2 <IndexTerm><Primary>language, GHC</Primary></IndexTerm>
3 <IndexTerm><Primary>extensions, GHC</Primary></IndexTerm>
4 As with all known Haskell systems, GHC implements some extensions to
5 the language. To use them, you'll need to give a <Option>-fglasgow-exts</Option>
6 <IndexTerm><Primary>-fglasgow-exts option</Primary></IndexTerm> option.
10 Virtually all of the Glasgow extensions serve to give you access to
11 the underlying facilities with which we implement Haskell. Thus, you
12 can get at the Raw Iron, if you are willing to write some non-standard
13 code at a more primitive level. You need not be “stuck” on
14 performance because of the implementation costs of Haskell's
15 “high-level” features—you can always code “under” them. In an extreme case, you can write all your time-critical code in C, and then just glue it together with Haskell!
19 Executive summary of our extensions:
26 <Term>Unboxed types and primitive operations:</Term>
29 You can get right down to the raw machine types and operations;
30 included in this are “primitive arrays” (direct access to Big Wads
31 of Bytes). Please see <XRef LinkEnd="glasgow-unboxed"> and following.
37 <Term>Multi-parameter type classes:</Term>
40 GHC's type system supports extended type classes with multiple
41 parameters. Please see <XRef LinkEnd="multi-param-type-classes">.
47 <Term>Local universal quantification:</Term>
50 GHC's type system supports explicit universal quantification in
51 constructor fields and function arguments. This is useful for things
52 like defining <Literal>runST</Literal> from the state-thread world. See <XRef LinkEnd="universal-quantification">.
58 <Term>Extistentially quantification in data types:</Term>
61 Some or all of the type variables in a datatype declaration may be
62 <Emphasis>existentially quantified</Emphasis>. More details in <XRef LinkEnd="existential-quantification">.
68 <Term>Scoped type variables:</Term>
71 Scoped type variables enable the programmer to supply type signatures
72 for some nested declarations, where this would not be legal in Haskell
73 98. Details in <XRef LinkEnd="scoped-type-variables">.
79 <Term>Pattern guards</Term>
82 Instead of being a boolean expression, a guard is a list of qualifiers, exactly as in a list comprehension. See <XRef LinkEnd="pattern-guards">.
88 <Term>Foreign calling:</Term>
91 Just what it sounds like. We provide <Emphasis>lots</Emphasis> of rope that you
92 can dangle around your neck. Please see <XRef LinkEnd="ffi">.
101 Pragmas are special instructions to the compiler placed in the source
102 file. The pragmas GHC supports are described in <XRef LinkEnd="pragmas">.
108 <Term>Rewrite rules:</Term>
111 The programmer can specify rewrite rules as part of the source program
112 (in a pragma). GHC applies these rewrite rules wherever it can.
113 Details in <XRef LinkEnd="rewrite-rules">.
121 Before you get too carried away working at the lowest level (e.g.,
122 sloshing <Literal>MutableByteArray#</Literal>s around your
123 program), you may wish to check if there are libraries that provide a
124 “Haskellised veneer” over the features you want. See
125 <xref linkend="book-hslibs">.
128 <Sect1 id="primitives">
129 <Title>Unboxed types and primitive operations
131 <IndexTerm><Primary>PrelGHC module</Primary></IndexTerm>
134 This module defines all the types which are primitive in Glasgow
135 Haskell, and the operations provided for them.
138 <Sect2 id="glasgow-unboxed">
143 <IndexTerm><Primary>Unboxed types (Glasgow extension)</Primary></IndexTerm>
146 <para>Most types in GHC are <firstterm>boxed</firstterm>, which means
147 that values of that type are represented by a pointer to a heap
148 object. The representation of a Haskell <literal>Int</literal>, for
149 example, is a two-word heap object. An <firstterm>unboxed</firstterm>
150 type, however, is represented by the value itself, no pointers or heap
151 allocation are involved.
155 Unboxed types correspond to the “raw machine” types you
156 would use in C: <Literal>Int#</Literal> (long int),
157 <Literal>Double#</Literal> (double), <Literal>Addr#</Literal>
158 (void *), etc. The <Emphasis>primitive operations</Emphasis>
159 (PrimOps) on these types are what you might expect; e.g.,
160 <Literal>(+#)</Literal> is addition on
161 <Literal>Int#</Literal>s, and is the machine-addition that we all
162 know and love—usually one instruction.
166 Primitive (unboxed) types cannot be defined in Haskell, and are
167 therefore built into the language and compiler. Primitive types are
168 always unlifted; that is, a value of a primitive type cannot be
169 bottom. We use the convention that primitive types, values, and
170 operations have a <Literal>#</Literal> suffix.
174 Primitive values are often represented by a simple bit-pattern, such
175 as <Literal>Int#</Literal>, <Literal>Float#</Literal>,
176 <Literal>Double#</Literal>. But this is not necessarily the case:
177 a primitive value might be represented by a pointer to a
178 heap-allocated object. Examples include
179 <Literal>Array#</Literal>, the type of primitive arrays. A
180 primitive array is heap-allocated because it is too big a value to fit
181 in a register, and would be too expensive to copy around; in a sense,
182 it is accidental that it is represented by a pointer. If a pointer
183 represents a primitive value, then it really does point to that value:
184 no unevaluated thunks, no indirections…nothing can be at the
185 other end of the pointer than the primitive value.
189 There are some restrictions on the use of primitive types, the main
190 one being that you can't pass a primitive value to a polymorphic
191 function or store one in a polymorphic data type. This rules out
192 things like <Literal>[Int#]</Literal> (i.e. lists of primitive
193 integers). The reason for this restriction is that polymorphic
194 arguments and constructor fields are assumed to be pointers: if an
195 unboxed integer is stored in one of these, the garbage collector would
196 attempt to follow it, leading to unpredictable space leaks. Or a
197 <Function>seq</Function> operation on the polymorphic component may
198 attempt to dereference the pointer, with disastrous results. Even
199 worse, the unboxed value might be larger than a pointer
200 (<Literal>Double#</Literal> for instance).
204 Nevertheless, A numerically-intensive program using unboxed types can
205 go a <Emphasis>lot</Emphasis> faster than its “standard”
206 counterpart—we saw a threefold speedup on one example.
211 <Sect2 id="unboxed-tuples">
212 <Title>Unboxed Tuples
216 Unboxed tuples aren't really exported by <Literal>PrelGHC</Literal>,
217 they're available by default with <Option>-fglasgow-exts</Option>. An
218 unboxed tuple looks like this:
230 where <Literal>e_1..e_n</Literal> are expressions of any
231 type (primitive or non-primitive). The type of an unboxed tuple looks
236 Unboxed tuples are used for functions that need to return multiple
237 values, but they avoid the heap allocation normally associated with
238 using fully-fledged tuples. When an unboxed tuple is returned, the
239 components are put directly into registers or on the stack; the
240 unboxed tuple itself does not have a composite representation. Many
241 of the primitive operations listed in this section return unboxed
246 There are some pretty stringent restrictions on the use of unboxed tuples:
255 Unboxed tuple types are subject to the same restrictions as
256 other unboxed types; i.e. they may not be stored in polymorphic data
257 structures or passed to polymorphic functions.
264 Unboxed tuples may only be constructed as the direct result of
265 a function, and may only be deconstructed with a <Literal>case</Literal> expression.
266 eg. the following are valid:
270 f x y = (# x+1, y-1 #)
271 g x = case f x x of { (# a, b #) -> a + b }
275 but the following are invalid:
289 No variable can have an unboxed tuple type. This is illegal:
293 f :: (# Int, Int #) -> (# Int, Int #)
298 because <VarName>x</VarName> has an unboxed tuple type.
308 Note: we may relax some of these restrictions in the future.
312 The <Literal>IO</Literal> and <Literal>ST</Literal> monads use unboxed tuples to avoid unnecessary
313 allocation during sequences of operations.
319 <Title>Character and numeric types</Title>
322 <IndexTerm><Primary>character types, primitive</Primary></IndexTerm>
323 <IndexTerm><Primary>numeric types, primitive</Primary></IndexTerm>
324 <IndexTerm><Primary>integer types, primitive</Primary></IndexTerm>
325 <IndexTerm><Primary>floating point types, primitive</Primary></IndexTerm>
326 There are the following obvious primitive types:
342 <IndexTerm><Primary><literal>Char#</literal></Primary></IndexTerm>
343 <IndexTerm><Primary><literal>Int#</literal></Primary></IndexTerm>
344 <IndexTerm><Primary><literal>Word#</literal></Primary></IndexTerm>
345 <IndexTerm><Primary><literal>Addr#</literal></Primary></IndexTerm>
346 <IndexTerm><Primary><literal>Float#</literal></Primary></IndexTerm>
347 <IndexTerm><Primary><literal>Double#</literal></Primary></IndexTerm>
348 <IndexTerm><Primary><literal>Int64#</literal></Primary></IndexTerm>
349 <IndexTerm><Primary><literal>Word64#</literal></Primary></IndexTerm>
353 If you really want to know their exact equivalents in C, see
354 <Filename>ghc/includes/StgTypes.h</Filename> in the GHC source tree.
358 Literals for these types may be written as follows:
367 'a'# a Char#; for weird characters, use '\o<octal>'#
368 "a"# an Addr# (a `char *')
371 <IndexTerm><Primary>literals, primitive</Primary></IndexTerm>
372 <IndexTerm><Primary>constants, primitive</Primary></IndexTerm>
373 <IndexTerm><Primary>numbers, primitive</Primary></IndexTerm>
379 <Title>Comparison operations</Title>
382 <IndexTerm><Primary>comparisons, primitive</Primary></IndexTerm>
383 <IndexTerm><Primary>operators, comparison</Primary></IndexTerm>
389 {>,>=,==,/=,<,<=}# :: Int# -> Int# -> Bool
391 {gt,ge,eq,ne,lt,le}Char# :: Char# -> Char# -> Bool
392 -- ditto for Word# and Addr#
395 <IndexTerm><Primary><literal>>#</literal></Primary></IndexTerm>
396 <IndexTerm><Primary><literal>>=#</literal></Primary></IndexTerm>
397 <IndexTerm><Primary><literal>==#</literal></Primary></IndexTerm>
398 <IndexTerm><Primary><literal>/=#</literal></Primary></IndexTerm>
399 <IndexTerm><Primary><literal><#</literal></Primary></IndexTerm>
400 <IndexTerm><Primary><literal><=#</literal></Primary></IndexTerm>
401 <IndexTerm><Primary><literal>gt{Char,Word,Addr}#</literal></Primary></IndexTerm>
402 <IndexTerm><Primary><literal>ge{Char,Word,Addr}#</literal></Primary></IndexTerm>
403 <IndexTerm><Primary><literal>eq{Char,Word,Addr}#</literal></Primary></IndexTerm>
404 <IndexTerm><Primary><literal>ne{Char,Word,Addr}#</literal></Primary></IndexTerm>
405 <IndexTerm><Primary><literal>lt{Char,Word,Addr}#</literal></Primary></IndexTerm>
406 <IndexTerm><Primary><literal>le{Char,Word,Addr}#</literal></Primary></IndexTerm>
412 <Title>Primitive-character operations</Title>
415 <IndexTerm><Primary>characters, primitive operations</Primary></IndexTerm>
416 <IndexTerm><Primary>operators, primitive character</Primary></IndexTerm>
422 ord# :: Char# -> Int#
423 chr# :: Int# -> Char#
426 <IndexTerm><Primary><literal>ord#</literal></Primary></IndexTerm>
427 <IndexTerm><Primary><literal>chr#</literal></Primary></IndexTerm>
433 <Title>Primitive-<Literal>Int</Literal> operations</Title>
436 <IndexTerm><Primary>integers, primitive operations</Primary></IndexTerm>
437 <IndexTerm><Primary>operators, primitive integer</Primary></IndexTerm>
443 {+,-,*,quotInt,remInt,gcdInt}# :: Int# -> Int# -> Int#
444 negateInt# :: Int# -> Int#
446 iShiftL#, iShiftRA#, iShiftRL# :: Int# -> Int# -> Int#
447 -- shift left, right arithmetic, right logical
449 addIntC#, subIntC#, mulIntC# :: Int# -> Int# -> (# Int#, Int# #)
450 -- add, subtract, multiply with carry
453 <IndexTerm><Primary><literal>+#</literal></Primary></IndexTerm>
454 <IndexTerm><Primary><literal>-#</literal></Primary></IndexTerm>
455 <IndexTerm><Primary><literal>*#</literal></Primary></IndexTerm>
456 <IndexTerm><Primary><literal>quotInt#</literal></Primary></IndexTerm>
457 <IndexTerm><Primary><literal>remInt#</literal></Primary></IndexTerm>
458 <IndexTerm><Primary><literal>gcdInt#</literal></Primary></IndexTerm>
459 <IndexTerm><Primary><literal>iShiftL#</literal></Primary></IndexTerm>
460 <IndexTerm><Primary><literal>iShiftRA#</literal></Primary></IndexTerm>
461 <IndexTerm><Primary><literal>iShiftRL#</literal></Primary></IndexTerm>
462 <IndexTerm><Primary><literal>addIntC#</literal></Primary></IndexTerm>
463 <IndexTerm><Primary><literal>subIntC#</literal></Primary></IndexTerm>
464 <IndexTerm><Primary><literal>mulIntC#</literal></Primary></IndexTerm>
465 <IndexTerm><Primary>shift operations, integer</Primary></IndexTerm>
469 <Emphasis>Note:</Emphasis> No error/overflow checking!
