1 %************************************************************************
3 \section[glasgow-exts]{Glasgow extensions to Haskell}
4 \index{Haskell, Glasgow extensions}
5 \index{extensions, Glasgow Haskell}
7 %************************************************************************
9 As with all known Haskell systems, GHC implements some extensions to
11 To use them, you'll need to give
12 a \tr{-fglasgow-exts}%
13 \index{-fglasgow-exts option} option.
15 Virtually all of the Glasgow extensions serve to give you access to the
16 underlying facilities with which we implement Haskell. Thus, you can
17 get at the Raw Iron, if you are willing to write some non-standard
18 code at a more primitive level. You need not be ``stuck'' on
19 performance because of the implementation costs of Haskell's
20 ``high-level'' features---you can always code ``under'' them. In an
21 extreme case, you can write all your time-critical code in C, and then
22 just glue it together with Haskell!
24 Executive summary of our extensions:
26 \item[Unboxed types and primitive operations:] You can get right down
27 to the raw machine types and operations; included in this are
28 ``primitive arrays'' (direct access to Big Wads of Bytes).
29 Please see \Sectionref{glasgow-unboxed} and following.
31 \item[Calling out to C:] Just what it sounds like. We provide {\em
32 lots} of rope that you can dangle around your neck.
33 Please see \Sectionref{glasgow-ccalls}.
35 \item[Low-level monadic I/O:] Monadic I/O is now standard with Haskell~1.3;
36 you can still get access to the system at a lower level (the ``PrimIO'' level).
38 \item[``HBC-ish'' extensions:] Extensions implemented because people said,
39 ``HBC does Y. Could you teach GHC to do the same?'' Please see
40 \Sectionref{glasgow-hbc-exts} for a quick list.
43 Before you get too carried away working at the lowest level (e.g.,
44 sloshing \tr{MutableByteArray#}s around your program), you may wish to
45 check if there are system libraries that provide a ``Haskellised
46 veneer'' over the features you want. See \Sectionref{syslibs}.
48 The definitive guide for many of the low-level facilities in GHC is
49 the ``state interface document'' (distributed in
50 \tr{ghc/docs/state-interface.dvi}). We do not repeat its details here.
52 %Pieter Hartel led an interesting comparison-of-many-compilers (and
53 %many languages) in which GHC got to show off its extensions. We did
54 %very well! For the full details, check out
55 %\tr{pub/computer-systems/functional/packages/pseudoknot.tar.Z} on \tr{ftp.fwi.uva.nl}.
58 %************************************************************************
60 \subsection[glasgow-unboxed]{Unboxed types}
61 \index{Unboxed types (Glasgow extension)}
63 %************************************************************************
65 These types correspond to the ``raw machine'' types you would use in
66 C: \tr{Int#} (long int), \tr{Double#} (double),
67 \tr{Addr#} (void *), etc. The {\em primitive
68 operations} (PrimOps) on these types are what you might expect; e.g.,
69 \tr{(+#)} is addition on \tr{Int#}s, and is the machine-addition that
70 we all know and love---usually one instruction.
72 A numerically-intensive program using unboxed types can go a {\em lot}
73 faster than its ``standard'' counterpart---we saw a threefold speedup
76 Please see the very first part of the ``state interface document''
77 (distributed in \tr{ghc/docs/state-interface.dvi}) for the details of
78 unboxed types and the operations on them.
80 %************************************************************************
82 \subsection[glasgow-ST-monad]{Primitive state-transformer monad}
83 \index{state transformers (Glasgow extensions)}
85 %************************************************************************
87 This monad underlies our implementation of arrays, mutable and immutable,
88 and our implementation of I/O, including ``C calls''.
90 You probably won't use the monad directly, but you might use all those
93 The ``state interface document'' defines the state-related types in
94 sections~1.4 and~1.5, and the monad itself in section~2.1.
