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10 <title>The Hugs-GHC Extension Libraries
11 <author>Alastair Reid <tt/reid-alastair@cs.yale.edu/
12 Simon Marlow <tt/simonm@dcs.gla.ac.uk/
13 <date>v0.8, 28 January 1998
15 Hugs and GHC provide a common set of libraries to aid portability.
16 This document specifies the interfaces to these libraries and documents
17 known differences. We hope that these modules will be adopted for inclusion
18 as Standard Haskell Libraries sometime soon.
23 <sect> <idx/Naming conventions/
24 <label id="sec:Naming conventions">
27 The set of interfaces specified in this document try to adhere to the
28 following naming conventions:
32 Actions that create a new values have the prefix <tt/new/ followed by
33 the name of the type of object they're creating, e.g., <tt/newIORef/,
36 Operations that read a value from a mutable object are prefixed with
37 <tt/read/, and operations that update the contents have the prefix
38 <tt/write/, e.g., <tt/readChan/, <tt/readIOArray/.
43 This differs from the convention used to name the operations for
44 reading and writing to a file <tt/Handle/, where <tt/get/ and <tt/put/
47 Operations provided by various concurrency abstractions, e.g., <tt/MVar/,
48 <tt/CVar/ , also deviate from this naming scheme. This is perhaps
49 defensible, since the read and write operations have additional
50 behaviour, e.g., <tt/takeMVar/ tries to read the current value
51 of an <tt/MVar/, locking it if it succeeds.
54 Conversions operators have the form <tt/AToB/ where <tt/A/ and <tt/B/
55 are the types we're converting between.
57 Operations that lazily read values from a mutable object/handle, have
58 the form <tt/getXContents/, e.g., <tt/Channel.getChanContents/ and
59 <tt/IO.hGetContents/. (OK, so the latter isn't called
60 <tt/getHandleContents/, but you hopefully get the picture.)
63 <sect> <idx/LazyST/ <p>
65 This library provides support for both <em/lazy/ and <em/strict/ state
66 threads, as described in the PLDI '94 paper by John Launchbury and
67 Simon Peyton Jones <cite id="LazyStateThreads">. In addition to the
68 monad <tt/ST/, it also provides mutable variables <tt/STRef/ and
69 mutable arrays <tt/STArray/. As the name suggests, the monad <tt/ST/
70 instance is <em/lazy/.
76 This library provides support for <em/strict/ state threads, as
77 described in the PLDI '94 paper by John Launchbury and Simon Peyton
78 Jones <cite id="LazyStateThreads">. In addition to the monad <tt/ST/,
79 it also provides mutable variables <tt/STRef/ and mutable arrays
83 module ST( module ST, module Monad ) where
86 data ST s a -- abstract type
87 runST :: forall a. (forall s. ST s a) -> a
88 fixST :: (a -> ST s a) -> ST s a
89 unsafeInterleaveST :: ST s a -> ST s a
90 instance Functor (ST s)
93 data STRef s a -- mutable variables in state thread s
94 -- containing values of type a.
95 newSTRef :: a -> ST s (STRef s a)
96 readSTRef :: STRef s a -> ST s a
97 writeSTRef :: STRef s a -> a -> ST s ()
98 instance Eq (STRef s a)
100 data STArray s ix elt -- mutable arrays in state thread s
101 -- indexed by values of type ix
102 -- containing values of type a.
103 newSTArray :: Ix ix => (ix,ix) -> elt -> ST s (STArray s ix elt)
104 boundsSTArray :: Ix ix => STArray s ix elt -> (ix, ix)
105 readSTArray :: Ix ix => STArray s ix elt -> ix -> ST s elt
106 writeSTArray :: Ix ix => STArray s ix elt -> ix -> elt -> ST s ()
107 thawSTArray :: Ix ix => Array ix elt -> ST s (STArray s ix elt)
108 freezeSTArray :: Ix ix => STArray s ix elt -> ST s (Array ix elt)
109 unsafeFreezeSTArray :: Ix ix => STArray s ix elt -> ST s (Array ix elt)
110 instance Eq (STArray s ix elt)
117 GHC also supports ByteArrays --- these aren't supported by Hugs yet.
120 The operations <tt/freezeSTArray/ and <tt/thawSTArray/ convert mutable
121 arrays to and from immutable arrays. Semantically, they are identical
122 to copying the array and they are usually implemented that way. The
123 operation <tt/unsafeFreezeSTArray/ is a faster version of
124 <tt/freezeSTArray/ which omits the copying step. It's a safe substitute for
125 <tt/freezeSTArray/ if you don't modify the mutable array after freezing it.