475 <Title>Primitive-<Literal>Double</Literal> and <Literal>Float</Literal> operations</Title>
478 <IndexTerm><Primary>floating point numbers, primitive</Primary></IndexTerm>
479 <IndexTerm><Primary>operators, primitive floating point</Primary></IndexTerm>
485 {+,-,*,/}## :: Double# -> Double# -> Double#
486 {<,<=,==,/=,>=,>}## :: Double# -> Double# -> Bool
487 negateDouble# :: Double# -> Double#
488 double2Int# :: Double# -> Int#
489 int2Double# :: Int# -> Double#
491 {plus,minux,times,divide}Float# :: Float# -> Float# -> Float#
492 {gt,ge,eq,ne,lt,le}Float# :: Float# -> Float# -> Bool
493 negateFloat# :: Float# -> Float#
494 float2Int# :: Float# -> Int#
495 int2Float# :: Int# -> Float#
501 <IndexTerm><Primary><literal>+##</literal></Primary></IndexTerm>
502 <IndexTerm><Primary><literal>-##</literal></Primary></IndexTerm>
503 <IndexTerm><Primary><literal>*##</literal></Primary></IndexTerm>
504 <IndexTerm><Primary><literal>/##</literal></Primary></IndexTerm>
505 <IndexTerm><Primary><literal><##</literal></Primary></IndexTerm>
506 <IndexTerm><Primary><literal><=##</literal></Primary></IndexTerm>
507 <IndexTerm><Primary><literal>==##</literal></Primary></IndexTerm>
508 <IndexTerm><Primary><literal>=/##</literal></Primary></IndexTerm>
509 <IndexTerm><Primary><literal>>=##</literal></Primary></IndexTerm>
510 <IndexTerm><Primary><literal>>##</literal></Primary></IndexTerm>
511 <IndexTerm><Primary><literal>negateDouble#</literal></Primary></IndexTerm>
512 <IndexTerm><Primary><literal>double2Int#</literal></Primary></IndexTerm>
513 <IndexTerm><Primary><literal>int2Double#</literal></Primary></IndexTerm>
517 <IndexTerm><Primary><literal>plusFloat#</literal></Primary></IndexTerm>
518 <IndexTerm><Primary><literal>minusFloat#</literal></Primary></IndexTerm>
519 <IndexTerm><Primary><literal>timesFloat#</literal></Primary></IndexTerm>
520 <IndexTerm><Primary><literal>divideFloat#</literal></Primary></IndexTerm>
521 <IndexTerm><Primary><literal>gtFloat#</literal></Primary></IndexTerm>
522 <IndexTerm><Primary><literal>geFloat#</literal></Primary></IndexTerm>
523 <IndexTerm><Primary><literal>eqFloat#</literal></Primary></IndexTerm>
524 <IndexTerm><Primary><literal>neFloat#</literal></Primary></IndexTerm>
525 <IndexTerm><Primary><literal>ltFloat#</literal></Primary></IndexTerm>
526 <IndexTerm><Primary><literal>leFloat#</literal></Primary></IndexTerm>
527 <IndexTerm><Primary><literal>negateFloat#</literal></Primary></IndexTerm>
528 <IndexTerm><Primary><literal>float2Int#</literal></Primary></IndexTerm>
529 <IndexTerm><Primary><literal>int2Float#</literal></Primary></IndexTerm>
533 And a full complement of trigonometric functions:
539 expDouble# :: Double# -> Double#
540 logDouble# :: Double# -> Double#
541 sqrtDouble# :: Double# -> Double#
542 sinDouble# :: Double# -> Double#
543 cosDouble# :: Double# -> Double#
544 tanDouble# :: Double# -> Double#
545 asinDouble# :: Double# -> Double#
546 acosDouble# :: Double# -> Double#
547 atanDouble# :: Double# -> Double#
548 sinhDouble# :: Double# -> Double#
549 coshDouble# :: Double# -> Double#
550 tanhDouble# :: Double# -> Double#
551 powerDouble# :: Double# -> Double# -> Double#
554 <IndexTerm><Primary>trigonometric functions, primitive</Primary></IndexTerm>
558 similarly for <Literal>Float#</Literal>.
562 There are two coercion functions for <Literal>Float#</Literal>/<Literal>Double#</Literal>:
568 float2Double# :: Float# -> Double#
569 double2Float# :: Double# -> Float#
572 <IndexTerm><Primary><literal>float2Double#</literal></Primary></IndexTerm>
573 <IndexTerm><Primary><literal>double2Float#</literal></Primary></IndexTerm>
577 The primitive version of <Function>decodeDouble</Function>
578 (<Function>encodeDouble</Function> is implemented as an external C
585 decodeDouble# :: Double# -> PrelNum.ReturnIntAndGMP
588 <IndexTerm><Primary><literal>encodeDouble#</literal></Primary></IndexTerm>
589 <IndexTerm><Primary><literal>decodeDouble#</literal></Primary></IndexTerm>
593 (And the same for <Literal>Float#</Literal>s.)
598 <Sect2 id="integer-operations">
599 <Title>Operations on/for <Literal>Integers</Literal> (interface to GMP)
603 <IndexTerm><Primary>arbitrary precision integers</Primary></IndexTerm>
604 <IndexTerm><Primary>Integer, operations on</Primary></IndexTerm>
608 We implement <Literal>Integers</Literal> (arbitrary-precision
609 integers) using the GNU multiple-precision (GMP) package (version
614 The data type for <Literal>Integer</Literal> is either a small
615 integer, represented by an <Literal>Int</Literal>, or a large integer
616 represented using the pieces required by GMP's
617 <Literal>MP_INT</Literal> in <Filename>gmp.h</Filename> (see
618 <Filename>gmp.info</Filename> in
619 <Filename>ghc/includes/runtime/gmp</Filename>). It comes out as:
625 data Integer = S# Int# -- small integers
626 | J# Int# ByteArray# -- large integers
629 <IndexTerm><Primary>Integer type</Primary></IndexTerm> The primitive
630 ops to support large <Literal>Integers</Literal> use the
631 “pieces” of the representation, and are as follows:
637 negateInteger# :: Int# -> ByteArray# -> Integer
639 {plus,minus,times}Integer#, gcdInteger#,
640 quotInteger#, remInteger#, divExactInteger#
641 :: Int# -> ByteArray#
642 -> Int# -> ByteArray#
643 -> (# Int#, ByteArray# #)
646 :: Int# -> ByteArray#
647 -> Int# -> ByteArray#
648 -> Int# -- -1 for <; 0 for ==; +1 for >
651 :: Int# -> ByteArray#
653 -> Int# -- -1 for <; 0 for ==; +1 for >
656 :: Int# -> ByteArray#
660 divModInteger#, quotRemInteger#
661 :: Int# -> ByteArray#
662 -> Int# -> ByteArray#
663 -> (# Int#, ByteArray#,
666 integer2Int# :: Int# -> ByteArray# -> Int#
668 int2Integer# :: Int# -> Integer -- NB: no error-checking on these two!
669 word2Integer# :: Word# -> Integer
671 addr2Integer# :: Addr# -> Integer
672 -- the Addr# is taken to be a `char *' string
673 -- to be converted into an Integer.
676 <IndexTerm><Primary><literal>negateInteger#</literal></Primary></IndexTerm>
677 <IndexTerm><Primary><literal>plusInteger#</literal></Primary></IndexTerm>
678 <IndexTerm><Primary><literal>minusInteger#</literal></Primary></IndexTerm>
679 <IndexTerm><Primary><literal>timesInteger#</literal></Primary></IndexTerm>
680 <IndexTerm><Primary><literal>quotInteger#</literal></Primary></IndexTerm>
681 <IndexTerm><Primary><literal>remInteger#</literal></Primary></IndexTerm>
682 <IndexTerm><Primary><literal>gcdInteger#</literal></Primary></IndexTerm>
683 <IndexTerm><Primary><literal>gcdIntegerInt#</literal></Primary></IndexTerm>
684 <IndexTerm><Primary><literal>divExactInteger#</literal></Primary></IndexTerm>
685 <IndexTerm><Primary><literal>cmpInteger#</literal></Primary></IndexTerm>
686 <IndexTerm><Primary><literal>divModInteger#</literal></Primary></IndexTerm>
687 <IndexTerm><Primary><literal>quotRemInteger#</literal></Primary></IndexTerm>
688 <IndexTerm><Primary><literal>integer2Int#</literal></Primary></IndexTerm>
689 <IndexTerm><Primary><literal>int2Integer#</literal></Primary></IndexTerm>
690 <IndexTerm><Primary><literal>word2Integer#</literal></Primary></IndexTerm>
691 <IndexTerm><Primary><literal>addr2Integer#</literal></Primary></IndexTerm>
697 <Title>Words and addresses</Title>
700 <IndexTerm><Primary>word, primitive type</Primary></IndexTerm>
701 <IndexTerm><Primary>address, primitive type</Primary></IndexTerm>
702 <IndexTerm><Primary>unsigned integer, primitive type</Primary></IndexTerm>
703 <IndexTerm><Primary>pointer, primitive type</Primary></IndexTerm>
707 A <Literal>Word#</Literal> is used for bit-twiddling operations.
708 It is the same size as an <Literal>Int#</Literal>, but has no sign
709 nor any arithmetic operations.
712 type Word# -- Same size/etc as Int# but *unsigned*
713 type Addr# -- A pointer from outside the "Haskell world" (from C, probably);
714 -- described under "arrays"
717 <IndexTerm><Primary><literal>Word#</literal></Primary></IndexTerm>
718 <IndexTerm><Primary><literal>Addr#</literal></Primary></IndexTerm>
722 <Literal>Word#</Literal>s and <Literal>Addr#</Literal>s have
723 the usual comparison operations. Other
724 unboxed-<Literal>Word</Literal> ops (bit-twiddling and coercions):
730 {gt,ge,eq,ne,lt,le}Word# :: Word# -> Word# -> Bool
732 and#, or#, xor# :: Word# -> Word# -> Word#
735 quotWord#, remWord# :: Word# -> Word# -> Word#
736 -- word (i.e. unsigned) versions are different from int
737 -- versions, so we have to provide these explicitly.
739 not# :: Word# -> Word#
741 shiftL#, shiftRL# :: Word# -> Int# -> Word#
742 -- shift left, right logical
744 int2Word# :: Int# -> Word# -- just a cast, really
745 word2Int# :: Word# -> Int#
748 <IndexTerm><Primary>bit operations, Word and Addr</Primary></IndexTerm>
749 <IndexTerm><Primary><literal>gtWord#</literal></Primary></IndexTerm>
750 <IndexTerm><Primary><literal>geWord#</literal></Primary></IndexTerm>
751 <IndexTerm><Primary><literal>eqWord#</literal></Primary></IndexTerm>
752 <IndexTerm><Primary><literal>neWord#</literal></Primary></IndexTerm>
753 <IndexTerm><Primary><literal>ltWord#</literal></Primary></IndexTerm>
754 <IndexTerm><Primary><literal>leWord#</literal></Primary></IndexTerm>
755 <IndexTerm><Primary><literal>and#</literal></Primary></IndexTerm>
756 <IndexTerm><Primary><literal>or#</literal></Primary></IndexTerm>
757 <IndexTerm><Primary><literal>xor#</literal></Primary></IndexTerm>
758 <IndexTerm><Primary><literal>not#</literal></Primary></IndexTerm>
759 <IndexTerm><Primary><literal>quotWord#</literal></Primary></IndexTerm>
760 <IndexTerm><Primary><literal>remWord#</literal></Primary></IndexTerm>
761 <IndexTerm><Primary><literal>shiftL#</literal></Primary></IndexTerm>
762 <IndexTerm><Primary><literal>shiftRA#</literal></Primary></IndexTerm>
763 <IndexTerm><Primary><literal>shiftRL#</literal></Primary></IndexTerm>
764 <IndexTerm><Primary><literal>int2Word#</literal></Primary></IndexTerm>
765 <IndexTerm><Primary><literal>word2Int#</literal></Primary></IndexTerm>
769 Unboxed-<Literal>Addr</Literal> ops (C casts, really):
772 {gt,ge,eq,ne,lt,le}Addr# :: Addr# -> Addr# -> Bool
774 int2Addr# :: Int# -> Addr#
775 addr2Int# :: Addr# -> Int#
776 addr2Integer# :: Addr# -> (# Int#, ByteArray# #)
779 <IndexTerm><Primary><literal>gtAddr#</literal></Primary></IndexTerm>
780 <IndexTerm><Primary><literal>geAddr#</literal></Primary></IndexTerm>
781 <IndexTerm><Primary><literal>eqAddr#</literal></Primary></IndexTerm>
782 <IndexTerm><Primary><literal>neAddr#</literal></Primary></IndexTerm>
783 <IndexTerm><Primary><literal>ltAddr#</literal></Primary></IndexTerm>
784 <IndexTerm><Primary><literal>leAddr#</literal></Primary></IndexTerm>
785 <IndexTerm><Primary><literal>int2Addr#</literal></Primary></IndexTerm>
786 <IndexTerm><Primary><literal>addr2Int#</literal></Primary></IndexTerm>
787 <IndexTerm><Primary><literal>addr2Integer#</literal></Primary></IndexTerm>
791 The casts between <Literal>Int#</Literal>,
792 <Literal>Word#</Literal> and <Literal>Addr#</Literal>
793 correspond to null operations at the machine level, but are required
794 to keep the Haskell type checker happy.
798 Operations for indexing off of C pointers
799 (<Literal>Addr#</Literal>s) to snatch values are listed under
800 “arrays”.
806 <Title>Arrays</Title>
809 <IndexTerm><Primary>arrays, primitive</Primary></IndexTerm>
813 The type <Literal>Array# elt</Literal> is the type of primitive,
814 unpointed arrays of values of type <Literal>elt</Literal>.
823 <IndexTerm><Primary><literal>Array#</literal></Primary></IndexTerm>
827 <Literal>Array#</Literal> is more primitive than a Haskell
828 array—indeed, the Haskell <Literal>Array</Literal> interface is
829 implemented using <Literal>Array#</Literal>—in that an
830 <Literal>Array#</Literal> is indexed only by
831 <Literal>Int#</Literal>s, starting at zero. It is also more
832 primitive by virtue of being unboxed. That doesn't mean that it isn't
833 a heap-allocated object—of course, it is. Rather, being unboxed
834 means that it is represented by a pointer to the array itself, and not
835 to a thunk which will evaluate to the array (or to bottom). The
836 components of an <Literal>Array#</Literal> are themselves boxed.
840 The type <Literal>ByteArray#</Literal> is similar to
841 <Literal>Array#</Literal>, except that it contains just a string
842 of (non-pointer) bytes.
851 <IndexTerm><Primary><literal>ByteArray#</literal></Primary></IndexTerm>
855 Arrays of these types are useful when a Haskell program wishes to
856 construct a value to pass to a C procedure. It is also possible to use
857 them to build (say) arrays of unboxed characters for internal use in a
858 Haskell program. Given these uses, <Literal>ByteArray#</Literal>
859 is deliberately a bit vague about the type of its components.
860 Operations are provided to extract values of type
861 <Literal>Char#</Literal>, <Literal>Int#</Literal>,
862 <Literal>Float#</Literal>, <Literal>Double#</Literal>, and
863 <Literal>Addr#</Literal> from arbitrary offsets within a
864 <Literal>ByteArray#</Literal>. (For type
865 <Literal>Foo#</Literal>, the $i$th offset gets you the $i$th
866 <Literal>Foo#</Literal>, not the <Literal>Foo#</Literal> at
867 byte-position $i$. Mumble.) (If you want a
868 <Literal>Word#</Literal>, grab an <Literal>Int#</Literal>,
873 Lastly, we have static byte-arrays, of type
874 <Literal>Addr#</Literal> [mentioned previously]. (Remember
875 the duality between arrays and pointers in C.) Arrays of this types
876 are represented by a pointer to an array in the world outside Haskell,
877 so this pointer is not followed by the garbage collector. In other
878 respects they are just like <Literal>ByteArray#</Literal>. They
879 are only needed in order to pass values from C to Haskell.
