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 e.g. '\o<octal>'#
368 "a"# an Addr# (a `char *'); only characters '\0'..'\255' allowed
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="sec-stable-pointers"> 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="sec-ForeignObj"> 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>
1482 <para>The foreign interface consists of the following components:</para>
1486 <para>The Foreign Function Interface language specification
1487 (included in this manual, in <xref linkend="ffi">).</para>
1491 <para>The <literal>Foreign</literal> module (see <xref
1492 linkend="sec-Foreign">) collects together several interfaces
1493 which are useful in specifying foreign language
1494 interfaces, including the following:</para>
1498 <para>The <literal>ForeignObj</literal> module (see <xref
1499 linkend="sec-ForeignObj">), for managing pointers from
1500 Haskell into the outside world.</para>
1504 <para>The <literal>StablePtr</literal> module (see <xref
1505 linkend="sec-stable-pointers">), for managing pointers
1506 into Haskell from the outside world.</para>
1510 <para>The <literal>CTypes</literal> module (see <xref
1511 linkend="sec-CTypes">) gives Haskell equivalents for the
1512 standard C datatypes, for use in making Haskell bindings
1513 to existing C libraries.</para>
1517 <para>The <literal>CTypesISO</literal> module (see <xref
1518 linkend="sec-CTypesISO">) gives Haskell equivalents for C
1519 types defined by the ISO C standard.</para>
1523 <para>The <literal>Storable</literal> library, for
1524 primitive marshalling of data types between Haskell and
1525 the foreign language.</para>
1532 <para>The following sections also give some hints and tips on the use
1533 of the foreign function interface in GHC.</para>
1535 <Sect2 id="glasgow-foreign-headers">
1536 <Title>Using function headers
1540 <IndexTerm><Primary>C calls, function headers</Primary></IndexTerm>
1544 When generating C (using the <Option>-fvia-C</Option> directive), one can assist the
1545 C compiler in detecting type errors by using the <Command>-#include</Command> directive
1546 to provide <Filename>.h</Filename> files containing function headers.
1558 void initialiseEFS (HsInt size);
1559 HsInt terminateEFS (void);
1560 HsForeignObj emptyEFS(void);
1561 HsForeignObj updateEFS (HsForeignObj a, HsInt i, HsInt x);
1562 HsInt lookupEFS (HsForeignObj a, HsInt i);
1566 <para>The types <literal>HsInt</literal>,
1567 <literal>HsForeignObj</literal> etc. are described in <xref
1568 linkend="sec-mapping-table">.</Para>
1570 <Para>Note that this approach is only
1571 <Emphasis>essential</Emphasis> for returning
1572 <Literal>float</Literal>s (or if <Literal>sizeof(int) !=
1573 sizeof(int *)</Literal> on your architecture) but is a Good
1574 Thing for anyone who cares about writing solid code. You're
1575 crazy not to do it.</Para>
1581 <Sect1 id="multi-param-type-classes">
1582 <Title>Multi-parameter type classes
1586 This section documents GHC's implementation of multi-parameter type
1587 classes. There's lots of background in the paper <ULink
1588 URL="http://research.microsoft.com/~simonpj/multi.ps.gz" >Type
1589 classes: exploring the design space</ULink > (Simon Peyton Jones, Mark
1590 Jones, Erik Meijer).
1594 I'd like to thank people who reported shorcomings in the GHC 3.02
1595 implementation. Our default decisions were all conservative ones, and
1596 the experience of these heroic pioneers has given useful concrete
1597 examples to support several generalisations. (These appear below as
1598 design choices not implemented in 3.02.)
1602 I've discussed these notes with Mark Jones, and I believe that Hugs
1603 will migrate towards the same design choices as I outline here.
1604 Thanks to him, and to many others who have offered very useful
1609 <Title>Types</Title>
1612 There are the following restrictions on the form of a qualified
1619 forall tv1..tvn (c1, ...,cn) => type
1625 (Here, I write the "foralls" explicitly, although the Haskell source
1626 language omits them; in Haskell 1.4, all the free type variables of an
1627 explicit source-language type signature are universally quantified,
1628 except for the class type variables in a class declaration. However,
1629 in GHC, you can give the foralls if you want. See <XRef LinkEnd="universal-quantification">).
1638 <Emphasis>Each universally quantified type variable
1639 <Literal>tvi</Literal> must be mentioned (i.e. appear free) in <Literal>type</Literal></Emphasis>.
1641 The reason for this is that a value with a type that does not obey
1642 this restriction could not be used without introducing
1643 ambiguity. Here, for example, is an illegal type:
1647 forall a. Eq a => Int
1651 When a value with this type was used, the constraint <Literal>Eq tv</Literal>
1652 would be introduced where <Literal>tv</Literal> is a fresh type variable, and
1653 (in the dictionary-translation implementation) the value would be
1654 applied to a dictionary for <Literal>Eq tv</Literal>. The difficulty is that we
1655 can never know which instance of <Literal>Eq</Literal> to use because we never
1656 get any more information about <Literal>tv</Literal>.
1663 <Emphasis>Every constraint <Literal>ci</Literal> must mention at least one of the
1664 universally quantified type variables <Literal>tvi</Literal></Emphasis>.