96 %************************************************************************
98 \subsection[glasgow-prim-arrays]{Primitive arrays, mutable and otherwise}
99 \index{primitive arrays (Glasgow extension)}
100 \index{arrays, primitive (Glasgow extension)}
102 %************************************************************************
104 GHC knows about quite a few flavours of Large Swathes of Bytes.
106 First, GHC distinguishes between primitive arrays of (boxed) Haskell
107 objects (type \tr{Array# obj}) and primitive arrays of bytes (type
110 Second, it distinguishes between...
113 Arrays that do not change (as with ``standard'' Haskell arrays); you
114 can only read from them. Obviously, they do not need the care and
115 attention of the state-transformer monad.
118 Arrays that may be changed or ``mutated.'' All the operations on them
119 live within the state-transformer monad and the updates happen {\em
122 \item[``Static'' (in C land):]
123 A C~routine may pass an \tr{Addr#} pointer back into Haskell land.
124 There are then primitive operations with which you may merrily grab
125 values over in C land, by indexing off the ``static'' pointer.
127 \item[``Stable'' pointers:]
128 If, for some reason, you wish to hand a Haskell pointer (i.e., {\em
129 not} an unboxed value) to a C~routine, you first make the pointer
130 ``stable,'' so that the garbage collector won't forget that it exists.
131 That is, GHC provides a safe way to pass Haskell pointers to C.
133 Please see \Sectionref{glasgow-stablePtrs} for more details.
135 \item[``Foreign objects'':]
136 A ``foreign object'' is a safe way to pass a C~pointer to Haskell and
137 have Haskell do the Right Thing when it no longer references the
138 object. So, for example, C could pass a large bitmap over to Haskell
139 and say ``please free this memory when you're done with it.''
141 Please see \Sectionref{glasgow-foreignObjs} for more details.
144 See sections~1.4 and~1.6 of the ``state interface document'' for the
145 details of all these ``primitive array'' types and the operations on
149 %************************************************************************
151 \subsection[own-mainPrimIO]{Using your own @mainPrimIO@}
152 \index{mainPrimIO, rolling your own}
154 %************************************************************************
156 Normally, the GHC runtime system begins things by called an internal
157 function @mainPrimIO :: PrimIO ()@ which, in turn, fires up
160 To subvert the above process, you need only provide a
161 @mainPrimIO :: PrimIO ()@ of your own (in a module named \tr{GHCmain}).
163 Here's a little example, stolen from Alastair Reid:
165 module GHCmain ( mainPrimIO ) where
169 mainPrimIO :: PrimIO ()
172 _ccall_ printf "%d\n" (14::Int)
174 sleep :: Int -> PrimIO ()
175 sleep t = _ccall_ sleep t
178 %************************************************************************
180 \subsection[glasgow-ccalls]{Calling~C directly from Haskell}
181 \index{C calls (Glasgow extension)}
182 \index{_ccall_ (Glasgow extension)}
183 \index{_casm_ (Glasgow extension)}
185 %************************************************************************
187 %Besides using a \tr{-fglasgow-exts} flag, your modules need to include...
189 %import PreludePrimIO
192 GOOD ADVICE: Because this stuff is not Entirely Stable as far as names
193 and things go, you would be well-advised to keep your C-callery
194 corraled in a few modules, rather than sprinkled all over your code.
195 It will then be quite easy to update later on.
197 WARNING AS OF 2.01: Yes, the \tr{_ccall_} stuff probably {\em will
198 change}, to something better, of course! We are still at the
199 musing-about-it stage, however...
201 %************************************************************************
203 \subsubsection[ccall-intro]{\tr{_ccall_} and \tr{_casm_}: an introduction}
205 %************************************************************************
207 The simplest way to use a simple C function
209 double fooC( FILE *in, char c, int i, double d, unsigned int u )
211 is to provide a Haskell wrapper:
213 fooH :: Char -> Int -> Double -> Word -> PrimIO Double
214 fooH c i d w = _ccall_ fooC ``stdin'' c i d w
216 The function @fooH@ will unbox all of its arguments, call the C
217 function \tr{fooC} and box the corresponding arguments.