885 <Title>Reading and writing</Title>
888 Primitive arrays are linear, and indexed starting at zero.
892 The size and indices of a <Literal>ByteArray#</Literal>, <Literal>Addr#</Literal>, and
893 <Literal>MutableByteArray#</Literal> are all in bytes. It's up to the program to
894 calculate the correct byte offset from the start of the array. This
895 allows a <Literal>ByteArray#</Literal> to contain a mixture of values of different
896 type, which is often needed when preparing data for and unpicking
897 results from C. (Umm…not true of indices…WDP 95/09)
901 <Emphasis>Should we provide some <Literal>sizeOfDouble#</Literal> constants?</Emphasis>
905 Out-of-range errors on indexing should be caught by the code which
906 uses the primitive operation; the primitive operations themselves do
907 <Emphasis>not</Emphasis> check for out-of-range indexes. The intention is that the
908 primitive ops compile to one machine instruction or thereabouts.
912 We use the terms “reading” and “writing” to refer to accessing
913 <Emphasis>mutable</Emphasis> arrays (see <XRef LinkEnd="sect-mutable">), and
914 “indexing” to refer to reading a value from an <Emphasis>immutable</Emphasis>
919 Immutable byte arrays are straightforward to index (all indices in bytes):
922 indexCharArray# :: ByteArray# -> Int# -> Char#
923 indexIntArray# :: ByteArray# -> Int# -> Int#
924 indexAddrArray# :: ByteArray# -> Int# -> Addr#
925 indexFloatArray# :: ByteArray# -> Int# -> Float#
926 indexDoubleArray# :: ByteArray# -> Int# -> Double#
928 indexCharOffAddr# :: Addr# -> Int# -> Char#
929 indexIntOffAddr# :: Addr# -> Int# -> Int#
930 indexFloatOffAddr# :: Addr# -> Int# -> Float#
931 indexDoubleOffAddr# :: Addr# -> Int# -> Double#
932 indexAddrOffAddr# :: Addr# -> Int# -> Addr#
933 -- Get an Addr# from an Addr# offset
936 <IndexTerm><Primary><literal>indexCharArray#</literal></Primary></IndexTerm>
937 <IndexTerm><Primary><literal>indexIntArray#</literal></Primary></IndexTerm>
938 <IndexTerm><Primary><literal>indexAddrArray#</literal></Primary></IndexTerm>
939 <IndexTerm><Primary><literal>indexFloatArray#</literal></Primary></IndexTerm>
940 <IndexTerm><Primary><literal>indexDoubleArray#</literal></Primary></IndexTerm>
941 <IndexTerm><Primary><literal>indexCharOffAddr#</literal></Primary></IndexTerm>
942 <IndexTerm><Primary><literal>indexIntOffAddr#</literal></Primary></IndexTerm>
943 <IndexTerm><Primary><literal>indexFloatOffAddr#</literal></Primary></IndexTerm>
944 <IndexTerm><Primary><literal>indexDoubleOffAddr#</literal></Primary></IndexTerm>
945 <IndexTerm><Primary><literal>indexAddrOffAddr#</literal></Primary></IndexTerm>
949 The last of these, <Function>indexAddrOffAddr#</Function>, extracts an <Literal>Addr#</Literal> using an offset
950 from another <Literal>Addr#</Literal>, thereby providing the ability to follow a chain of
955 Something a bit more interesting goes on when indexing arrays of boxed
956 objects, because the result is simply the boxed object. So presumably
957 it should be entered—we never usually return an unevaluated
958 object! This is a pain: primitive ops aren't supposed to do
959 complicated things like enter objects. The current solution is to
960 return a single element unboxed tuple (see <XRef LinkEnd="unboxed-tuples">).
966 indexArray# :: Array# elt -> Int# -> (# elt #)
969 <IndexTerm><Primary><literal>indexArray#</literal></Primary></IndexTerm>
975 <Title>The state type</Title>
978 <IndexTerm><Primary><literal>state, primitive type</literal></Primary></IndexTerm>
979 <IndexTerm><Primary><literal>State#</literal></Primary></IndexTerm>
983 The primitive type <Literal>State#</Literal> represents the state of a state
984 transformer. It is parameterised on the desired type of state, which
985 serves to keep states from distinct threads distinct from one another.
986 But the <Emphasis>only</Emphasis> effect of this parameterisation is in the type
987 system: all values of type <Literal>State#</Literal> are represented in the same way.
988 Indeed, they are all represented by nothing at all! The code
989 generator “knows” to generate no code, and allocate no registers
990 etc, for primitive states.
1002 The type <Literal>GHC.RealWorld</Literal> is truly opaque: there are no values defined
1003 of this type, and no operations over it. It is “primitive” in that
1004 sense - but it is <Emphasis>not unlifted!</Emphasis> Its only role in life is to be
1005 the type which distinguishes the <Literal>IO</Literal> state transformer.
1019 <Title>State of the world</Title>
1022 A single, primitive, value of type <Literal>State# RealWorld</Literal> is provided.
1028 realWorld# :: State# RealWorld
1031 <IndexTerm><Primary>realWorld# state object</Primary></IndexTerm>
1035 (Note: in the compiler, not a <Literal>PrimOp</Literal>; just a mucho magic
1036 <Literal>Id</Literal>. Exported from <Literal>GHC</Literal>, though).
1041 <Sect2 id="sect-mutable">
1042 <Title>Mutable arrays</Title>
1045 <IndexTerm><Primary>mutable arrays</Primary></IndexTerm>
1046 <IndexTerm><Primary>arrays, mutable</Primary></IndexTerm>
1047 Corresponding to <Literal>Array#</Literal> and <Literal>ByteArray#</Literal>, we have the types of
1048 mutable versions of each. In each case, the representation is a
1049 pointer to a suitable block of (mutable) heap-allocated storage.
1055 type MutableArray# s elt
1056 type MutableByteArray# s
1059 <IndexTerm><Primary><literal>MutableArray#</literal></Primary></IndexTerm>
1060 <IndexTerm><Primary><literal>MutableByteArray#</literal></Primary></IndexTerm>
1064 <Title>Allocation</Title>
1067 <IndexTerm><Primary>mutable arrays, allocation</Primary></IndexTerm>
1068 <IndexTerm><Primary>arrays, allocation</Primary></IndexTerm>
1069 <IndexTerm><Primary>allocation, of mutable arrays</Primary></IndexTerm>
1073 Mutable arrays can be allocated. Only pointer-arrays are initialised;
1074 arrays of non-pointers are filled in by “user code” rather than by
1075 the array-allocation primitive. Reason: only the pointer case has to
1076 worry about GC striking with a partly-initialised array.
1082 newArray# :: Int# -> elt -> State# s -> (# State# s, MutableArray# s elt #)
1084 newCharArray# :: Int# -> State# s -> (# State# s, MutableByteArray# s elt #)
1085 newIntArray# :: Int# -> State# s -> (# State# s, MutableByteArray# s elt #)
1086 newAddrArray# :: Int# -> State# s -> (# State# s, MutableByteArray# s elt #)
1087 newFloatArray# :: Int# -> State# s -> (# State# s, MutableByteArray# s elt #)
1088 newDoubleArray# :: Int# -> State# s -> (# State# s, MutableByteArray# s elt #)
1091 <IndexTerm><Primary><literal>newArray#</literal></Primary></IndexTerm>
1092 <IndexTerm><Primary><literal>newCharArray#</literal></Primary></IndexTerm>
1093 <IndexTerm><Primary><literal>newIntArray#</literal></Primary></IndexTerm>
1094 <IndexTerm><Primary><literal>newAddrArray#</literal></Primary></IndexTerm>
1095 <IndexTerm><Primary><literal>newFloatArray#</literal></Primary></IndexTerm>
1096 <IndexTerm><Primary><literal>newDoubleArray#</literal></Primary></IndexTerm>
1100 The size of a <Literal>ByteArray#</Literal> is given in bytes.
1106 <Title>Reading and writing</Title>
1109 <IndexTerm><Primary>arrays, reading and writing</Primary></IndexTerm>
1115 readArray# :: MutableArray# s elt -> Int# -> State# s -> (# State# s, elt #)
1116 readCharArray# :: MutableByteArray# s -> Int# -> State# s -> (# State# s, Char# #)
1117 readIntArray# :: MutableByteArray# s -> Int# -> State# s -> (# State# s, Int# #)
1118 readAddrArray# :: MutableByteArray# s -> Int# -> State# s -> (# State# s, Addr# #)
1119 readFloatArray# :: MutableByteArray# s -> Int# -> State# s -> (# State# s, Float# #)
1120 readDoubleArray# :: MutableByteArray# s -> Int# -> State# s -> (# State# s, Double# #)
1122 writeArray# :: MutableArray# s elt -> Int# -> elt -> State# s -> State# s
1123 writeCharArray# :: MutableByteArray# s -> Int# -> Char# -> State# s -> State# s
1124 writeIntArray# :: MutableByteArray# s -> Int# -> Int# -> State# s -> State# s
1125 writeAddrArray# :: MutableByteArray# s -> Int# -> Addr# -> State# s -> State# s
1126 writeFloatArray# :: MutableByteArray# s -> Int# -> Float# -> State# s -> State# s
1127 writeDoubleArray# :: MutableByteArray# s -> Int# -> Double# -> State# s -> State# s
1130 <IndexTerm><Primary><literal>readArray#</literal></Primary></IndexTerm>
1131 <IndexTerm><Primary><literal>readCharArray#</literal></Primary></IndexTerm>
1132 <IndexTerm><Primary><literal>readIntArray#</literal></Primary></IndexTerm>
1133 <IndexTerm><Primary><literal>readAddrArray#</literal></Primary></IndexTerm>
1134 <IndexTerm><Primary><literal>readFloatArray#</literal></Primary></IndexTerm>
1135 <IndexTerm><Primary><literal>readDoubleArray#</literal></Primary></IndexTerm>
1136 <IndexTerm><Primary><literal>writeArray#</literal></Primary></IndexTerm>
1137 <IndexTerm><Primary><literal>writeCharArray#</literal></Primary></IndexTerm>
1138 <IndexTerm><Primary><literal>writeIntArray#</literal></Primary></IndexTerm>
1139 <IndexTerm><Primary><literal>writeAddrArray#</literal></Primary></IndexTerm>
1140 <IndexTerm><Primary><literal>writeFloatArray#</literal></Primary></IndexTerm>
1141 <IndexTerm><Primary><literal>writeDoubleArray#</literal></Primary></IndexTerm>
1147 <Title>Equality</Title>
1150 <IndexTerm><Primary>arrays, testing for equality</Primary></IndexTerm>
1154 One can take “equality” of mutable arrays. What is compared is the
1155 <Emphasis>name</Emphasis> or reference to the mutable array, not its contents.
1161 sameMutableArray# :: MutableArray# s elt -> MutableArray# s elt -> Bool
1162 sameMutableByteArray# :: MutableByteArray# s -> MutableByteArray# s -> Bool
1165 <IndexTerm><Primary><literal>sameMutableArray#</literal></Primary></IndexTerm>
1166 <IndexTerm><Primary><literal>sameMutableByteArray#</literal></Primary></IndexTerm>
1172 <Title>Freezing mutable arrays</Title>
1175 <IndexTerm><Primary>arrays, freezing mutable</Primary></IndexTerm>
1176 <IndexTerm><Primary>freezing mutable arrays</Primary></IndexTerm>
1177 <IndexTerm><Primary>mutable arrays, freezing</Primary></IndexTerm>
1181 Only unsafe-freeze has a primitive. (Safe freeze is done directly in Haskell
1182 by copying the array and then using <Function>unsafeFreeze</Function>.)
1188 unsafeFreezeArray# :: MutableArray# s elt -> State# s -> (# State# s, Array# s elt #)
1189 unsafeFreezeByteArray# :: MutableByteArray# s -> State# s -> (# State# s, ByteArray# #)
1192 <IndexTerm><Primary><literal>unsafeFreezeArray#</literal></Primary></IndexTerm>
1193 <IndexTerm><Primary><literal>unsafeFreezeByteArray#</literal></Primary></IndexTerm>
1201 <Title>Synchronizing variables (M-vars)</Title>
1204 <IndexTerm><Primary>synchronising variables (M-vars)</Primary></IndexTerm>
1205 <IndexTerm><Primary>M-Vars</Primary></IndexTerm>
1209 Synchronising variables are the primitive type used to implement
1210 Concurrent Haskell's MVars (see the Concurrent Haskell paper for
1211 the operational behaviour of these operations).
1217 type MVar# s elt -- primitive
1219 newMVar# :: State# s -> (# State# s, MVar# s elt #)
1220 takeMVar# :: SynchVar# s elt -> State# s -> (# State# s, elt #)
1221 putMVar# :: SynchVar# s elt -> State# s -> State# s
1224 <IndexTerm><Primary><literal>SynchVar#</literal></Primary></IndexTerm>
1225 <IndexTerm><Primary><literal>newSynchVar#</literal></Primary></IndexTerm>
1226 <IndexTerm><Primary><literal>takeMVar</literal></Primary></IndexTerm>
1227 <IndexTerm><Primary><literal>putMVar</literal></Primary></IndexTerm>
1234 <Sect1 id="glasgow-ST-monad">
1235 <Title>Primitive state-transformer monad
1239 <IndexTerm><Primary>state transformers (Glasgow extensions)</Primary></IndexTerm>
1240 <IndexTerm><Primary>ST monad (Glasgow extension)</Primary></IndexTerm>
1244 This monad underlies our implementation of arrays, mutable and
1245 immutable, and our implementation of I/O, including “C calls”.
1249 The <Literal>ST</Literal> library, which provides access to the
1250 <Function>ST</Function> monad, is described in <xref
1256 <Sect1 id="glasgow-prim-arrays">
1257 <Title>Primitive arrays, mutable and otherwise
1261 <IndexTerm><Primary>primitive arrays (Glasgow extension)</Primary></IndexTerm>
1262 <IndexTerm><Primary>arrays, primitive (Glasgow extension)</Primary></IndexTerm>
1266 GHC knows about quite a few flavours of Large Swathes of Bytes.
1270 First, GHC distinguishes between primitive arrays of (boxed) Haskell
1271 objects (type <Literal>Array# obj</Literal>) and primitive arrays of bytes (type
1272 <Literal>ByteArray#</Literal>).
1276 Second, it distinguishes between…
1280 <Term>Immutable:</Term>
1283 Arrays that do not change (as with “standard” Haskell arrays); you
1284 can only read from them. Obviously, they do not need the care and
1285 attention of the state-transformer monad.
1290 <Term>Mutable:</Term>
1293 Arrays that may be changed or “mutated.” All the operations on them
1294 live within the state-transformer monad and the updates happen
1295 <Emphasis>in-place</Emphasis>.
1300 <Term>“Static” (in C land):</Term>
1303 A C routine may pass an <Literal>Addr#</Literal> pointer back into Haskell land. There
1304 are then primitive operations with which you may merrily grab values
1305 over in C land, by indexing off the “static” pointer.