128 In the current version of Hugs, the <tt/<idx/runST// operation,
129 used to specify encapsulation, is implemented as a language construct,
130 and <tt/runST/ is treated as a keyword. We plan to change this to match
135 Note that it is possible to install Hugs 1.4 without support for lazy
136 state threads, and hence the primitives described here may not be
137 available in all implementations. Also, in contrast with the
138 implementation of lazy state threads in previous releases of Hugs and
139 Gofer, there is no direct relationship between the
140 <tt/<idx/ST monad// and the <tt/<idx/IO monad//.
144 Hugs provides <tt/thenLazyST/ and <tt/thenStrictST/ so that you can
145 import <tt/LazyST/ (say) and still use the strict instance in those
146 places where it matters. GHC implements LazyST and ST using different
147 types, so this isn't possible.
153 <label id="sec:LazyST">
156 This library is identical to <tt/ST/ except that the <tt/ST/ monad
157 instance is <em/lazy/. The lazy ST monad tends to be more prone to
158 space leaks than the strict version, so most programmers will use the
159 former unless laziness is explicitly required. <tt/LazyST/ provides
160 two additional operations:
163 lazyToStrictST :: LazyST.ST s a -> ST.ST s a
164 strictToLazyST :: ST.ST s a -> LazyST.ST s a
167 These are used to convert between lazy and strict state threads. The
168 semantics with respect to laziness are as you would expect: the strict
169 state thread passed to <tt/strictToLazyST/ is not performed until the
170 result of the lazy state thread it returns is demanded.
173 <label id="sec:IOExts">
176 This library provides the following extensions to the IO monad:
179 The operations <tt/fixIO/, <tt/unsafePerformIO/ and <tt/unsafeInterleaveIO/
180 described in <cite id="ImperativeFP">
183 References (aka mutable variables) and mutable arrays (but no form of
187 <tt/openFileEx/ extends the standard <tr/openFile/ action with support
188 for opening binary files.
191 <tt/performGC/ triggers an immediate garbage collection
194 When called, <tt/trace/ prints the string in its first argument, and then
195 returns the second argument as its result. The <tt/trace/ function is not
196 referentially transparent, and should only be used for debugging, or for
197 monitoring execution.
200 You should also be warned that, unless you understand some of the
201 details about the way that Haskell programs are executed, results
202 obtained using <tt/trace/ can be rather confusing. For example, the
203 messages may not appear in the order that you expect. Even ignoring the
204 output that they produce, adding calls to <tt/trace/ can change the
205 semantics of your program. Consider this a warning!
209 <tt/unsafePtrEq/ compares two values for pointer equality without
210 evaluating them. The results are not referentially transparent and
211 may vary significantly from one compiler to another or in the face of
212 semantics-preserving program changes. However, pointer equality is useful
213 in creating a number of referentially transparent constructs such as this
214 simplified memoisation function:
217 > cache :: (a -> b) -> (a -> b)
218 > cache f = \x -> unsafePerformIO (check x)
220 > ref = unsafePerformIO (newIORef (error "cache", error "cache"))
221 > check x = readIORef ref >>= \ (x',a) ->
222 > if x `unsafePtrEq` x' then
226 > writeIORef ref (x, a) >>
236 fixIO :: (a -> IO a) -> IO a
237 unsafePerformIO :: IO a -> a
238 unsafeInterleaveIO :: IO a -> IO a
240 data IORef a -- mutable variables containing values of type a
241 newIORef :: a -> IO (IORef a)
242 readIORef :: IORef a -> IO a
243 writeIORef :: IORef a -> a -> IO ()
244 instance Eq (IORef a)
246 data IOArray ix elt -- mutable arrays indexed by values of type ix
247 -- containing values of type a.