219 So, if you want to do C-calling, you have to confront the underlying
220 I/O system (at the ``PrimIO'' level).
222 %The code in \tr{ghc/lib/glaExts/*.lhs} is not too obtuse.
223 %That code, plus \tr{lib/prelude/Builtin.hs}, give examples
224 %of its use. The latter includes the implementations of \tr{error} and
227 One of the annoyances about \tr{_ccall_}s is when the C types don't quite
228 match the Haskell compiler's ideas. For this, the \tr{_casm_} variant
229 may be just the ticket (NB: {\em no chance} of such code going through
230 a native-code generator):
233 = _casm_ ``%r = getenv((char *) %0);'' name >>= \ litstring@(A# str#) ->
235 if (litstring == ``NULL'') then
236 Left ("Fail:oldGetEnv:"++name)
238 Right (unpackCString# str#)
242 The first literal-literal argument to a \tr{_casm_} is like a
243 \tr{printf} format: \tr{%r} is replaced with the ``result,''
244 \tr{%0}--\tr{%n-1} are replaced with the 1st--nth arguments. As you
245 can see above, it is an easy way to do simple C~casting. Everything
246 said about \tr{_ccall_} goes for \tr{_casm_} as well.
248 %************************************************************************
250 \subsubsection[glasgow-foreign-headers]{Using function headers}
251 \index{C calls---function headers}
253 %************************************************************************
255 When generating C (using the \tr{-fvia-C} directive), one can assist
256 the C compiler in detecting type errors by using the \tr{-#include}
257 directive to provide \tr{.h} files containing function headers.
261 typedef unsigned long *StgForeignObj;
264 void initialiseEFS (StgInt size);
265 StgInt terminateEFS (void);
266 StgForeignObj emptyEFS(void);
267 StgForeignObj updateEFS (StgForeignObj a, StgInt i, StgInt x);
268 StgInt lookupEFS (StgForeignObj a, StgInt i);
271 You can find appropriate definitions for \tr{StgInt},
272 \tr{StgForeignObj}, etc using \tr{gcc} on your architecture by
273 consulting \tr{ghc/includes/StgTypes.lh}. The following table
274 summarises the relationship between Haskell types and C types.
277 C type name & Haskell Type \\ \hline
278 %----- & ---------------
279 \tr{StgChar} & \tr{Char#}\\
280 \tr{StgInt} & \tr{Int#}\\
281 \tr{StgWord} & \tr{Word#}\\
282 \tr{StgAddr} & \tr{Addr#}\\
283 \tr{StgFloat} & \tr{Float#}\\
284 \tr{StgDouble} & \tr{Double#}\\
286 \tr{StgArray} & \tr{Array#}\\
287 \tr{StgByteArray} & \tr{ByteArray#}\\
288 \tr{StgArray} & \tr{MutableArray#}\\
289 \tr{StgByteArray} & \tr{MutableByteArray#}\\
291 \tr{StgStablePtr} & \tr{StablePtr#}\\
292 \tr{StgForeignObj} & \tr{MallocPtr#}
295 Note that this approach is only {\em essential\/} for returning
296 \tr{float}s (or if \tr{sizeof(int) != sizeof(int *)} on your
297 architecture) but is a Good Thing for anyone who cares about writing
298 solid code. You're crazy not to do it.
300 %************************************************************************
302 \subsubsection[glasgow-stablePtrs]{Subverting automatic unboxing with ``stable pointers''}
303 \index{stable pointers (Glasgow extension)}
305 %************************************************************************
307 The arguments of a \tr{_ccall_} are automatically unboxed before the
308 call. There are two reasons why this is usually the Right Thing to do:
311 C is a strict language: it would be excessively tedious to pass
312 unevaluated arguments and require the C programmer to force their
313 evaluation before using them.
315 \item Boxed values are stored on the Haskell heap and may be moved
316 within the heap if a garbage collection occurs---that is, pointers
317 to boxed objects are not {\em stable\/}.