1310 <Term>“Stable” pointers:</Term>
1313 If, for some reason, you wish to hand a Haskell pointer (i.e.,
1314 <Emphasis>not</Emphasis> an unboxed value) to a C routine, you first make the
1315 pointer “stable,” so that the garbage collector won't forget that it
1316 exists. That is, GHC provides a safe way to pass Haskell pointers to
1321 Please see <XRef LinkEnd="glasgow-stablePtrs"> for more details.
1326 <Term>“Foreign objects”:</Term>
1329 A “foreign object” is a safe way to pass an external object (a
1330 C-allocated pointer, say) to Haskell and have Haskell do the Right
1331 Thing when it no longer references the object. So, for example, C
1332 could pass a large bitmap over to Haskell and say “please free this
1333 memory when you're done with it.”
1337 Please see <XRef LinkEnd="glasgow-foreignObjs"> for more details.
1345 The libraries documentatation gives more details on all these
1346 “primitive array” types and the operations on them.
1352 <Sect1 id="pattern-guards">
1353 <Title>Pattern guards</Title>
1356 <IndexTerm><Primary>Pattern guards (Glasgow extension)</Primary></IndexTerm>
1357 The discussion that follows is an abbreviated version of Simon Peyton Jones's original <ULink URL="http://research.microsoft.com/~simonpj/Haskell/guards.html">proposal</ULink>. (Note that the proposal was written before pattern guards were implemented, so refers to them as unimplemented.)
1361 Suppose we have an abstract data type of finite maps, with a
1365 lookup :: FiniteMap -> Int -> Maybe Int
1368 The lookup returns <Function>Nothing</Function> if the supplied key is not in the domain of the mapping, and <Function>(Just v)</Function> otherwise,
1369 where <VarName>v</VarName> is the value that the key maps to. Now consider the following definition:
1373 clunky env var1 var2 | ok1 && ok2 = val1 + val2
1374 | otherwise = var1 + var2
1376 m1 = lookup env var1
1377 m2 = lookup env var2
1378 ok1 = maybeToBool m1
1379 ok2 = maybeToBool m2
1380 val1 = expectJust m1
1381 val2 = expectJust m2
1385 The auxiliary functions are
1389 maybeToBool :: Maybe a -> Bool
1390 maybeToBool (Just x) = True
1391 maybeToBool Nothing = False
1393 expectJust :: Maybe a -> a
1394 expectJust (Just x) = x
1395 expectJust Nothing = error "Unexpected Nothing"
1399 What is <Function>clunky</Function> doing? The guard <Literal>ok1 &&
1400 ok2</Literal> checks that both lookups succeed, using
1401 <Function>maybeToBool</Function> to convert the <Function>Maybe</Function>
1402 types to booleans. The (lazily evaluated) <Function>expectJust</Function>
1403 calls extract the values from the results of the lookups, and binds the
1404 returned values to <VarName>val1</VarName> and <VarName>val2</VarName>
1405 respectively. If either lookup fails, then clunky takes the
1406 <Literal>otherwise</Literal> case and returns the sum of its arguments.
1410 This is certainly legal Haskell, but it is a tremendously verbose and
1411 un-obvious way to achieve the desired effect. Arguably, a more direct way
1412 to write clunky would be to use case expressions:
1416 clunky env var1 var1 = case lookup env var1 of
1418 Just val1 -> case lookup env var2 of
1420 Just val2 -> val1 + val2
1426 This is a bit shorter, but hardly better. Of course, we can rewrite any set
1427 of pattern-matching, guarded equations as case expressions; that is
1428 precisely what the compiler does when compiling equations! The reason that
1429 Haskell provides guarded equations is because they allow us to write down
1430 the cases we want to consider, one at a time, independently of each other.
1431 This structure is hidden in the case version. Two of the right-hand sides
1432 are really the same (<Function>fail</Function>), and the whole expression
1433 tends to become more and more indented.
1437 Here is how I would write clunky:
1441 clunky env var1 var1
1442 | Just val1 <- lookup env var1
1443 , Just val2 <- lookup env var2
1445 ...other equations for clunky...
1449 The semantics should be clear enough. The qualifers are matched in order.
1450 For a <Literal><-</Literal> qualifier, which I call a pattern guard, the
1451 right hand side is evaluated and matched against the pattern on the left.
1452 If the match fails then the whole guard fails and the next equation is
1453 tried. If it succeeds, then the appropriate binding takes place, and the
1454 next qualifier is matched, in the augmented environment. Unlike list
1455 comprehensions, however, the type of the expression to the right of the
1456 <Literal><-</Literal> is the same as the type of the pattern to its
1457 left. The bindings introduced by pattern guards scope over all the
1458 remaining guard qualifiers, and over the right hand side of the equation.
1462 Just as with list comprehensions, boolean expressions can be freely mixed
1463 with among the pattern guards. For example:
1474 Haskell's current guards therefore emerge as a special case, in which the
1475 qualifier list has just one element, a boolean expression.
1479 <Sect1 id="sec-ffi">
1480 <Title>The foreign interface</Title>
1483 The foreign interface consists of language and library support. The former
1484 is described later in <XRef LinkEnd="ffi">; the latter is outlined below,
1485 and detailed in <XRef LinkEnd="sec-Foreign">.
1488 <Sect2 id="glasgow-foreign-headers">
1489 <Title>Using function headers
1493 <IndexTerm><Primary>C calls, function headers</Primary></IndexTerm>
1497 When generating C (using the <Option>-fvia-C</Option> directive), one can assist the
1498 C compiler in detecting type errors by using the <Command>-#include</Command> directive
1499 to provide <Filename>.h</Filename> files containing function headers.
1511 void initialiseEFS (HsInt size);
1512 HsInt terminateEFS (void);
1513 HsForeignObj emptyEFS(void);
1514 HsForeignObj updateEFS (HsForeignObj a, HsInt i, HsInt x);
1515 HsInt lookupEFS (HsForeignObj a, HsInt i);
1519 <para>The types <literal>HsInt</literal>,
1520 <literal>HsForeignObj</literal> etc. are described in <xref
1521 linkend="sec-mapping-table">.</Para>
1523 <Para>Note that this approach is only
1524 <Emphasis>essential</Emphasis> for returning
1525 <Literal>float</Literal>s (or if <Literal>sizeof(int) !=
1526 sizeof(int *)</Literal> on your architecture) but is a Good
1527 Thing for anyone who cares about writing solid code. You're
1528 crazy not to do it.</Para>
1532 <Sect2 id="glasgow-stablePtrs">
1533 <Title>Subverting automatic unboxing with “stable pointers”
1537 <IndexTerm><Primary>stable pointers (Glasgow extension)</Primary></IndexTerm>
1541 The arguments of a <Function>_ccall_</Function> automatically unboxed before the
1542 call. There are two reasons why this is usually the Right Thing to
1552 C is a strict language: it would be excessively tedious to pass
1553 unevaluated arguments and require the C programmer to force their
1554 evaluation before using them.
1561 Boxed values are stored on the Haskell heap and may be moved
1562 within the heap if a garbage collection occurs—that is, pointers
1563 to boxed objects are not <Emphasis>stable</Emphasis>.
1572 It is possible to subvert the unboxing process by creating a “stable
1573 pointer” to a value and passing the stable pointer instead. For
1574 example, to pass/return an integer lazily to C functions <Function>storeC</Function> and
1575 <Function>fetchC</Function> might write:
1581 storeH :: Int -> IO ()
1582 storeH x = makeStablePtr x >>= \ stable_x ->
1583 _ccall_ storeC stable_x
1586 fetchH x = _ccall_ fetchC >>= \ stable_x ->
1587 deRefStablePtr stable_x >>= \ x ->
1588 freeStablePtr stable_x >>
1595 The garbage collector will refrain from throwing a stable pointer away
1596 until you explicitly call one of the following from C or Haskell.
1602 void freeStablePointer( StgStablePtr stablePtrToToss )
1603 freeStablePtr :: StablePtr a -> IO ()
1609 As with the use of <Function>free</Function> in C programs, GREAT CARE SHOULD BE
1610 EXERCISED to ensure these functions are called at the right time: too
1611 early and you get dangling references (and, if you're lucky, an error
1612 message from the runtime system); too late and you get space leaks.
1616 And to force evaluation of the argument within <Function>fooC</Function>, one would
1617 call one of the following C functions (according to type of argument).
1623 void performIO ( StgStablePtr stableIndex /* StablePtr s (IO ()) */ );
1624 StgInt enterInt ( StgStablePtr stableIndex /* StablePtr s Int */ );
1625 StgFloat enterFloat ( StgStablePtr stableIndex /* StablePtr s Float */ );
1631 <IndexTerm><Primary>performIO</Primary></IndexTerm>
1632 <IndexTerm><Primary>enterInt</Primary></IndexTerm>
1633 <IndexTerm><Primary>enterFloat</Primary></IndexTerm>
1637 Nota Bene: <Function>_ccall_GC_</Function><IndexTerm><Primary>_ccall_GC_</Primary></IndexTerm> must be used if any of
1638 these functions are used.
1643 <Sect2 id="glasgow-foreignObjs">
1644 <Title>Foreign objects: pointing outside the Haskell heap
1648 <IndexTerm><Primary>foreign objects (Glasgow extension)</Primary></IndexTerm>
1652 There are two types that GHC programs can use to reference
1653 (heap-allocated) objects outside the Haskell world: <Literal>Addr</Literal> and
1654 <Literal>ForeignObj</Literal>.
1658 If you use <Literal>Addr</Literal>, it is up to you to the programmer to arrange
1659 allocation and deallocation of the objects.
1663 If you use <Literal>ForeignObj</Literal>, GHC's garbage collector will call upon the
1664 user-supplied <Emphasis>finaliser</Emphasis> function to free the object when the
1665 Haskell world no longer can access the object. (An object is
1666 associated with a finaliser function when the abstract
1667 Haskell type <Literal>ForeignObj</Literal> is created). The finaliser function is
1668 expressed in C, and is passed as argument the object:
1674 void foreignFinaliser ( StgForeignObj fo )
1680 when the Haskell world can no longer access the object. Since
1681 <Literal>ForeignObj</Literal>s only get released when a garbage collection occurs, we
1682 provide ways of triggering a garbage collection from within C and from
1689 void GarbageCollect()
1696 More information on the programmers' interface to <Literal>ForeignObj</Literal> can be
1697 found in the library documentation.
1702 <Sect2 id="glasgow-avoiding-monads">
1703 <Title>Avoiding monads
1707 <IndexTerm><Primary>C calls to `pure C'</Primary></IndexTerm>
1708 <IndexTerm><Primary>unsafePerformIO</Primary></IndexTerm>
1712 The <Function>_ccall_</Function> construct is part of the <Literal>IO</Literal> monad because 9 out of 10
1713 uses will be to call imperative functions with side effects such as
1714 <Function>printf</Function>. Use of the monad ensures that these operations happen in a
1715 predictable order in spite of laziness and compiler optimisations.
1719 To avoid having to be in the monad to call a C function, it is
1720 possible to use <Function>unsafePerformIO</Function>, which is available from the
1721 <Literal>IOExts</Literal> module. There are three situations where one might like to
1722 call a C function from outside the IO world:
1731 Calling a function with no side-effects:
1734 atan2d :: Double -> Double -> Double
1735 atan2d y x = unsafePerformIO (_ccall_ atan2d y x)
1737 sincosd :: Double -> (Double, Double)
1738 sincosd x = unsafePerformIO $ do
1739 da <- newDoubleArray (0, 1)
1740 _casm_ “sincosd( %0, &((double *)%1[0]), &((double *)%1[1]) );” x da
1741 s <- readDoubleArray da 0
1742 c <- readDoubleArray da 1
1752 Calling a set of functions which have side-effects but which can
1753 be used in a purely functional manner.
1755 For example, an imperative implementation of a purely functional
1756 lookup-table might be accessed using the following functions.
1761 update :: EFS x -> Int -> x -> EFS x
1762 lookup :: EFS a -> Int -> a
1764 empty = unsafePerformIO (_ccall_ emptyEFS)
1766 update a i x = unsafePerformIO $
1767 makeStablePtr x >>= \ stable_x ->
1768 _ccall_ updateEFS a i stable_x
1770 lookup a i = unsafePerformIO $
1771 _ccall_ lookupEFS a i >>= \ stable_x ->
1772 deRefStablePtr stable_x
1776 You will almost always want to use <Literal>ForeignObj</Literal>s with this.
1783 Calling a side-effecting function even though the results will
1784 be unpredictable. For example the <Function>trace</Function> function is defined by:
1788 trace :: String -> a -> a
1791 ((_ccall_ PreTraceHook sTDERR{-msg-}):: IO ()) >>
1792 fputs sTDERR string >>
1793 ((_ccall_ PostTraceHook sTDERR{-msg-}):: IO ()) >>
1796 sTDERR = (“stderr” :: Addr)
1800 (This kind of use is not highly recommended—it is only really
1801 useful in debugging code.)
1811 <Sect2 id="ccall-gotchas">
1812 <Title>C-calling “gotchas” checklist
1816 <IndexTerm><Primary>C call dangers</Primary></IndexTerm>
1817 <IndexTerm><Primary>CCallable</Primary></IndexTerm>
1818 <IndexTerm><Primary>CReturnable</Primary></IndexTerm>
1822 And some advice, too.
1831 For modules that use <Function>_ccall_</Function>s, etc., compile with
1832 <Option>-fvia-C</Option>.<IndexTerm><Primary>-fvia-C option</Primary></IndexTerm> You don't have to, but you should.
1834 Also, use the <Option>-#include "prototypes.h"</Option> flag (hack) to inform the C
1835 compiler of the fully-prototyped types of all the C functions you
1836 call. (<XRef LinkEnd="glasgow-foreign-headers"> says more about this…)
1838 This scheme is the <Emphasis>only</Emphasis> way that you will get <Emphasis>any</Emphasis>
1839 typechecking of your <Function>_ccall_</Function>s. (It shouldn't be that way, but…).
1840 GHC will pass the flag <Option>-Wimplicit</Option> to <Command>gcc</Command> so that you'll get warnings
1841 if any <Function>_ccall_</Function>ed functions have no prototypes.
1848 Try to avoid <Function>_ccall_</Function>s to C functions that take <Literal>float</Literal>
1849 arguments or return <Literal>float</Literal> results. Reason: if you do, you will
1850 become entangled in (ANSI?) C's rules for when arguments/results are
1851 promoted to <Literal>doubles</Literal>. It's a nightmare and just not worth it.
1852 Use <Literal>doubles</Literal> if possible.
1854 If you do use <Literal>floats</Literal>, check and re-check that the right thing is
1855 happening. Perhaps compile with <Option>-keep-hc-file-too</Option> and look at
1856 the intermediate C (<Function>.hc</Function>).
