monad can produce code that is impossible
386to understand and maintain.
387</P
388></TD
389></TR
390><TR
391><TD CLASS="s15"
392></TD
393></TR
394><TR
395><TD CLASS="s15"
396></TD
397></TR
398><TR
399><TD CLASS="section1"
400>Documentation</TD
401></TR
402><TR
403><TD CLASS="s15"
404></TD
405></TR
406><TR
407><TD CLASS="decl"
408><SPAN CLASS="keyword"
409>class</SPAN
410> Monad m =&gt; <A NAME="t%3AMonadCont"
411></A
412><B
413>MonadCont</B
414> m  <SPAN CLASS="keyword"
415>where</SPAN
416></TD
417></TR
418><TR
419><TD CLASS="body"
420><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
421><TR
422><TD CLASS="s8"
423></TD
424></TR
425><TR
426><TD CLASS="section4"
427>Methods</TD
428></TR
429><TR
430><TD CLASS="body"
431><TABLE CLASS="vanilla" CELLSPACING="0" CELLPADDING="0"
432><TR
433><TD CLASS="decl"
434><A NAME="v%3AcallCC"
435></A
436><B
437>callCC</B
438> :: ((a -&gt; m b) -&gt; m a) -&gt; m a</TD
439></TR
440><TR
441><TD CLASS="doc"
442><P
443><TT
444>callCC</TT
445> (call-with-current-continuation)
446    calls a function with the current continuation as its argument.
447    Provides an escape continuation mechanism for use with Continuation monads.
448    Escape continuations allow to abort the current computation and return
449    a value immediately.
450    They achieve a similar effect to <TT
451><A HREF="Control-Monad-Error.html#v%3AthrowError"
452>throwError</A
453></TT
454>
455    and <TT
456><A HREF="Control-Monad-Error.html#v%3AcatchError"
457>catchError</A
458></TT
459>
460    within an <TT
461><A HREF="Control-Monad-Error.html#t%3AError"
462>Error</A
463></TT
464> monad.
465    Advantage of this function over calling <TT
466>return</TT
467> is that it makes
468    the continuation explicit,
469    allowing more flexibility and better control
470    (see examples in <A HREF="Control-Monad-Cont.html"
471>Control.Monad.Cont</A
472>).
473</P
474><P
475>The standard idiom used with <TT
476>callCC</TT
477> is to provide a lambda-expression
478    to name the continuation. Then calling the named continuation anywhere
479    within its scope will escape from the computation,
480    even if it is many layers deep within nested computations.
481</P
482></TD
483></TR
484></TABLE
485></TD
486></TR
487><TR
488><TD CLASS="s8"
489></TD
490></TR
491><TR
492><TD CLASS="section4"
493><IMG SRC="minus.gif" CLASS="coll" ONCLICK="toggle(this,'i:MonadCont')" ALT="show/hide"
494> Instances</TD
495></TR
496><TR
497><TD CLASS="body"
498><DIV ID="i:MonadCont" STYLE="display:block;"
499><TABLE CLASS="vanilla" CELLSPACING="1" CELLPADDING="0"
500><TR
501><TD CLASS="decl"
502><A HREF="Control-Monad-Cont-Class.html#t%3AMonadCont"
503>MonadCont</A
504> (<A HREF="Control-Monad-Cont.html#t%3ACont"
505>Cont</A
506> r)</TD
507></TR
508><TR
509><TD CLASS="decl"
510>Monad m =&gt; <A HREF="Control-Monad-Cont-Class.html#t%3AMonadCont"
511>MonadCont</A
512> (<A HREF="Control-Monad-Cont.html#t%3AContT"
513>ContT</A
514> r m)</TD
515></TR
516><TR
517><TD CLASS="decl"
518>(<A HREF="Control-Monad-Error-Class.html#t%3AError"
519>Error</A
520> e, <A HREF="Control-Monad-Cont-Class.html#t%3AMonadCont"
521>MonadCont</A
522> m) =&gt; <A HREF="Control-Monad-Cont-Class.html#t%3AMonadCont"
523>MonadCont</A
524> (<A HREF="Control-Monad-Error.html#t%3AErrorT"
525>ErrorT</A
526> e m)</TD
527></TR
528><TR
529><TD CLASS="decl"
530><A HREF="Control-Monad-Cont-Class.html#t%3AMonadCont"
531>MonadCont</A
532> m =&gt; <A HREF="Control-Monad-Cont-Class.html#t%3AMonadCont"
533>MonadCont</A
534> (<A HREF="Control-Monad-Reader.html#t%3AReaderT"
535>ReaderT</A
536> r m)</TD
537></TR
538></TABLE
539></DIV
540></TD
541></TR
542></TABLE
543></TD
544></TR
545><TR
546><TD CLASS="s15"
547></TD
548></TR
549><TR
550><TD CLASS="botbar"
551>Produced by <A HREF="http://www.haskell.org/haddock/"
552>Haddock</A
553> version 0.7</TD
554></TR
555></TABLE
556></BODY
557></HTML
558>