320 It is possible to subvert the unboxing process by creating a ``stable
321 pointer'' to a value and passing the stable pointer instead. For example, to
322 pass/return an integer lazily to C functions \tr{storeC} and
323 \tr{fetchC}, one might write:
325 storeH :: Int -> PrimIO ()
326 storeH x = makeStablePtr x >>= \ stable_x ->
327 _ccall_ storeC stable_x
330 fetchH x = _ccall_ fetchC >>= \ stable_x ->
331 deRefStablePtr stable_x >>= \ x ->
332 freeStablePtr stable_x >>
336 The garbage collector will refrain from throwing a stable pointer away
337 until you explicitly call one of the following from C or Haskell.
339 void freeStablePointer( StgStablePtr stablePtrToToss )
340 freeStablePtr :: StablePtr a -> PrimIO ()
343 As with the use of \tr{free} in C programs, GREAT CARE SHOULD BE
344 EXERCISED to ensure these functions are called at the right time: too
345 early and you get dangling references (and, if you're lucky, an error
346 message from the runtime system); too late and you get space leaks.
348 %Doesn't work in ghc-0.23 - best to just keep quiet about them.
350 %And to force evaluation of the argument within \tr{fooC}, one would
351 %call one of the following C functions (according to type of argument).
354 %void performIO ( StgStablePtr stableIndex /* StablePtr s (PrimIO ()) */ );
355 %StgInt enterInt ( StgStablePtr stableIndex /* StablePtr s Int */ );
356 %StgFloat enterFloat ( StgStablePtr stableIndex /* StablePtr s Float */ );
359 %ToDo ADR: test these functions!
361 %Note Bene: \tr{_ccall_GC_} must be used if any of these functions are used.
364 %************************************************************************
366 \subsubsection[glasgow-foreignObjs]{Pointing outside the Haskell heap}
367 \index{foreign objects (Glasgow extension)}
369 %************************************************************************
371 There are two types that \tr{ghc} programs can use to reference
372 (heap-allocated) objects outside the Haskell world: \tr{Addr} and
375 If you use \tr{Addr}, it is up to you to the programmer to arrange
376 allocation and deallocation of the objects.
378 If you use \tr{ForeignObj}, \tr{ghc}'s garbage collector will
379 call the user-supplied C function
381 void freeForeignObj( StgForeignObj garbageMallocPtr )
383 when the Haskell world can no longer access the object. Since
384 \tr{ForeignObj}s only get released when a garbage collection occurs,
385 we provide ways of triggering a garbage collection from within C and
388 void StgPerformGarbageCollection()
389 performGC :: PrimIO ()
392 %************************************************************************
394 \subsubsection[glasgow-avoiding-monads]{Avoiding monads}
395 \index{C calls to `pure C'}
396 \index{unsafePerformPrimIO (PreludeGlaST)}
398 %************************************************************************
400 The \tr{_ccall_} construct is part of the \tr{PrimIO} monad because 9
401 out of 10 uses will be to call imperative functions with side effects
402 such as \tr{printf}. Use of the monad ensures that these operations
403 happen in a predictable order in spite of laziness and compiler
406 There are three situations where one might like to use
407 @unsafePerformPrimIO@ to avoid the monad:
410 Calling a function with no side-effects:
412 atan2d :: Double -> Double -> Double
413 atan2d y x = unsafePerformPrimIO (_ccall_ atan2d y x)
415 sincosd :: Double -> (Double, Double)
416 sincosd x = unsafePerformPrimIO $
417 newDoubleArray (0, 1) >>= \ da ->
418 _casm_ ``sincosd( %0, &((double *)%1[0]), &((double *)%1[1]) );'' x da
420 readDoubleArray da 0 >>= \ s ->
421 readDoubleArray da 1 >>= \ c ->
425 \item Calling a set of functions which have side-effects but which can
426 be used in a purely functional manner.
428 For example, an imperative implementation of a purely functional
429 lookup-table might be accessed using the following functions.