1863 The compiler uses two non-standard type-classes when
1864 type-checking the arguments and results of <Function>_ccall_</Function>: the arguments
1865 (respectively result) of <Function>_ccall_</Function> must be instances of the class
1866 <Literal>CCallable</Literal> (respectively <Literal>CReturnable</Literal>). Both classes may be
1867 imported from the module <Literal>CCall</Literal>, but this should only be
1868 necessary if you want to define a new instance. (Neither class
1869 defines any methods—their only function is to keep the
1870 type-checker happy.)
1872 The type checker must be able to figure out just which of the
1873 C-callable/returnable types is being used. If it can't, you have to
1874 add type signatures. For example,
1882 is not good enough, because the compiler can't work out what type <VarName>x</VarName>
1883 is, nor what type the <Function>_ccall_</Function> returns. You have to write, say:
1887 f :: Int -> IO Double
1892 This table summarises the standard instances of these classes.
1896 <ColSpec Align="Left" Colsep="0">
1897 <ColSpec Align="Left" Colsep="0">
1898 <ColSpec Align="Left" Colsep="0">
1899 <ColSpec Align="Left" Colsep="0">
1902 <Entry><Emphasis>Type</Emphasis> </Entry>
1903 <Entry><Emphasis>CCallable</Emphasis></Entry>
1904 <Entry><Emphasis>CReturnable</Emphasis> </Entry>
1905 <Entry><Emphasis>Which is probably…</Emphasis> </Entry>
1909 <Literal>Char</Literal> </Entry>
1910 <Entry> Yes </Entry>
1911 <Entry> Yes </Entry>
1912 <Entry> <Literal>unsigned char</Literal> </Entry>
1916 <Literal>Int</Literal> </Entry>
1917 <Entry> Yes </Entry>
1918 <Entry> Yes </Entry>
1919 <Entry> <Literal>long int</Literal> </Entry>
1923 <Literal>Word</Literal> </Entry>
1924 <Entry> Yes </Entry>
1925 <Entry> Yes </Entry>
1926 <Entry> <Literal>unsigned long int</Literal> </Entry>
1930 <Literal>Addr</Literal> </Entry>
1931 <Entry> Yes </Entry>
1932 <Entry> Yes </Entry>
1933 <Entry> <Literal>void *</Literal> </Entry>
1937 <Literal>Float</Literal> </Entry>
1938 <Entry> Yes </Entry>
1939 <Entry> Yes </Entry>
1940 <Entry> <Literal>float</Literal> </Entry>
1944 <Literal>Double</Literal> </Entry>
1945 <Entry> Yes </Entry>
1946 <Entry> Yes </Entry>
1947 <Entry> <Literal>double</Literal> </Entry>
1951 <Literal>()</Literal> </Entry>
1953 <Entry> Yes </Entry>
1954 <Entry> <Literal>void</Literal> </Entry>
1958 <Literal>[Char]</Literal> </Entry>
1959 <Entry> Yes </Entry>
1961 <Entry> <Literal>char *</Literal> (null-terminated) </Entry>
1965 <Literal>Array</Literal> </Entry>
1966 <Entry> Yes </Entry>
1968 <Entry> <Literal>unsigned long *</Literal> </Entry>
1972 <Literal>ByteArray</Literal> </Entry>
1973 <Entry> Yes </Entry>
1975 <Entry> <Literal>unsigned long *</Literal> </Entry>
1979 <Literal>MutableArray</Literal> </Entry>
1980 <Entry> Yes </Entry>
1982 <Entry> <Literal>unsigned long *</Literal> </Entry>
1986 <Literal>MutableByteArray</Literal> </Entry>
1987 <Entry> Yes </Entry>
1989 <Entry> <Literal>unsigned long *</Literal> </Entry>
1993 <Literal>State</Literal> </Entry>
1994 <Entry> Yes </Entry>
1995 <Entry> Yes </Entry>
1996 <Entry> nothing!</Entry>
2000 <Literal>StablePtr</Literal> </Entry>
2001 <Entry> Yes </Entry>
2002 <Entry> Yes </Entry>
2003 <Entry> <Literal>unsigned long *</Literal> </Entry>
2007 <Literal>ForeignObjs</Literal> </Entry>
2008 <Entry> Yes </Entry>
2009 <Entry> Yes </Entry>
2010 <Entry> see later </Entry>
2018 Actually, the <Literal>Word</Literal> type is defined as being the same size as a
2019 pointer on the target architecture, which is <Emphasis>probably</Emphasis>
2020 <Literal>unsigned long int</Literal>.
2022 The brave and careful programmer can add their own instances of these
2023 classes for the following types:
2030 A <Emphasis>boxed-primitive</Emphasis> type may be made an instance of both
2031 <Literal>CCallable</Literal> and <Literal>CReturnable</Literal>.
2033 A boxed primitive type is any data type with a
2034 single unary constructor with a single primitive argument. For
2035 example, the following are all boxed primitive types:
2041 data XDisplay = XDisplay Addr#
2042 data EFS a = EFS# ForeignObj#
2048 instance CCallable (EFS a)
2049 instance CReturnable (EFS a)
2058 Any datatype with a single nullary constructor may be made an
2059 instance of <Literal>CReturnable</Literal>. For example:
2063 data MyVoid = MyVoid
2064 instance CReturnable MyVoid
2073 As at version 2.09, <Literal>String</Literal> (i.e., <Literal>[Char]</Literal>) is still
2074 not a <Literal>CReturnable</Literal> type.
2076 Also, the now-builtin type <Literal>PackedString</Literal> is neither
2077 <Literal>CCallable</Literal> nor <Literal>CReturnable</Literal>. (But there are functions in
2078 the PackedString interface to let you get at the necessary bits…)
2090 The code-generator will complain if you attempt to use <Literal>%r</Literal> in
2091 a <Literal>_casm_</Literal> whose result type is <Literal>IO ()</Literal>; or if you don't use <Literal>%r</Literal>
2092 <Emphasis>precisely</Emphasis> once for any other result type. These messages are
2093 supposed to be helpful and catch bugs—please tell us if they wreck
2101 If you call out to C code which may trigger the Haskell garbage
2102 collector or create new threads (examples of this later…), then you
2103 must use the <Function>_ccall_GC_</Function><IndexTerm><Primary>_ccall_GC_ primitive</Primary></IndexTerm> or
2104 <Function>_casm_GC_</Function><IndexTerm><Primary>_casm_GC_ primitive</Primary></IndexTerm> variant of C-calls. (This
2105 does not work with the native code generator—use <Option>-fvia-C</Option>.) This
2106 stuff is hairy with a capital H!
2118 <Sect1 id="multi-param-type-classes">
2119 <Title>Multi-parameter type classes
2123 This section documents GHC's implementation of multi-parameter type
2124 classes. There's lots of background in the paper <ULink
2125 URL="http://research.microsoft.com/~simonpj/multi.ps.gz" >Type
2126 classes: exploring the design space</ULink > (Simon Peyton Jones, Mark
2127 Jones, Erik Meijer).
2131 I'd like to thank people who reported shorcomings in the GHC 3.02
2132 implementation. Our default decisions were all conservative ones, and
2133 the experience of these heroic pioneers has given useful concrete
2134 examples to support several generalisations. (These appear below as
2135 design choices not implemented in 3.02.)
2139 I've discussed these notes with Mark Jones, and I believe that Hugs
2140 will migrate towards the same design choices as I outline here.
2141 Thanks to him, and to many others who have offered very useful
2146 <Title>Types</Title>
2149 There are the following restrictions on the form of a qualified
2156 forall tv1..tvn (c1, ...,cn) => type
2162 (Here, I write the "foralls" explicitly, although the Haskell source
2163 language omits them; in Haskell 1.4, all the free type variables of an
2164 explicit source-language type signature are universally quantified,
2165 except for the class type variables in a class declaration. However,
2166 in GHC, you can give the foralls if you want. See <XRef LinkEnd="universal-quantification">).
2175 <Emphasis>Each universally quantified type variable
2176 <Literal>tvi</Literal> must be mentioned (i.e. appear free) in <Literal>type</Literal></Emphasis>.
2178 The reason for this is that a value with a type that does not obey
2179 this restriction could not be used without introducing
2180 ambiguity. Here, for example, is an illegal type:
2184 forall a. Eq a => Int
2188 When a value with this type was used, the constraint <Literal>Eq tv</Literal>
2189 would be introduced where <Literal>tv</Literal> is a fresh type variable, and
2190 (in the dictionary-translation implementation) the value would be
2191 applied to a dictionary for <Literal>Eq tv</Literal>. The difficulty is that we
2192 can never know which instance of <Literal>Eq</Literal> to use because we never
2193 get any more information about <Literal>tv</Literal>.
2200 <Emphasis>Every constraint <Literal>ci</Literal> must mention at least one of the
2201 universally quantified type variables <Literal>tvi</Literal></Emphasis>.
2203 For example, this type is OK because <Literal>C a b</Literal> mentions the
2204 universally quantified type variable <Literal>b</Literal>:
2208 forall a. C a b => burble
2212 The next type is illegal because the constraint <Literal>Eq b</Literal> does not
2213 mention <Literal>a</Literal>:
2217 forall a. Eq b => burble
2221 The reason for this restriction is milder than the other one. The
2222 excluded types are never useful or necessary (because the offending
2223 context doesn't need to be witnessed at this point; it can be floated
2224 out). Furthermore, floating them out increases sharing. Lastly,
2225 excluding them is a conservative choice; it leaves a patch of
2226 territory free in case we need it later.
2236 These restrictions apply to all types, whether declared in a type signature
2241 Unlike Haskell 1.4, constraints in types do <Emphasis>not</Emphasis> have to be of
2242 the form <Emphasis>(class type-variables)</Emphasis>. Thus, these type signatures
2249 f :: Eq (m a) => [m a] -> [m a]
2256 This choice recovers principal types, a property that Haskell 1.4 does not have.
2262 <Title>Class declarations</Title>
2270 <Emphasis>Multi-parameter type classes are permitted</Emphasis>. For example:
2274 class Collection c a where
2275 union :: c a -> c a -> c a
2286 <Emphasis>The class hierarchy must be acyclic</Emphasis>. However, the definition
2287 of "acyclic" involves only the superclass relationships. For example,
2293 op :: D b => a -> b -> b
2296 class C a => D a where { ... }
2300 Here, <Literal>C</Literal> is a superclass of <Literal>D</Literal>, but it's OK for a
2301 class operation <Literal>op</Literal> of <Literal>C</Literal> to mention <Literal>D</Literal>. (It
2302 would not be OK for <Literal>D</Literal> to be a superclass of <Literal>C</Literal>.)
2309 <Emphasis>There are no restrictions on the context in a class declaration
2310 (which introduces superclasses), except that the class hierarchy must
2311 be acyclic</Emphasis>. So these class declarations are OK:
2315 class Functor (m k) => FiniteMap m k where
2318 class (Monad m, Monad (t m)) => Transform t m where
2319 lift :: m a -> (t m) a
2328 <Emphasis>In the signature of a class operation, every constraint
2329 must mention at least one type variable that is not a class type
2330 variable</Emphasis>.
2336 class Collection c a where
2337 mapC :: Collection c b => (a->b) -> c a -> c b
2341 is OK because the constraint <Literal>(Collection a b)</Literal> mentions
2342 <Literal>b</Literal>, even though it also mentions the class variable
2343 <Literal>a</Literal>. On the other hand:
2348 op :: Eq a => (a,b) -> (a,b)
2352 is not OK because the constraint <Literal>(Eq a)</Literal> mentions on the class
2353 type variable <Literal>a</Literal>, but not <Literal>b</Literal>. However, any such
2354 example is easily fixed by moving the offending context up to the
2359 class Eq a => C a where
2364 A yet more relaxed rule would allow the context of a class-op signature
2365 to mention only class type variables. However, that conflicts with
2366 Rule 1(b) for types above.
2373 <Emphasis>The type of each class operation must mention <Emphasis>all</Emphasis> of
2374 the class type variables</Emphasis>. For example:
2378 class Coll s a where
2380 insert :: s -> a -> s
2384 is not OK, because the type of <Literal>empty</Literal> doesn't mention
2385 <Literal>a</Literal>. This rule is a consequence of Rule 1(a), above, for
2386 types, and has the same motivation.
2388 Sometimes, offending class declarations exhibit misunderstandings. For
2389 example, <Literal>Coll</Literal> might be rewritten
2393 class Coll s a where
2395 insert :: s a -> a -> s a
2399 which makes the connection between the type of a collection of
2400 <Literal>a</Literal>'s (namely <Literal>(s a)</Literal>) and the element type <Literal>a</Literal>.
2401 Occasionally this really doesn't work, in which case you can split the
2409 class CollE s => Coll s a where
2410 insert :: s -> a -> s
2424 <Title>Instance declarations</Title>
2432 <Emphasis>Instance declarations may not overlap</Emphasis>. The two instance
2437 instance context1 => C type1 where ...
2438 instance context2 => C type2 where ...
2442 "overlap" if <Literal>type1</Literal> and <Literal>type2</Literal> unify
2444 However, if you give the command line option
2445 <Option>-fallow-overlapping-instances</Option><IndexTerm><Primary>-fallow-overlapping-instances
2446 option</Primary></IndexTerm> then two overlapping instance declarations are permitted
2454 EITHER <Literal>type1</Literal> and <Literal>type2</Literal> do not unify
2460 OR <Literal>type2</Literal> is a substitution instance of <Literal>type1</Literal>
2461 (but not identical to <Literal>type1</Literal>)
2474 Notice that these rules
2481 make it clear which instance decl to use
2482 (pick the most specific one that matches)
2489 do not mention the contexts <Literal>context1</Literal>, <Literal>context2</Literal>
2490 Reason: you can pick which instance decl
2491 "matches" based on the type.
2498 Regrettably, GHC doesn't guarantee to detect overlapping instance
2499 declarations if they appear in different modules. GHC can "see" the
2500 instance declarations in the transitive closure of all the modules
2501 imported by the one being compiled, so it can "see" all instance decls
2502 when it is compiling <Literal>Main</Literal>. However, it currently chooses not
2503 to look at ones that can't possibly be of use in the module currently
2504 being compiled, in the interests of efficiency. (Perhaps we should
2505 change that decision, at least for <Literal>Main</Literal>.)
2512 <Emphasis>There are no restrictions on the type in an instance
2513 <Emphasis>head</Emphasis>, except that at least one must not be a type variable</Emphasis>.
2514 The instance "head" is the bit after the "=>" in an instance decl. For
2515 example, these are OK:
2519 instance C Int a where ...
2521 instance D (Int, Int) where ...
2523 instance E [[a]] where ...
2527 Note that instance heads <Emphasis>may</Emphasis> contain repeated type variables.
2528 For example, this is OK:
2532 instance Stateful (ST s) (MutVar s) where ...