248 newIOArray :: Ix ix => (ix,ix) -> elt -> IO (IOArray ix elt)
249 boundsIOArray :: Ix ix => IOArray ix elt -> (ix, ix)
250 readIOArray :: Ix ix => IOArray ix elt -> ix -> IO elt
251 writeIOArray :: Ix ix => IOArray ix elt -> ix -> elt -> IO ()
252 freezeIOArray :: Ix ix => IOArray ix elt -> IO (Array ix elt)
253 instance Eq (IOArray ix elt)
255 openFileEx :: FilePath -> IOModeEx -> IO Handle
256 data IOModeEx = BinaryMode IO.IOMode | TextMode IO.IOMode
258 instance Read IOModeEx
259 instance Show IOModeEx
262 trace :: String -> a -> a
263 unsafePtrEq :: a -> a -> Bool
267 <sect> <idx/GlaExts/ <p>
269 This library provides a convenient bundle of most of the extensions
270 available in GHC and Hugs. This module is generally more stable than
271 the other modules of non-standard extensions so you might choose to
272 import them from here rather than going straight to the horses mouth.
275 module GlaExts( module GlaExts, module IOExts, module ST, module Addr ) where
279 trace :: String -> a -> a
283 The GHC version also provides the types <tt/PrimIO/, <tt/RealWorld/,
284 <tt/ByteArray/, <tt/Lift/ and operations on these types. It also
285 provides the unboxed views of the types
293 and a number of ``primitive operations'' (<tt/+&num/,
294 <tt/plusFloat&num/, etc.).
299 <label id="sec:Bits">
302 This library defines bitwise operations for signed and unsigned ints.
306 infixl 8 `shift`, `rotate`
312 (.&.), (.|.), xor :: a -> a -> a
314 shift :: a -> Int -> a
315 rotate :: a -> Int -> a
317 setBit :: a -> Int -> a
318 clearBit :: a -> Int -> a
319 complementBit :: a -> Int -> a
320 testBit :: a -> Int -> Bool
322 isSigned :: a -> Bool
324 shiftL, shiftR :: Bits a => a -> Int -> a
325 rotateL, rotateR :: Bits a => a -> Int -> a
326 shiftL a i = shift a i
327 shiftR a i = shift a (-i)
328 rotateL a i = rotate a i
329 rotateR a i = rotate a (-i)
335 <tt/bitSize/ and <tt/isSigned/ are like <tt/floatRadix/ and <tt/floatDigits/
336 -- they return parameters of the <em/type/ of their argument rather than
337 of the particular argument they are applied to. <tt/bitSize/ returns
338 the number of bits in the type (or <tt/Nothing/ for unbounded types); and
339 <tt/isSigned/ returns whether the type is signed or not.
341 <tt/shift/ performs sign extension on signed number types.
342 That is, right shifts fill the top bits with 1 if the number is negative
343 and with 0 otherwise.
345 Bits are numbered from 0 with bit 0 being the least significant bit.
347 <tt/shift x i/ and <tt/rotate x i/ shift to the left if <tt/i/ is
348 positive and to the right otherwise.
351 <tt/rotate/ is well defined only if bitSize returns a number.
352 (Maybe we should impose a Bounded constraint on it?)
355 <tt/bit i/ is the value with the i'th bit set.
359 <label id="sec:Word">
362 This library provides unsigned integers of various sizes.
363 The types supported are as follows:
366 type | number of bits @
375 For each type <it/W/ above, we provide the following functions and
376 instances. The type <it/I/ refers to the signed integer type of the
380 data W -- Unsigned Ints
395 word8ToWord32 :: Word8 -> Word32
396 word32ToWord8 :: Word32 -> Word8
397 word16ToWord32 :: Word16 -> Word32
398 word32ToWord16 :: Word32 -> Word16
400 word8ToInt :: Word8 -> Int
401 intToWord8 :: Int -> Word8
402 word16ToInt :: Word16 -> Int
403 intToWord16 :: Int -> Word16
404 word32ToInt :: Word32 -> Int
405 intToWord32 :: Int -> Word32
411 All arithmetic is performed modulo 2^n
413 One non-obvious consequequence of this is that <tt/negate/
414 should <em/not/ raise an error on negative arguments.