433 update :: EFS x -> Int -> x -> EFS x
434 lookup :: EFS a -> Int -> a
436 empty = unsafePerformPrimIO (_ccall_ emptyEFS)
438 update a i x = unsafePerformPrimIO $
439 makeStablePtr x >>= \ stable_x ->
440 _ccall_ updateEFS a i stable_x
442 lookup a i = unsafePerformPrimIO $
443 _ccall_ lookupEFS a i >>= \ stable_x ->
444 deRefStablePtr stable_x
447 You will almost always want to use \tr{ForeignObj}s with this.
449 \item Calling a side-effecting function even though the results will
450 be unpredictable. For example the \tr{trace} function is defined by:
453 trace :: String -> a -> a
455 = unsafePerformPrimIO (
456 ((_ccall_ PreTraceHook sTDERR{-msg-}):: PrimIO ()) >>
457 fputs sTDERR string >>
458 ((_ccall_ PostTraceHook sTDERR{-msg-}):: PrimIO ()) >>
461 sTDERR = (``stderr'' :: Addr)
464 (This kind of use is not highly recommended --- it is only really
465 useful in debugging code.)
468 %************************************************************************
470 \subsubsection[ccall-gotchas]{C-calling ``gotchas'' checklist}
471 \index{C call dangers}
473 %************************************************************************
475 And some advice, too.
479 \tr{_ccall_} is part of the \tr{PrimIO} monad --- not the 1.3 \tr{IO} Monad.
482 primIOToIO :: PrimIO a -> IO a
484 to promote a \tr{_ccall_} to the \tr{IO} monad.
487 For modules that use \tr{_ccall_}s, etc., compile with \tr{-fvia-C}.\index{-fvia-C option}
488 You don't have to, but you should.
490 Also, use the \tr{-#include "prototypes.h"} flag (hack) to inform the
491 C compiler of the fully-prototyped types of all the C functions you
492 call. (\Sectionref{glasgow-foreign-headers} says more about this...)
494 This scheme is the {\em only} way that you will get {\em any}
495 typechecking of your \tr{_ccall_}s. (It shouldn't be that way,
499 Try to avoid \tr{_ccall_}s to C~functions that take \tr{float}
500 arguments or return \tr{float} results. Reason: if you do, you will
501 become entangled in (ANSI?) C's rules for when arguments/results are
502 promoted to \tr{doubles}. It's a nightmare and just not worth it.
503 Use \tr{doubles} if possible.
505 If you do use \tr{floats}, check and re-check that the right thing is
506 happening. Perhaps compile with \tr{-keep-hc-file-too} and look at
507 the intermediate C (\tr{.hc} file).
510 The compiler uses two non-standard type-classes when
511 type-checking the arguments and results of \tr{_ccall_}: the arguments
512 (respectively result) of \tr{_ccall_} must be instances of the class
513 \tr{CCallable} (respectively \tr{CReturnable}). (Neither class
514 defines any methods --- their only function is to keep the
517 The type checker must be able to figure out just which of the
518 C-callable/returnable types is being used. If it can't, you have to
519 add type signatures. For example,
523 is not good enough, because the compiler can't work out what type @x@ is, nor
524 what type the @_ccall_@ returns. You have to write, say:
526 f :: Int -> PrimIO Double
530 This table summarises the standard instances of these classes.
532 % ToDo: check this table against implementation!
534 \begin{tabular}{llll}
535 Type &CCallable&CReturnable & Which is probably... \\ \hline
536 %------ ---------- ------------ -------------
537 \tr{Char} & Yes & Yes & \tr{unsigned char} \\
538 \tr{Int} & Yes & Yes & \tr{long int} \\
539 \tr{Word} & Yes & Yes & \tr{unsigned long int} \\
540 \tr{Addr} & Yes & Yes & \tr{char *} \\
541 \tr{Float} & Yes & Yes & \tr{float} \\
542 \tr{Double} & Yes & Yes & \tr{double} \\
543 \tr{()} & No & Yes & \tr{void} \\
544 \tr{[Char]} & Yes & No & \tr{char *} (null-terminated) \\
546 \tr{Array} & Yes & No & \tr{unsigned long *}\\
547 \tr{ByteArray} & Yes & No & \tr{unsigned long *}\\
548 \tr{MutableArray} & Yes & No & \tr{unsigned long *}\\
549 \tr{MutableByteArray} & Yes & No & \tr{unsigned long *}\\
551 \tr{State} & Yes & Yes & nothing!\\
553 \tr{StablePtr} & Yes & Yes & \tr{unsigned long *}\\
554 \tr{ForeignObjs} & Yes & Yes & see later\\
557 The brave and careful programmer can add their own instances of these
558 classes for the following types:
561 A {\em boxed-primitive} type may be made an instance of both
562 \tr{CCallable} and \tr{CReturnable}.