2536 The "at least one not a type variable" restriction is to ensure that
2537 context reduction terminates: each reduction step removes one type
2538 constructor. For example, the following would make the type checker
2539 loop if it wasn't excluded:
2543 instance C a => C a where ...
2547 There are two situations in which the rule is a bit of a pain. First,
2548 if one allows overlapping instance declarations then it's quite
2549 convenient to have a "default instance" declaration that applies if
2550 something more specific does not:
2559 Second, sometimes you might want to use the following to get the
2560 effect of a "class synonym":
2564 class (C1 a, C2 a, C3 a) => C a where { }
2566 instance (C1 a, C2 a, C3 a) => C a where { }
2570 This allows you to write shorter signatures:
2582 f :: (C1 a, C2 a, C3 a) => ...
2586 I'm on the lookout for a simple rule that preserves decidability while
2587 allowing these idioms. The experimental flag
2588 <Option>-fallow-undecidable-instances</Option><IndexTerm><Primary>-fallow-undecidable-instances
2589 option</Primary></IndexTerm> lifts this restriction, allowing all the types in an
2590 instance head to be type variables.
2597 <Emphasis>Unlike Haskell 1.4, instance heads may use type
2598 synonyms</Emphasis>. As always, using a type synonym is just shorthand for
2599 writing the RHS of the type synonym definition. For example:
2603 type Point = (Int,Int)
2604 instance C Point where ...
2605 instance C [Point] where ...
2609 is legal. However, if you added
2613 instance C (Int,Int) where ...
2617 as well, then the compiler will complain about the overlapping
2618 (actually, identical) instance declarations. As always, type synonyms
2619 must be fully applied. You cannot, for example, write:
2624 instance Monad P where ...
2628 This design decision is independent of all the others, and easily
2629 reversed, but it makes sense to me.
2636 <Emphasis>The types in an instance-declaration <Emphasis>context</Emphasis> must all
2637 be type variables</Emphasis>. Thus
2641 instance C a b => Eq (a,b) where ...
2649 instance C Int b => Foo b where ...
2653 is not OK. Again, the intent here is to make sure that context
2654 reduction terminates.
2656 Voluminous correspondence on the Haskell mailing list has convinced me
2657 that it's worth experimenting with a more liberal rule. If you use
2658 the flag <Option>-fallow-undecidable-instances</Option> can use arbitrary
2659 types in an instance context. Termination is ensured by having a
2660 fixed-depth recursion stack. If you exceed the stack depth you get a
2661 sort of backtrace, and the opportunity to increase the stack depth
2662 with <Option>-fcontext-stack</Option><Emphasis>N</Emphasis>.
2675 <Sect1 id="universal-quantification">
2676 <Title>Explicit universal quantification
2680 GHC now allows you to write explicitly quantified types. GHC's
2681 syntax for this now agrees with Hugs's, namely:
2687 forall a b. (Ord a, Eq b) => a -> b -> a
2693 The context is, of course, optional. You can't use <Literal>forall</Literal> as
2694 a type variable any more!
2698 Haskell type signatures are implicitly quantified. The <Literal>forall</Literal>
2699 allows us to say exactly what this means. For example:
2717 g :: forall b. (b -> b)
2723 The two are treated identically.
2727 <Title>Universally-quantified data type fields
2731 In a <Literal>data</Literal> or <Literal>newtype</Literal> declaration one can quantify
2732 the types of the constructor arguments. Here are several examples:
2738 data T a = T1 (forall b. b -> b -> b) a
2740 data MonadT m = MkMonad { return :: forall a. a -> m a,
2741 bind :: forall a b. m a -> (a -> m b) -> m b
2744 newtype Swizzle = MkSwizzle (Ord a => [a] -> [a])
2750 The constructors now have so-called <Emphasis>rank 2</Emphasis> polymorphic
2751 types, in which there is a for-all in the argument types.:
2757 T1 :: forall a. (forall b. b -> b -> b) -> a -> T a
2758 MkMonad :: forall m. (forall a. a -> m a)
2759 -> (forall a b. m a -> (a -> m b) -> m b)
2761 MkSwizzle :: (Ord a => [a] -> [a]) -> Swizzle
2767 Notice that you don't need to use a <Literal>forall</Literal> if there's an
2768 explicit context. For example in the first argument of the
2769 constructor <Function>MkSwizzle</Function>, an implicit "<Literal>forall a.</Literal>" is
2770 prefixed to the argument type. The implicit <Literal>forall</Literal>
2771 quantifies all type variables that are not already in scope, and are
2772 mentioned in the type quantified over.
2776 As for type signatures, implicit quantification happens for non-overloaded
2777 types too. So if you write this:
2780 data T a = MkT (Either a b) (b -> b)
2783 it's just as if you had written this:
2786 data T a = MkT (forall b. Either a b) (forall b. b -> b)
2789 That is, since the type variable <Literal>b</Literal> isn't in scope, it's
2790 implicitly universally quantified. (Arguably, it would be better
2791 to <Emphasis>require</Emphasis> explicit quantification on constructor arguments
2792 where that is what is wanted. Feedback welcomed.)
2798 <Title>Construction </Title>
2801 You construct values of types <Literal>T1, MonadT, Swizzle</Literal> by applying
2802 the constructor to suitable values, just as usual. For example,
2808 (T1 (\xy->x) 3) :: T Int
2810 (MkSwizzle sort) :: Swizzle
2811 (MkSwizzle reverse) :: Swizzle
2818 MkMonad r b) :: MonadT Maybe
2824 The type of the argument can, as usual, be more general than the type
2825 required, as <Literal>(MkSwizzle reverse)</Literal> shows. (<Function>reverse</Function>
2826 does not need the <Literal>Ord</Literal> constraint.)
2832 <Title>Pattern matching</Title>
2835 When you use pattern matching, the bound variables may now have
2836 polymorphic types. For example:
2842 f :: T a -> a -> (a, Char)
2843 f (T1 f k) x = (f k x, f 'c' 'd')
2845 g :: (Ord a, Ord b) => Swizzle -> [a] -> (a -> b) -> [b]
2846 g (MkSwizzle s) xs f = s (map f (s xs))
2848 h :: MonadT m -> [m a] -> m [a]
2849 h m [] = return m []
2850 h m (x:xs) = bind m x $ \y ->
2851 bind m (h m xs) $ \ys ->
2858 In the function <Function>h</Function> we use the record selectors <Literal>return</Literal>
2859 and <Literal>bind</Literal> to extract the polymorphic bind and return functions
2860 from the <Literal>MonadT</Literal> data structure, rather than using pattern
2865 You cannot pattern-match against an argument that is polymorphic.
2869 newtype TIM s a = TIM (ST s (Maybe a))
2871 runTIM :: (forall s. TIM s a) -> Maybe a
2872 runTIM (TIM m) = runST m
2878 Here the pattern-match fails, because you can't pattern-match against
2879 an argument of type <Literal>(forall s. TIM s a)</Literal>. Instead you
2880 must bind the variable and pattern match in the right hand side:
2883 runTIM :: (forall s. TIM s a) -> Maybe a
2884 runTIM tm = case tm of { TIM m -> runST m }
2887 The <Literal>tm</Literal> on the right hand side is (invisibly) instantiated, like
2888 any polymorphic value at its occurrence site, and now you can pattern-match
2895 <Title>The partial-application restriction</Title>
2898 There is really only one way in which data structures with polymorphic
2899 components might surprise you: you must not partially apply them.
2900 For example, this is illegal:
2906 map MkSwizzle [sort, reverse]
2912 The restriction is this: <Emphasis>every subexpression of the program must
2913 have a type that has no for-alls, except that in a function
2914 application (f e1…en) the partial applications are not subject to
2915 this rule</Emphasis>. The restriction makes type inference feasible.
2919 In the illegal example, the sub-expression <Literal>MkSwizzle</Literal> has the
2920 polymorphic type <Literal>(Ord b => [b] -> [b]) -> Swizzle</Literal> and is not
2921 a sub-expression of an enclosing application. On the other hand, this
2928 map (T1 (\a b -> a)) [1,2,3]
2934 even though it involves a partial application of <Function>T1</Function>, because
2935 the sub-expression <Literal>T1 (\a b -> a)</Literal> has type <Literal>Int -> T
2942 <Title>Type signatures
2946 Once you have data constructors with universally-quantified fields, or
2947 constants such as <Constant>runST</Constant> that have rank-2 types, it isn't long
2948 before you discover that you need more! Consider:
2954 mkTs f x y = [T1 f x, T1 f y]
2960 <Function>mkTs</Function> is a fuction that constructs some values of type
2961 <Literal>T</Literal>, using some pieces passed to it. The trouble is that since
2962 <Literal>f</Literal> is a function argument, Haskell assumes that it is
2963 monomorphic, so we'll get a type error when applying <Function>T1</Function> to
2964 it. This is a rather silly example, but the problem really bites in
2965 practice. Lots of people trip over the fact that you can't make
2966 "wrappers functions" for <Constant>runST</Constant> for exactly the same reason.
2967 In short, it is impossible to build abstractions around functions with
2972 The solution is fairly clear. We provide the ability to give a rank-2
2973 type signature for <Emphasis>ordinary</Emphasis> functions (not only data
2974 constructors), thus:
2980 mkTs :: (forall b. b -> b -> b) -> a -> [T a]
2981 mkTs f x y = [T1 f x, T1 f y]
2987 This type signature tells the compiler to attribute <Literal>f</Literal> with
2988 the polymorphic type <Literal>(forall b. b -> b -> b)</Literal> when type
2989 checking the body of <Function>mkTs</Function>, so now the application of
2990 <Function>T1</Function> is fine.
2994 There are two restrictions:
3003 You can only define a rank 2 type, specified by the following
3008 rank2type ::= [forall tyvars .] [context =>] funty
3009 funty ::= ([forall tyvars .] [context =>] ty) -> funty
3011 ty ::= ...current Haskell monotype syntax...
3015 Informally, the universal quantification must all be right at the beginning,
3016 or at the top level of a function argument.
3023 There is a restriction on the definition of a function whose
3024 type signature is a rank-2 type: the polymorphic arguments must be
3025 matched on the left hand side of the "<Literal>=</Literal>" sign. You can't
3026 define <Function>mkTs</Function> like this:
3030 mkTs :: (forall b. b -> b -> b) -> a -> [T a]
3031 mkTs = \ f x y -> [T1 f x, T1 f y]
3036 The same partial-application rule applies to ordinary functions with
3037 rank-2 types as applied to data constructors.
3050 <Title>Type synonyms and hoisting
3054 GHC also allows you to write a <Literal>forall</Literal> in a type synonym, thus:
3056 type Discard a = forall b. a -> b -> a
3061 However, it is often convenient to use these sort of synonyms at the right hand
3062 end of an arrow, thus:
3064 type Discard a = forall b. a -> b -> a
3066 g :: Int -> Discard Int
3069 Simply expanding the type synonym would give
3071 g :: Int -> (forall b. Int -> b -> Int)
3073 but GHC "hoists" the <Literal>forall</Literal> to give the isomorphic type
3075 g :: forall b. Int -> Int -> b -> Int
3077 In general, the rule is this: <Emphasis>to determine the type specified by any explicit
3078 user-written type (e.g. in a type signature), GHC expands type synonyms and then repeatedly
3079 performs the transformation:</Emphasis>
3081 <Emphasis>type1</Emphasis> -> forall a. <Emphasis>type2</Emphasis>
3083 forall a. <Emphasis>type1</Emphasis> -> <Emphasis>type2</Emphasis>
3085 (In fact, GHC tries to retain as much synonym information as possible for use in
3086 error messages, but that is a usability issue.) This rule applies, of course, whether
3087 or not the <Literal>forall</Literal> comes from a synonym. For example, here is another
3088 valid way to write <Literal>g</Literal>'s type signature:
3090 g :: Int -> Int -> forall b. b -> Int
3097 <Sect1 id="existential-quantification">
3098 <Title>Existentially quantified data constructors
3102 The idea of using existential quantification in data type declarations
3103 was suggested by Laufer (I believe, thought doubtless someone will
3104 correct me), and implemented in Hope+. It's been in Lennart
3105 Augustsson's <Command>hbc</Command> Haskell compiler for several years, and
3106 proved very useful. Here's the idea. Consider the declaration:
3112 data Foo = forall a. MkFoo a (a -> Bool)
3119 The data type <Literal>Foo</Literal> has two constructors with types:
3125 MkFoo :: forall a. a -> (a -> Bool) -> Foo
3132 Notice that the type variable <Literal>a</Literal> in the type of <Function>MkFoo</Function>
3133 does not appear in the data type itself, which is plain <Literal>Foo</Literal>.
3134 For example, the following expression is fine:
3140 [MkFoo 3 even, MkFoo 'c' isUpper] :: [Foo]
3146 Here, <Literal>(MkFoo 3 even)</Literal> packages an integer with a function
3147 <Function>even</Function> that maps an integer to <Literal>Bool</Literal>; and <Function>MkFoo 'c'
3148 isUpper</Function> packages a character with a compatible function. These
3149 two things are each of type <Literal>Foo</Literal> and can be put in a list.
3153 What can we do with a value of type <Literal>Foo</Literal>?. In particular,
3154 what happens when we pattern-match on <Function>MkFoo</Function>?
3160 f (MkFoo val fn) = ???
3166 Since all we know about <Literal>val</Literal> and <Function>fn</Function> is that they
3167 are compatible, the only (useful) thing we can do with them is to
3168 apply <Function>fn</Function> to <Literal>val</Literal> to get a boolean. For example:
3175 f (MkFoo val fn) = fn val
3181 What this allows us to do is to package heterogenous values
3182 together with a bunch of functions that manipulate them, and then treat
3183 that collection of packages in a uniform manner. You can express
3184 quite a bit of object-oriented-like programming this way.
3187 <Sect2 id="existential">
3188 <Title>Why existential?
3192 What has this to do with <Emphasis>existential</Emphasis> quantification?
3193 Simply that <Function>MkFoo</Function> has the (nearly) isomorphic type
3199 MkFoo :: (exists a . (a, a -> Bool)) -> Foo
3205 But Haskell programmers can safely think of the ordinary
3206 <Emphasis>universally</Emphasis> quantified type given above, thereby avoiding
3207 adding a new existential quantification construct.
3213 <Title>Type classes</Title>
3216 An easy extension (implemented in <Command>hbc</Command>) is to allow
3217 arbitrary contexts before the constructor. For example:
3223 data Baz = forall a. Eq a => Baz1 a a
3224 | forall b. Show b => Baz2 b (b -> b)
3230 The two constructors have the types you'd expect:
3236 Baz1 :: forall a. Eq a => a -> a -> Baz
3237 Baz2 :: forall b. Show b => b -> (b -> b) -> Baz
3243 But when pattern matching on <Function>Baz1</Function> the matched values can be compared
3244 for equality, and when pattern matching on <Function>Baz2</Function> the first matched
3245 value can be converted to a string (as well as applying the function to it).