417 The coercion <tt/wToI/ converts an unsigned n-bit value to the
418 signed n-bit value with the same representation. For example,
419 <tt/word8ToInt8 0xff = -1/.
420 Likewise, <tt/iToW/ converts signed n-bit values to the
421 corresponding unsigned n-bit value.
424 ToDo: complete the set of coercion functions.
427 Use <tt/Prelude.fromIntegral :: (Integral a, Num b) => a -> b/ to
428 coerce between different sizes or to preserve sign when converting
429 between values of the same size.
432 It would be very natural to add a type a type <tt/Natural/ providing
433 an unbounded size unsigned integer --- just as <tt/Integer/ provides
434 unbounded size signed integers. We do not do that yet since there is
435 no demand for it. Doing so would require <tt/Bits.bitSize/ to return
439 The <tt/Enum/ instances stop when they reach their upper or lower
440 bound --- they don't overflow the way the <tt/Int/ and <tt/Float/
444 It would be useful to provide a function (or a family of functions?)
445 which coerced between any two Word types (without going through
450 Hugs only provides <tt/Eq/, <tt/Ord/, <tt/Read/ and <tt/Show/
451 instances for <tt/Word64/ at the moment.
457 This library provides signed integers of various sizes. The types
458 supported are as follows:
461 type | number of bits @
470 For each type <it/I/ above, we provide the following instances.
473 data I -- Signed Ints
474 iToInt :: I -> Int -- not provided for Int64
475 intToi :: Int -> I -- not provided for Int64
490 int8ToInt :: Int8 -> Int
491 intToInt8 :: Int -> Int8
492 int16ToInt :: Int16 -> Int
493 intToInt16 :: Int -> Int16
494 int32ToInt :: Int32 -> Int
495 intToInt32 :: Int -> Int32
500 Hugs does not provide <tt/Int64/ at the moment.
503 ToDo: complete the set of coercion functions.
508 <label id="sec:Addr">
511 This library provides machine addresses and is primarily intended for
512 use in creating foreign function interfaces using GreenCard.
516 data Addr -- Address type
520 plusAddr :: Addr -> Int -> Addr
522 -- read value out of _immutable_ memory
523 indexCharOffAddr :: Addr -> Int -> Char
524 indexIntOffAddr :: Addr -> Int -> Int -- should we drop this?
525 indexAddrOffAddr :: Addr -> Int -> Addr
526 indexFloatOffAddr :: Addr -> Int -> Float
527 indexDoubleOffAddr :: Addr -> Int -> Double
528 indexWord8OffAddr :: Addr -> Int -> Word8
529 indexWord16OffAddr :: Addr -> Int -> Word16
530 indexWord32OffAddr :: Addr -> Int -> Word32
531 indexWord64OffAddr :: Addr -> Int -> Word64
532 indexInt8OffAddr :: Addr -> Int -> Int8
533 indexInt16OffAddr :: Addr -> Int -> Int16
534 indexInt32OffAddr :: Addr -> Int -> Int32
535 indexInt64OffAddr :: Addr -> Int -> Int64
537 -- read value out of mutable memory
538 readCharOffAddr :: Addr -> Int -> IO Char
539 readIntOffAddr :: Addr -> Int -> IO Int -- should we drop this?
540 readAddrOffAddr :: Addr -> Int -> IO Addr
541 readFloatOffAddr :: Addr -> Int -> IO Float
542 readDoubleOffAddr :: Addr -> Int -> IO Double
543 readWord8OffAddr :: Addr -> Int -> IO Word8
544 readWord16OffAddr :: Addr -> Int -> IO Word16
545 readWord32OffAddr :: Addr -> Int -> IO Word32
546 readWord64OffAddr :: Addr -> Int -> IO Word64
547 readInt8OffAddr :: Addr -> Int -> IO Int8
548 readInt16OffAddr :: Addr -> Int -> IO Int16
549 readInt32OffAddr :: Addr -> Int -> IO Int32
550 readInt64OffAddr :: Addr -> Int -> IO Int64
552 -- write value into mutable memory
553 writeCharOffAddr :: Addr -> Int -> Char -> IO ()
554 writeIntOffAddr :: Addr -> Int -> Int -> IO () -- should we drop this?