564 A boxed primitive type is any data type with a
565 single unary constructor with a single primitive argument. For
566 example, the following are all boxed primitive types:
571 data XDisplay = XDisplay Addr#
572 data EFS a = EFS# ForeignObj#
576 instance CCallable (EFS a)
577 instance CReturnable (EFS a)
580 \item Any datatype with a single nullary constructor may be made an
581 instance of \tr{CReturnable}. For example:
585 instance CReturnable MyVoid
588 \item As at version 2.01, \tr{String} (i.e., \tr{[Char]}) is still
589 not a \tr{CReturnable} type.
591 Also, the now-builtin type \tr{PackedString} is neither
592 \tr{CCallable} nor \tr{CReturnable}. (But there are functions in
593 the PackedString interface to let you get at the necessary bits...)
597 The code-generator will complain if you attempt to use \tr{%r}
598 in a \tr{_casm_} whose result type is \tr{PrimIO ()}; or if you don't
599 use \tr{%r} {\em precisely\/} once for any other result type. These
600 messages are supposed to be helpful and catch bugs---please tell us
601 if they wreck your life.
604 If you call out to C code which may trigger the Haskell garbage
605 collector (examples of this later...), then you must use the
606 \tr{_ccall_GC_} or \tr{_casm_GC_} variant of C-calls. (This does not
607 work with the native code generator - use \tr{\fvia-C}.) This stuff is
608 hairy with a capital H!
611 %************************************************************************
613 %\subsubsection[ccall-good-practice]{C-calling ``good practice'' checklist}
615 %************************************************************************
617 %************************************************************************
619 \subsubsection[glasgow-prim-interface]{Access to the \tr{PrimIO} monad}
620 \index{PrimIO monad (Glasgow extension)}
621 \index{I/O, primitive (Glasgow extension)}
623 %************************************************************************
625 The \tr{IO} monad (new in Haskell~1.3) catches errors and passes them
626 along. It is built on top of the \tr{ST} state-transformer monad.
628 A related (and inter-operable-with) monad is the \tr{PrimIO} monad
629 (NB: the level at which @_ccall_@s work...), where you handle errors
632 Should you wish to use the \tr{PrimIO} monad directly, you can import
633 \tr{PreludeGlaST}. It makes available the usual monadic stuff (@>>=@,
634 @>>@, @return@, etc.), as well as these functions:
636 -- for backward compatibility:
637 returnPrimIO :: a -> PrimIO a
638 thenPrimIO :: PrimIO a -> (a -> PrimIO b) -> PrimIO b
639 seqPrimIO :: PrimIO a -> PrimIO b -> PrimIO b
642 fixPrimIO :: (a -> PrimIO a) -> PrimIO a
643 forkPrimIO :: PrimIO a -> PrimIO a
644 listPrimIO :: [PrimIO a] -> PrimIO [a]
645 mapAndUnzipPrimIO :: (a -> PrimIO (b,c)) -> [a] -> PrimIO ([b],[c])
646 mapPrimIO :: (a -> PrimIO b) -> [a] -> PrimIO [b]
648 unsafePerformPrimIO :: PrimIO a -> a
649 unsafeInterleavePrimIO :: PrimIO a -> PrimIO a
650 -- and they are not called "unsafe" for nothing!