3246 So this program is legal:
3253 f (Baz1 p q) | p == q = "Yes"
3255 f (Baz1 v fn) = show (fn v)
3261 Operationally, in a dictionary-passing implementation, the
3262 constructors <Function>Baz1</Function> and <Function>Baz2</Function> must store the
3263 dictionaries for <Literal>Eq</Literal> and <Literal>Show</Literal> respectively, and
3264 extract it on pattern matching.
3268 Notice the way that the syntax fits smoothly with that used for
3269 universal quantification earlier.
3275 <Title>Restrictions</Title>
3278 There are several restrictions on the ways in which existentially-quantified
3279 constructors can be use.
3288 When pattern matching, each pattern match introduces a new,
3289 distinct, type for each existential type variable. These types cannot
3290 be unified with any other type, nor can they escape from the scope of
3291 the pattern match. For example, these fragments are incorrect:
3299 Here, the type bound by <Function>MkFoo</Function> "escapes", because <Literal>a</Literal>
3300 is the result of <Function>f1</Function>. One way to see why this is wrong is to
3301 ask what type <Function>f1</Function> has:
3305 f1 :: Foo -> a -- Weird!
3309 What is this "<Literal>a</Literal>" in the result type? Clearly we don't mean
3314 f1 :: forall a. Foo -> a -- Wrong!
3318 The original program is just plain wrong. Here's another sort of error
3322 f2 (Baz1 a b) (Baz1 p q) = a==q
3326 It's ok to say <Literal>a==b</Literal> or <Literal>p==q</Literal>, but
3327 <Literal>a==q</Literal> is wrong because it equates the two distinct types arising
3328 from the two <Function>Baz1</Function> constructors.
3336 You can't pattern-match on an existentially quantified
3337 constructor in a <Literal>let</Literal> or <Literal>where</Literal> group of
3338 bindings. So this is illegal:
3342 f3 x = a==b where { Baz1 a b = x }
3346 You can only pattern-match
3347 on an existentially-quantified constructor in a <Literal>case</Literal> expression or
3348 in the patterns of a function definition.
3350 The reason for this restriction is really an implementation one.
3351 Type-checking binding groups is already a nightmare without
3352 existentials complicating the picture. Also an existential pattern
3353 binding at the top level of a module doesn't make sense, because it's
3354 not clear how to prevent the existentially-quantified type "escaping".
3355 So for now, there's a simple-to-state restriction. We'll see how
3363 You can't use existential quantification for <Literal>newtype</Literal>
3364 declarations. So this is illegal:
3368 newtype T = forall a. Ord a => MkT a
3372 Reason: a value of type <Literal>T</Literal> must be represented as a pair
3373 of a dictionary for <Literal>Ord t</Literal> and a value of type <Literal>t</Literal>.
3374 That contradicts the idea that <Literal>newtype</Literal> should have no
3375 concrete representation. You can get just the same efficiency and effect
3376 by using <Literal>data</Literal> instead of <Literal>newtype</Literal>. If there is no
3377 overloading involved, then there is more of a case for allowing
3378 an existentially-quantified <Literal>newtype</Literal>, because the <Literal>data</Literal>
3379 because the <Literal>data</Literal> version does carry an implementation cost,
3380 but single-field existentially quantified constructors aren't much
3381 use. So the simple restriction (no existential stuff on <Literal>newtype</Literal>)
3382 stands, unless there are convincing reasons to change it.
3390 You can't use <Literal>deriving</Literal> to define instances of a
3391 data type with existentially quantified data constructors.
3393 Reason: in most cases it would not make sense. For example:#
3396 data T = forall a. MkT [a] deriving( Eq )
3399 To derive <Literal>Eq</Literal> in the standard way we would need to have equality
3400 between the single component of two <Function>MkT</Function> constructors:
3404 (MkT a) == (MkT b) = ???
3407 But <VarName>a</VarName> and <VarName>b</VarName> have distinct types, and so can't be compared.
3408 It's just about possible to imagine examples in which the derived instance
3409 would make sense, but it seems altogether simpler simply to prohibit such
3410 declarations. Define your own instances!
3422 <Sect1 id="sec-assertions">
3424 <IndexTerm><Primary>Assertions</Primary></IndexTerm>
3428 If you want to make use of assertions in your standard Haskell code, you
3429 could define a function like the following:
3435 assert :: Bool -> a -> a
3436 assert False x = error "assertion failed!"
3443 which works, but gives you back a less than useful error message --
3444 an assertion failed, but which and where?
3448 One way out is to define an extended <Function>assert</Function> function which also
3449 takes a descriptive string to include in the error message and
3450 perhaps combine this with the use of a pre-processor which inserts
3451 the source location where <Function>assert</Function> was used.
3455 Ghc offers a helping hand here, doing all of this for you. For every
3456 use of <Function>assert</Function> in the user's source:
3462 kelvinToC :: Double -> Double
3463 kelvinToC k = assert (k &gt;= 0.0) (k+273.15)
3469 Ghc will rewrite this to also include the source location where the
3476 assert pred val ==> assertError "Main.hs|15" pred val
3482 The rewrite is only performed by the compiler when it spots
3483 applications of <Function>Exception.assert</Function>, so you can still define and
3484 use your own versions of <Function>assert</Function>, should you so wish. If not,
3485 import <Literal>Exception</Literal> to make use <Function>assert</Function> in your code.
3489 To have the compiler ignore uses of assert, use the compiler option
3490 <Option>-fignore-asserts</Option>. <IndexTerm><Primary>-fignore-asserts option</Primary></IndexTerm> That is,
3491 expressions of the form <Literal>assert pred e</Literal> will be rewritten to <Literal>e</Literal>.
3495 Assertion failures can be caught, see the documentation for the
3496 <literal>Exception</literal> library (<xref linkend="sec-Exception">)
3502 <Sect1 id="scoped-type-variables">
3503 <Title>Scoped Type Variables
3507 A <Emphasis>pattern type signature</Emphasis> can introduce a <Emphasis>scoped type
3508 variable</Emphasis>. For example
3514 f (xs::[a]) = ys ++ ys
3523 The pattern <Literal>(xs::[a])</Literal> includes a type signature for <VarName>xs</VarName>.
3524 This brings the type variable <Literal>a</Literal> into scope; it scopes over
3525 all the patterns and right hand sides for this equation for <Function>f</Function>.
3526 In particular, it is in scope at the type signature for <VarName>y</VarName>.
3530 At ordinary type signatures, such as that for <VarName>ys</VarName>, any type variables
3531 mentioned in the type signature <Emphasis>that are not in scope</Emphasis> are
3532 implicitly universally quantified. (If there are no type variables in
3533 scope, all type variables mentioned in the signature are universally
3534 quantified, which is just as in Haskell 98.) In this case, since <VarName>a</VarName>
3535 is in scope, it is not universally quantified, so the type of <VarName>ys</VarName> is
3536 the same as that of <VarName>xs</VarName>. In Haskell 98 it is not possible to declare
3537 a type for <VarName>ys</VarName>; a major benefit of scoped type variables is that
3538 it becomes possible to do so.
3542 Scoped type variables are implemented in both GHC and Hugs. Where the
3543 implementations differ from the specification below, those differences
3548 So much for the basic idea. Here are the details.
3552 <Title>Scope and implicit quantification</Title>
3560 All the type variables mentioned in the patterns for a single
3561 function definition equation, that are not already in scope,
3562 are brought into scope by the patterns. We describe this set as
3563 the <Emphasis>type variables bound by the equation</Emphasis>.
3570 The type variables thus brought into scope may be mentioned
3571 in ordinary type signatures or pattern type signatures anywhere within
3579 In ordinary type signatures, any type variable mentioned in the
3580 signature that is in scope is <Emphasis>not</Emphasis> universally quantified.
3587 Ordinary type signatures do not bring any new type variables
3588 into scope (except in the type signature itself!). So this is illegal:
3597 It's illegal because <VarName>a</VarName> is not in scope in the body of <Function>f</Function>,
3598 so the ordinary signature <Literal>x::a</Literal> is equivalent to <Literal>x::forall a.a</Literal>;
3599 and that is an incorrect typing.
3606 There is no implicit universal quantification on pattern type
3607 signatures, nor may one write an explicit <Literal>forall</Literal> type in a pattern
3608 type signature. The pattern type signature is a monotype.
3616 The type variables in the head of a <Literal>class</Literal> or <Literal>instance</Literal> declaration
3617 scope over the methods defined in the <Literal>where</Literal> part. For example:
3631 (Not implemented in Hugs yet, Dec 98).
3642 <Title>Polymorphism</Title>
3650 Pattern type signatures are completely orthogonal to ordinary, separate
3651 type signatures. The two can be used independently or together. There is
3652 no scoping associated with the names of the type variables in a separate type signature.
3657 f (xs::[b]) = reverse xs
3666 The function must be polymorphic in the type variables
3667 bound by all its equations. Operationally, the type variables bound
3668 by one equation must not:
3675 Be unified with a type (such as <Literal>Int</Literal>, or <Literal>[a]</Literal>).
3681 Be unified with a type variable free in the environment.
3687 Be unified with each other. (They may unify with the type variables
3688 bound by another equation for the same function, of course.)
3695 For example, the following all fail to type check:
3699 f (x::a) (y::b) = [x,y] -- a unifies with b
3701 g (x::a) = x + 1::Int -- a unifies with Int
3703 h x = let k (y::a) = [x,y] -- a is free in the
3704 in k x -- environment
3706 k (x::a) True = ... -- a unifies with Int
3707 k (x::Int) False = ...
3710 w (x::a) = x -- a unifies with [b]
3719 The pattern-bound type variable may, however, be constrained
3720 by the context of the principal type, thus:
3724 f (x::a) (y::a) = x+y*2
3728 gets the inferred type: <Literal>forall a. Num a => a -> a -> a</Literal>.
3739 <Title>Result type signatures</Title>
3747 The result type of a function can be given a signature,
3752 f (x::a) :: [a] = [x,x,x]
3756 The final <Literal>:: [a]</Literal> after all the patterns gives a signature to the
3757 result type. Sometimes this is the only way of naming the type variable
3762 f :: Int -> [a] -> [a]
3763 f n :: ([a] -> [a]) = let g (x::a, y::a) = (y,x)
3764 in \xs -> map g (reverse xs `zip` xs)
3776 Result type signatures are not yet implemented in Hugs.
3782 <Title>Pattern signatures on other constructs</Title>
3790 A pattern type signature can be on an arbitrary sub-pattern, not
3795 f ((x,y)::(a,b)) = (y,x) :: (b,a)
3804 Pattern type signatures, including the result part, can be used
3805 in lambda abstractions:
3809 (\ (x::a, y) :: a -> x)
3813 Type variables bound by these patterns must be polymorphic in
3814 the sense defined above.
3819 f1 (x::c) = f1 x -- ok
3820 f2 = \(x::c) -> f2 x -- not ok
3824 Here, <Function>f1</Function> is OK, but <Function>f2</Function> is not, because <VarName>c</VarName> gets unified
3825 with a type variable free in the environment, in this
3826 case, the type of <Function>f2</Function>, which is in the environment when
3827 the lambda abstraction is checked.
3834 Pattern type signatures, including the result part, can be used
3835 in <Literal>case</Literal> expressions:
3839 case e of { (x::a, y) :: a -> x }
3843 The pattern-bound type variables must, as usual,
3844 be polymorphic in the following sense: each case alternative,
3845 considered as a lambda abstraction, must be polymorphic.
3850 case (True,False) of { (x::a, y) -> x }
3854 Even though the context is that of a pair of booleans,
3855 the alternative itself is polymorphic. Of course, it is
3860 case (True,False) of { (x::Bool, y) -> x }
3869 To avoid ambiguity, the type after the “<Literal>::</Literal>” in a result
3870 pattern signature on a lambda or <Literal>case</Literal> must be atomic (i.e. a single
3871 token or a parenthesised type of some sort). To see why,
3872 consider how one would parse this:
3885 Pattern type signatures that bind new type variables
3886 may not be used in pattern bindings at all.
3891 f x = let (y, z::a) = x in ...
3895 But these are OK, because they do not bind fresh type variables:
3899 f1 x = let (y, z::Int) = x in ...
3900 f2 (x::(Int,a)) = let (y, z::a) = x in ...
3904 However a single variable is considered a degenerate function binding,
3905 rather than a degerate pattern binding, so this is permitted, even
3906 though it binds a type variable:
3910 f :: (b->b) = \(x::b) -> x
3919 Such degnerate function bindings do not fall under the monomorphism
3926 g :: a -> a -> Bool = \x y. x==y
3932 Here <Function>g</Function> has type <Literal>forall a. Eq a => a -> a -> Bool</Literal>, just as if
3933 <Function>g</Function> had a separate type signature. Lacking a type signature, <Function>g</Function>
3934 would get a monomorphic type.
3940 <Title>Existentials</Title>
3948 Pattern type signatures can bind existential type variables.
3953 data T = forall a. MkT [a]
3956 f (MkT [t::a]) = MkT t3
3973 <Sect1 id="pragmas">
3978 GHC supports several pragmas, or instructions to the compiler placed
3979 in the source code. Pragmas don't affect the meaning of the program,
3980 but they might affect the efficiency of the generated code.
3983 <Sect2 id="inline-pragma">
3984 <Title>INLINE pragma
3986 <IndexTerm><Primary>INLINE pragma</Primary></IndexTerm>
3987 <IndexTerm><Primary>pragma, INLINE</Primary></IndexTerm></Title>
3990 GHC (with <Option>-O</Option>, as always) tries to inline (or “unfold”)
3991 functions/values that are “small enough,” thus avoiding the call
3992 overhead and possibly exposing other more-wonderful optimisations.
3996 You will probably see these unfoldings (in Core syntax) in your
4001 Normally, if GHC decides a function is “too expensive” to inline, it
4002 will not do so, nor will it export that unfolding for other modules to
4007 The sledgehammer you can bring to bear is the
4008 <Literal>INLINE</Literal><IndexTerm><Primary>INLINE pragma</Primary></IndexTerm> pragma, used thusly:
4011 key_function :: Int -> String -> (Bool, Double)
4013 #ifdef __GLASGOW_HASKELL__
4014 {-# INLINE key_function #-}
4018 (You don't need to do the C pre-processor carry-on unless you're going
4019 to stick the code through HBC—it doesn't like <Literal>INLINE</Literal> pragmas.)
4023 The major effect of an <Literal>INLINE</Literal> pragma is to declare a function's
4024 “cost” to be very low. The normal unfolding machinery will then be
4025 very keen to inline it.
4029 An <Literal>INLINE</Literal> pragma for a function can be put anywhere its type
4030 signature could be put.
4034 <Literal>INLINE</Literal> pragmas are a particularly good idea for the
4035 <Literal>then</Literal>/<Literal>return</Literal> (or <Literal>bind</Literal>/<Literal>unit</Literal>) functions in a monad.