555 writeAddrOffAddr :: Addr -> Int -> Addr -> IO ()
556 writeFloatOffAddr :: Addr -> Int -> Float -> IO ()
557 writeDoubleOffAddr :: Addr -> Int -> Double -> IO ()
558 writeWord8OffAddr :: Addr -> Int -> Word8 -> IO ()
559 writeWord16OffAddr :: Addr -> Int -> Word16 -> IO ()
560 writeWord32OffAddr :: Addr -> Int -> Word32 -> IO ()
561 writeWord64OffAddr :: Addr -> Int -> Word64 -> IO ()
562 writeInt8OffAddr :: Addr -> Int -> Int8 -> IO ()
563 writeInt16OffAddr :: Addr -> Int -> Int16 -> IO ()
564 writeInt32OffAddr :: Addr -> Int -> Int32 -> IO ()
565 writeInt64OffAddr :: Addr -> Int -> Int64 -> IO ()
568 Hugs provides <tt/Addr/ and <tt/nullAddr/ but does not provide any of
569 the index, read or write functions. They can be implemented using
570 GreenCard if required.
573 <label id="sec:NumExts">
576 The <tt/NumExts/ interface collect together various numeric
577 operations that have proven to be commonly useful
580 -- Going between Doubles and Floats:
581 doubleToFloat :: Double -> Float
582 floatToDouble :: Float -> Double
584 showHex :: Integral a => a -> ShowS
585 showOct :: Integral a => a -> ShowS
591 If <tt/doubleToFloat/ is applied to a <tt/Double/ that is within
592 the representable range for <tt/Float/, the result may be the next
593 higher or lower representable <tt/Float/ value. If the <tt/Double/
594 is out of range, the result is undefined.
596 No loss of precision occurs in the other direction with
597 <tt/floatToDouble/, the floating value remains unchanged.
599 <tt/showOct/ and <tt/showHex/ will prefix <tt/0o/ and <tt/0x/
600 respectively. Like <tt/Numeric.showInt/, these show functions
601 work on positive numbers only.
605 <label id="sec:Foreign">
608 This module is provided by GHC but not by Hugs.
609 GreenCard for Hugs provides the <tt/ForeignObj/ type.
611 <sect> <idx/Concurrent/
612 <label id="sec:Concurrent">
615 This library provides the Concurrent Haskell extensions
616 <cite id="concurrentHaskell:popl96">.
618 We are grateful to the Glasgow Haskell Project for allowing us to
619 redistribute their implementation of this module.
622 module Concurrent where
624 data ThreadId -- thread identifiers
626 instance Ord ThreadId
628 forkIO :: IO () -> IO ThreadId
629 killThread :: ThreadId -> IO ()
631 data MVar a -- Synchronisation variables
632 newEmptyMVar :: IO (MVar a)
633 newMVar :: a -> IO (MVar a)
634 takeMVar :: MVar a -> IO a
635 putMVar :: MVar a -> a -> IO ()
636 swapMVar :: MVar a -> a -> IO a
637 readMVar :: MVar a -> IO a
640 data Chan a -- channels
641 newChan :: IO (Chan a)
642 writeChan :: Chan a -> a -> IO ()
643 readChan :: Chan a -> IO a
644 dupChan :: Chan a -> IO (Chan a)
645 unReadChan :: Chan a -> a -> IO ()
646 getChanContents :: Chan a -> IO [a]
647 writeList2Chan :: Chan a -> [a] -> IO ()
649 data CVar a -- one element channels
650 newCVar :: IO (CVar a)
651 putCVar :: CVar a -> a -> IO ()
652 getCVar :: CVar a -> IO a
654 data QSem -- General/quantity semaphores
655 newQSem :: Int -> IO QSem
656 waitQSem :: QSem -> IO ()
657 signalQSem :: QSem -> IO ()
659 data QSemN -- General/quantity semaphores
660 newQSemN :: Int -> IO QSemN
661 waitQSemN :: QSemN -> Int -> IO ()
662 signalQSemN :: QSemN -> Int -> IO ()
664 type SampleVar a -- Sample variables
665 newEmptySampleVar:: IO (SampleVar a)
666 newSampleVar :: a -> IO (SampleVar a)
667 emptySampleVar :: SampleVar a -> IO ()
668 readSampleVar :: SampleVar a -> IO a
669 writeSampleVar :: SampleVar a -> a -> IO ()
676 GHC uses preemptive multitasking:
677 Context switches can occur at any time, except if you call a C
678 function (like \verb"getchar") that blocks waiting for input.