652 -- to convert back and forth between IO and PrimIO
653 ioToPrimIO :: IO a -> PrimIO a
654 primIOToIO :: PrimIO a -> IO a
658 %************************************************************************
660 \subsection[glasgow-hbc-exts]{``HBC-ish'' extensions implemented by GHC}
661 \index{HBC-like Glasgow extensions}
662 \index{extensions, HBC-like}
664 %************************************************************************
667 %-------------------------------------------------------------------
668 \item[@fromInt@ method in class @Num@:]
669 It's there. Converts from an \tr{Int} to the type.
671 %-------------------------------------------------------------------
672 \item[@toInt@ method in class @Integral@:]
673 Converts from type type to an \tr{Int}.
675 %-------------------------------------------------------------------
676 \item[Overlapping instance declarations:]
677 \index{overlapping instances}
678 \index{instances, overlapping}
680 In \tr{instance <context> => Class (T x1 ... xn)}, the \tr{xi}s can be
681 {\em types}, rather than just {\em type variables}.
683 Thus, you can have an instance \tr{instance Foo [Char]}, as well as
684 the more general \tr{instance Foo [a]}; the former will be used in
685 preference to the latter, where applicable.
687 As Lennart says, ``This is a dubious feature and should not be used
690 See also: \tr{SPECIALIZE instance} pragmas, in \Sectionref{faster}.
692 %-------------------------------------------------------------------
693 % \item[Signal-handling I/O request:]
694 % \index{signal handling (extension)}
695 % \index{SigAction I/O request}
696 % The Haskell-1.2 I/O request \tr{SigAction n act} installs a signal handler for signal
697 % \tr{n :: Int}. The number is the usual UNIX signal number. The action
706 % The corresponding continuation-style I/O function is the unsurprising:
708 % sigAction :: Int -> SigAct -> FailCont -> SuccCont -> Dialogue
711 % When a signal handler is installed with \tr{SACatch}, receipt of the
712 % signal causes the current top-level computation to be abandoned, and
713 % the specified dialogue to be executed instead. The abandoned
714 % computation may leave some partially evaluated expressions in a
715 % non-resumable state. If you believe that your top-level computation
716 % and your signal handling dialogue may share subexpressions, you should
717 % execute your program with the \tr{-N} RTS option, to prevent
720 % The \tr{-N} option is not available with concurrent/parallel programs,
721 % so great care should be taken to avoid shared subexpressions between
722 % the top-level computation and any signal handlers when using threads.
724 %-------------------------------------------------------------------
725 %\item[Simple time-out mechanism, in ``monadic I/O'':]
726 %\index{time-outs (extension)}
728 %This function is available:
730 %timeoutIO :: Int -> IO Void -> IO (IO Void)
733 %Wait that many seconds, then abandon the current computation and
734 %perform the given I/O operation (second argument). Uses the
735 %signal-handling, so it returns the previous signal-handler (in case
736 %you want to re-install it). As above, you may need to execute your
737 %program with the RTS flag \tr{-N}, to prevent black-holing.
741 %************************************************************************
743 %\subsection[glasgow-compiler-namespace]{Fiddlings the compiler's built-in namespaces}
745 %************************************************************************
747 %This is really only used for compiling the prelude. It's turgid and
748 %will probably change.
750 % \begin{description}
751 % \item[\tr{-no-implicit-prelude}:]
752 % \index{-no-implicit-prelude option}
754 % ???? (Tells the parser not to read \tr{Prelude.hi}).
756 % \item[\tr{-fhide-builtin-names}:]
757 % \index{-fhide-builtin-names option}
758 % This hides {\em all} Prelude names built-in to the compiler.
760 % \item[\tr{-fmin-builtin-names}:]
761 % \index{-fmin-builtin-names option}
762 % This hides all but a few of the Prelude names that are built-in to the
763 % compiler. @:@ (cons) is an example of one that would remain visible.
765 % \item[\tr{-fhide-builtin-instances}:]
766 % \index{-fhide-builtin-instances option}
767 % This suppresses the compiler's own ideas about what instances already
768 % exist (e.g., \tr{instance Eq Int}).
770 % This flag is used when actually compiling the various instance
771 % declarations in the Prelude.