4036 For example, in GHC's own <Literal>UniqueSupply</Literal> monad code, we have:
4039 #ifdef __GLASGOW_HASKELL__
4040 {-# INLINE thenUs #-}
4041 {-# INLINE returnUs #-}
4049 <Sect2 id="noinline-pragma">
4050 <Title>NOINLINE pragma
4054 <IndexTerm><Primary>NOINLINE pragma</Primary></IndexTerm>
4055 <IndexTerm><Primary>pragma, NOINLINE</Primary></IndexTerm>
4059 The <Literal>NOINLINE</Literal> pragma does exactly what you'd expect: it stops the
4060 named function from being inlined by the compiler. You shouldn't ever
4061 need to do this, unless you're very cautious about code size.
4066 <Sect2 id="specialize-pragma">
4067 <Title>SPECIALIZE pragma
4071 <IndexTerm><Primary>SPECIALIZE pragma</Primary></IndexTerm>
4072 <IndexTerm><Primary>pragma, SPECIALIZE</Primary></IndexTerm>
4073 <IndexTerm><Primary>overloading, death to</Primary></IndexTerm>
4077 (UK spelling also accepted.) For key overloaded functions, you can
4078 create extra versions (NB: more code space) specialised to particular
4079 types. Thus, if you have an overloaded function:
4085 hammeredLookup :: Ord key => [(key, value)] -> key -> value
4091 If it is heavily used on lists with <Literal>Widget</Literal> keys, you could
4092 specialise it as follows:
4095 {-# SPECIALIZE hammeredLookup :: [(Widget, value)] -> Widget -> value #-}
4101 To get very fancy, you can also specify a named function to use for
4102 the specialised value, by adding <Literal>= blah</Literal>, as in:
4105 {-# SPECIALIZE hammeredLookup :: ...as before... = blah #-}
4108 It's <Emphasis>Your Responsibility</Emphasis> to make sure that <Function>blah</Function> really
4109 behaves as a specialised version of <Function>hammeredLookup</Function>!!!
4113 NOTE: the <Literal>=blah</Literal> feature isn't implemented in GHC 4.xx.
4117 An example in which the <Literal>= blah</Literal> form will Win Big:
4120 toDouble :: Real a => a -> Double
4121 toDouble = fromRational . toRational
4123 {-# SPECIALIZE toDouble :: Int -> Double = i2d #-}
4124 i2d (I# i) = D# (int2Double# i) -- uses Glasgow prim-op directly
4127 The <Function>i2d</Function> function is virtually one machine instruction; the
4128 default conversion—via an intermediate <Literal>Rational</Literal>—is obscenely
4129 expensive by comparison.
4133 By using the US spelling, your <Literal>SPECIALIZE</Literal> pragma will work with
4134 HBC, too. Note that HBC doesn't support the <Literal>= blah</Literal> form.
4138 A <Literal>SPECIALIZE</Literal> pragma for a function can be put anywhere its type
4139 signature could be put.
4144 <Sect2 id="specialize-instance-pragma">
4145 <Title>SPECIALIZE instance pragma
4149 <IndexTerm><Primary>SPECIALIZE pragma</Primary></IndexTerm>
4150 <IndexTerm><Primary>overloading, death to</Primary></IndexTerm>
4151 Same idea, except for instance declarations. For example:
4154 instance (Eq a) => Eq (Foo a) where { ... usual stuff ... }
4156 {-# SPECIALIZE instance Eq (Foo [(Int, Bar)] #-}
4159 Compatible with HBC, by the way.
4164 <Sect2 id="line-pragma">
4169 <IndexTerm><Primary>LINE pragma</Primary></IndexTerm>
4170 <IndexTerm><Primary>pragma, LINE</Primary></IndexTerm>
4174 This pragma is similar to C's <Literal>#line</Literal> pragma, and is mainly for use in
4175 automatically generated Haskell code. It lets you specify the line
4176 number and filename of the original code; for example
4182 {-# LINE 42 "Foo.vhs" #-}
4188 if you'd generated the current file from something called <Filename>Foo.vhs</Filename>
4189 and this line corresponds to line 42 in the original. GHC will adjust
4190 its error messages to refer to the line/file named in the <Literal>LINE</Literal>
4197 <Title>RULES pragma</Title>
4200 The RULES pragma lets you specify rewrite rules. It is described in
4201 <XRef LinkEnd="rewrite-rules">.
4208 <Sect1 id="rewrite-rules">
4209 <Title>Rewrite rules
4211 <IndexTerm><Primary>RULES pagma</Primary></IndexTerm>
4212 <IndexTerm><Primary>pragma, RULES</Primary></IndexTerm>
4213 <IndexTerm><Primary>rewrite rules</Primary></IndexTerm></Title>
4216 The programmer can specify rewrite rules as part of the source program
4217 (in a pragma). GHC applies these rewrite rules wherever it can.
4225 "map/map" forall f g xs. map f (map g xs) = map (f.g) xs
4232 <Title>Syntax</Title>
4235 From a syntactic point of view:
4241 Each rule has a name, enclosed in double quotes. The name itself has
4242 no significance at all. It is only used when reporting how many times the rule fired.
4248 There may be zero or more rules in a <Literal>RULES</Literal> pragma.
4254 Layout applies in a <Literal>RULES</Literal> pragma. Currently no new indentation level
4255 is set, so you must lay out your rules starting in the same column as the
4256 enclosing definitions.
4262 Each variable mentioned in a rule must either be in scope (e.g. <Function>map</Function>),
4263 or bound by the <Literal>forall</Literal> (e.g. <Function>f</Function>, <Function>g</Function>, <Function>xs</Function>). The variables bound by
4264 the <Literal>forall</Literal> are called the <Emphasis>pattern</Emphasis> variables. They are separated
4265 by spaces, just like in a type <Literal>forall</Literal>.
4271 A pattern variable may optionally have a type signature.
4272 If the type of the pattern variable is polymorphic, it <Emphasis>must</Emphasis> have a type signature.
4273 For example, here is the <Literal>foldr/build</Literal> rule:
4276 "fold/build" forall k z (g::forall b. (a->b->b) -> b -> b) .
4277 foldr k z (build g) = g k z
4280 Since <Function>g</Function> has a polymorphic type, it must have a type signature.
4287 The left hand side of a rule must consist of a top-level variable applied
4288 to arbitrary expressions. For example, this is <Emphasis>not</Emphasis> OK:
4291 "wrong1" forall e1 e2. case True of { True -> e1; False -> e2 } = e1
4292 "wrong2" forall f. f True = True
4295 In <Literal>"wrong1"</Literal>, the LHS is not an application; in <Literal>"wrong1"</Literal>, the LHS has a pattern variable
4302 A rule does not need to be in the same module as (any of) the
4303 variables it mentions, though of course they need to be in scope.
4309 Rules are automatically exported from a module, just as instance declarations are.
4320 <Title>Semantics</Title>
4323 From a semantic point of view:
4329 Rules are only applied if you use the <Option>-O</Option> flag.
4335 Rules are regarded as left-to-right rewrite rules.
4336 When GHC finds an expression that is a substitution instance of the LHS
4337 of a rule, it replaces the expression by the (appropriately-substituted) RHS.
4338 By "a substitution instance" we mean that the LHS can be made equal to the
4339 expression by substituting for the pattern variables.
4346 The LHS and RHS of a rule are typechecked, and must have the
4354 GHC makes absolutely no attempt to verify that the LHS and RHS
4355 of a rule have the same meaning. That is undecideable in general, and
4356 infeasible in most interesting cases. The responsibility is entirely the programmer's!
4363 GHC makes no attempt to make sure that the rules are confluent or
4364 terminating. For example:
4367 "loop" forall x,y. f x y = f y x
4370 This rule will cause the compiler to go into an infinite loop.
4377 If more than one rule matches a call, GHC will choose one arbitrarily to apply.
4383 GHC currently uses a very simple, syntactic, matching algorithm
4384 for matching a rule LHS with an expression. It seeks a substitution
4385 which makes the LHS and expression syntactically equal modulo alpha
4386 conversion. The pattern (rule), but not the expression, is eta-expanded if
4387 necessary. (Eta-expanding the epression can lead to laziness bugs.)
4388 But not beta conversion (that's called higher-order matching).
4392 Matching is carried out on GHC's intermediate language, which includes
4393 type abstractions and applications. So a rule only matches if the
4394 types match too. See <XRef LinkEnd="rule-spec"> below.
4400 GHC keeps trying to apply the rules as it optimises the program.
4401 For example, consider:
4410 The expression <Literal>s (t xs)</Literal> does not match the rule <Literal>"map/map"</Literal>, but GHC
4411 will substitute for <VarName>s</VarName> and <VarName>t</VarName>, giving an expression which does match.
4412 If <VarName>s</VarName> or <VarName>t</VarName> was (a) used more than once, and (b) large or a redex, then it would
4413 not be substituted, and the rule would not fire.
4420 In the earlier phases of compilation, GHC inlines <Emphasis>nothing
4421 that appears on the LHS of a rule</Emphasis>, because once you have substituted
4422 for something you can't match against it (given the simple minded
4423 matching). So if you write the rule
4426 "map/map" forall f,g. map f . map g = map (f.g)
4429 this <Emphasis>won't</Emphasis> match the expression <Literal>map f (map g xs)</Literal>.
4430 It will only match something written with explicit use of ".".
4431 Well, not quite. It <Emphasis>will</Emphasis> match the expression
4437 where <Function>wibble</Function> is defined:
4440 wibble f g = map f . map g
4443 because <Function>wibble</Function> will be inlined (it's small).
4445 Later on in compilation, GHC starts inlining even things on the
4446 LHS of rules, but still leaves the rules enabled. This inlining
4447 policy is controlled by the per-simplification-pass flag <Option>-finline-phase</Option><Emphasis>n</Emphasis>.
4454 All rules are implicitly exported from the module, and are therefore
4455 in force in any module that imports the module that defined the rule, directly
4456 or indirectly. (That is, if A imports B, which imports C, then C's rules are
4457 in force when compiling A.) The situation is very similar to that for instance
4469 <Title>List fusion</Title>
4472 The RULES mechanism is used to implement fusion (deforestation) of common list functions.
4473 If a "good consumer" consumes an intermediate list constructed by a "good producer", the
4474 intermediate list should be eliminated entirely.
4478 The following are good producers:
4490 Enumerations of <Literal>Int</Literal> and <Literal>Char</Literal> (e.g. <Literal>['a'..'z']</Literal>).
4496 Explicit lists (e.g. <Literal>[True, False]</Literal>)
4502 The cons constructor (e.g <Literal>3:4:[]</Literal>)
4508 <Function>++</Function>
4514 <Function>map</Function>
4520 <Function>filter</Function>
4526 <Function>iterate</Function>, <Function>repeat</Function>
4532 <Function>zip</Function>, <Function>zipWith</Function>
4541 The following are good consumers:
4553 <Function>array</Function> (on its second argument)
4559 <Function>length</Function>
4565 <Function>++</Function> (on its first argument)
4571 <Function>map</Function>
4577 <Function>filter</Function>
4583 <Function>concat</Function>
4589 <Function>unzip</Function>, <Function>unzip2</Function>, <Function>unzip3</Function>, <Function>unzip4</Function>
4595 <Function>zip</Function>, <Function>zipWith</Function> (but on one argument only; if both are good producers, <Function>zip</Function>
4596 will fuse with one but not the other)
4602 <Function>partition</Function>
4608 <Function>head</Function>
4614 <Function>and</Function>, <Function>or</Function>, <Function>any</Function>, <Function>all</Function>
4620 <Function>sequence_</Function>
4626 <Function>msum</Function>
4632 <Function>sortBy</Function>
4641 So, for example, the following should generate no intermediate lists:
4644 array (1,10) [(i,i*i) | i <- map (+ 1) [0..9]]
4650 This list could readily be extended; if there are Prelude functions that you use
4651 a lot which are not included, please tell us.
4655 If you want to write your own good consumers or producers, look at the
4656 Prelude definitions of the above functions to see how to do so.
4661 <Sect2 id="rule-spec">
4662 <Title>Specialisation
4666 Rewrite rules can be used to get the same effect as a feature
4667 present in earlier version of GHC:
4670 {-# SPECIALIZE fromIntegral :: Int8 -> Int16 = int8ToInt16 #-}
4673 This told GHC to use <Function>int8ToInt16</Function> instead of <Function>fromIntegral</Function> whenever
4674 the latter was called with type <Literal>Int8 -> Int16</Literal>. That is, rather than
4675 specialising the original definition of <Function>fromIntegral</Function> the programmer is
4676 promising that it is safe to use <Function>int8ToInt16</Function> instead.
4680 This feature is no longer in GHC. But rewrite rules let you do the
4685 "fromIntegral/Int8/Int16" fromIntegral = int8ToInt16
4689 This slightly odd-looking rule instructs GHC to replace <Function>fromIntegral</Function>
4690 by <Function>int8ToInt16</Function> <Emphasis>whenever the types match</Emphasis>. Speaking more operationally,
4691 GHC adds the type and dictionary applications to get the typed rule
4694 forall (d1::Integral Int8) (d2::Num Int16) .
4695 fromIntegral Int8 Int16 d1 d2 = int8ToInt16
4699 this rule does not need to be in the same file as fromIntegral,
4700 unlike the <Literal>SPECIALISE</Literal> pragmas which currently do (so that they
4701 have an original definition available to specialise).
4707 <Title>Controlling what's going on</Title>
4715 Use <Option>-ddump-rules</Option> to see what transformation rules GHC is using.
4721 Use <Option>-ddump-simpl-stats</Option> to see what rules are being fired.
4722 If you add <Option>-dppr-debug</Option> you get a more detailed listing.
4728 The defintion of (say) <Function>build</Function> in <FileName>PrelBase.lhs</FileName> looks llike this:
4731 build :: forall a. (forall b. (a -> b -> b) -> b -> b) -> [a]
4732 {-# INLINE build #-}
4736 Notice the <Literal>INLINE</Literal>! That prevents <Literal>(:)</Literal> from being inlined when compiling
4737 <Literal>PrelBase</Literal>, so that an importing module will “see” the <Literal>(:)</Literal>, and can
4738 match it on the LHS of a rule. <Literal>INLINE</Literal> prevents any inlining happening
4739 in the RHS of the <Literal>INLINE</Literal> thing. I regret the delicacy of this.
4746 In <Filename>ghc/lib/std/PrelBase.lhs</Filename> look at the rules for <Function>map</Function> to
4747 see how to write rules that will do fusion and yet give an efficient
4748 program even if fusion doesn't happen. More rules in <Filename>PrelList.lhs</Filename>.
4761 ;;; Local Variables: ***
4763 ;;; sgml-parent-document: ("users_guide.sgml" "book" "chapter" "sect1") ***
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