680 Hugs uses cooperative multitasking:
681 Context switches only occur when you use one of the primitives
682 defined in this module. This means that programs such as:
685 main = forkIO (write 'a') >> write 'b'
686 where write c = putChar c >> write c
689 will print either <tt/aaaaaaaaaaaaaa.../ or <tt/bbbbbbbbbbbb.../,
690 instead of some random interleaving of <tt/a/s and <tt/b/s.
692 In practice, cooperative multitasking is sufficient for writing
693 simple graphical user interfaces.
696 Hugs does not provide the functions <tt/mergeIO/ or <tt/nmergeIO/ since these
697 require preemptive multitasking.
700 Thread identities and <tt/killThread/ has not been implemented yet on
701 either system. The plan is that <tt/killThread/ will raise an IO
702 exception in the killed thread which it can catch --- perhaps allowing -->
703 --it to kill its children before exiting.
706 The <tt/Ord/ instance for <tt/ThreadId/s provides an arbitrary total ordering
707 which might be used to build an ordered binary tree, say.
712 <label id="sec:Pretty">
715 This library contains Simon Peyton Jones' implementation of John
716 Hughes's pretty printer combinators.
723 data Doc -- the Document datatype
725 -- The primitive Doc values
727 text :: String -> Doc
730 integer :: Integer -> Doc
731 float :: Float -> Doc
732 double :: Double -> Doc
733 rational :: Rational -> Doc
734 semi, comma, colon, space, equals :: Doc
735 lparen, rparen, lbrack, rbrack, lbrace, rbrace :: Doc
736 parens, brackets, braces :: Doc -> Doc
737 quotes, doubleQuotes :: Doc -> Doc
739 -- Combining Doc values
740 (<>) :: Doc -> Doc -> Doc -- Beside
741 hcat :: [Doc] -> Doc -- List version of <>
742 (<+>) :: Doc -> Doc -> Doc -- Beside, separated by space
743 hsep :: [Doc] -> Doc -- List version of <+>
744 ($$) :: Doc -> Doc -> Doc -- Above; if there is no
745 -- overlap it "dovetails" the two
746 vcat :: [Doc] -> Doc -- List version of $$
747 cat :: [Doc] -> Doc -- Either hcat or vcat
748 sep :: [Doc] -> Doc -- Either hsep or vcat
749 fcat :: [Doc] -> Doc -- ``Paragraph fill'' version of cat
750 fsep :: [Doc] -> Doc -- ``Paragraph fill'' version of sep
751 nest :: Int -> Doc -> Doc -- Nested
752 hang :: Doc -> Int -> Doc -> Doc
753 punctuate :: Doc -> [Doc] -> [Doc]
754 -- punctuate p [d1, ... dn] = [d1 <> p, d2 <> p, ... dn-1 <> p, dn]
756 -- Displaying Doc values
758 render :: Doc -> String -- Uses default style
759 renderStyle :: Style -> Doc -> String
760 data Style = Style { lineLength :: Int, -- In chars
761 ribbonsPerLine :: Float, -- Ratio of ribbon length
765 data Mode = PageMode -- Normal
766 | ZigZagMode -- With zig-zag cuts
767 | LeftMode -- No indentation, infinitely long lines
768 | OneLineMode -- All on one line
771 <biblio files="refs" style="abbrv">