1666 For example, this type is OK because <Literal>C a b</Literal> mentions the
1667 universally quantified type variable <Literal>b</Literal>:
1671 forall a. C a b => burble
1675 The next type is illegal because the constraint <Literal>Eq b</Literal> does not
1676 mention <Literal>a</Literal>:
1680 forall a. Eq b => burble
1684 The reason for this restriction is milder than the other one. The
1685 excluded types are never useful or necessary (because the offending
1686 context doesn't need to be witnessed at this point; it can be floated
1687 out). Furthermore, floating them out increases sharing. Lastly,
1688 excluding them is a conservative choice; it leaves a patch of
1689 territory free in case we need it later.
1699 These restrictions apply to all types, whether declared in a type signature
1704 Unlike Haskell 1.4, constraints in types do <Emphasis>not</Emphasis> have to be of
1705 the form <Emphasis>(class type-variables)</Emphasis>. Thus, these type signatures
1712 f :: Eq (m a) => [m a] -> [m a]
1719 This choice recovers principal types, a property that Haskell 1.4 does not have.
1725 <Title>Class declarations</Title>
1733 <Emphasis>Multi-parameter type classes are permitted</Emphasis>. For example:
1737 class Collection c a where
1738 union :: c a -> c a -> c a
1749 <Emphasis>The class hierarchy must be acyclic</Emphasis>. However, the definition
1750 of "acyclic" involves only the superclass relationships. For example,
1756 op :: D b => a -> b -> b
1759 class C a => D a where { ... }
1763 Here, <Literal>C</Literal> is a superclass of <Literal>D</Literal>, but it's OK for a
1764 class operation <Literal>op</Literal> of <Literal>C</Literal> to mention <Literal>D</Literal>. (It
1765 would not be OK for <Literal>D</Literal> to be a superclass of <Literal>C</Literal>.)
1772 <Emphasis>There are no restrictions on the context in a class declaration
1773 (which introduces superclasses), except that the class hierarchy must
1774 be acyclic</Emphasis>. So these class declarations are OK:
1778 class Functor (m k) => FiniteMap m k where
1781 class (Monad m, Monad (t m)) => Transform t m where
1782 lift :: m a -> (t m) a
1791 <Emphasis>In the signature of a class operation, every constraint
1792 must mention at least one type variable that is not a class type
1793 variable</Emphasis>.
1799 class Collection c a where
1800 mapC :: Collection c b => (a->b) -> c a -> c b
1804 is OK because the constraint <Literal>(Collection a b)</Literal> mentions
1805 <Literal>b</Literal>, even though it also mentions the class variable
1806 <Literal>a</Literal>. On the other hand:
1811 op :: Eq a => (a,b) -> (a,b)
1815 is not OK because the constraint <Literal>(Eq a)</Literal> mentions on the class
1816 type variable <Literal>a</Literal>, but not <Literal>b</Literal>. However, any such
1817 example is easily fixed by moving the offending context up to the
1822 class Eq a => C a where
1827 A yet more relaxed rule would allow the context of a class-op signature
1828 to mention only class type variables. However, that conflicts with
1829 Rule 1(b) for types above.
1836 <Emphasis>The type of each class operation must mention <Emphasis>all</Emphasis> of
1837 the class type variables</Emphasis>. For example:
1841 class Coll s a where
1843 insert :: s -> a -> s
1847 is not OK, because the type of <Literal>empty</Literal> doesn't mention
1848 <Literal>a</Literal>. This rule is a consequence of Rule 1(a), above, for
1849 types, and has the same motivation.
1851 Sometimes, offending class declarations exhibit misunderstandings. For
1852 example, <Literal>Coll</Literal> might be rewritten
1856 class Coll s a where
1858 insert :: s a -> a -> s a
1862 which makes the connection between the type of a collection of
1863 <Literal>a</Literal>'s (namely <Literal>(s a)</Literal>) and the element type <Literal>a</Literal>.
1864 Occasionally this really doesn't work, in which case you can split the
1872 class CollE s => Coll s a where
1873 insert :: s -> a -> s
1887 <Title>Instance declarations</Title>
1895 <Emphasis>Instance declarations may not overlap</Emphasis>. The two instance
1900 instance context1 => C type1 where ...
1901 instance context2 => C type2 where ...
1905 "overlap" if <Literal>type1</Literal> and <Literal>type2</Literal> unify
1907 However, if you give the command line option
1908 <Option>-fallow-overlapping-instances</Option><IndexTerm><Primary>-fallow-overlapping-instances
1909 option</Primary></IndexTerm> then two overlapping instance declarations are permitted
1917 EITHER <Literal>type1</Literal> and <Literal>type2</Literal> do not unify
1923 OR <Literal>type2</Literal> is a substitution instance of <Literal>type1</Literal>
1924 (but not identical to <Literal>type1</Literal>)
1937 Notice that these rules
1944 make it clear which instance decl to use
1945 (pick the most specific one that matches)
1952 do not mention the contexts <Literal>context1</Literal>, <Literal>context2</Literal>
1953 Reason: you can pick which instance decl
1954 "matches" based on the type.
1961 Regrettably, GHC doesn't guarantee to detect overlapping instance
1962 declarations if they appear in different modules. GHC can "see" the
1963 instance declarations in the transitive closure of all the modules
1964 imported by the one being compiled, so it can "see" all instance decls
1965 when it is compiling <Literal>Main</Literal>. However, it currently chooses not
1966 to look at ones that can't possibly be of use in the module currently
1967 being compiled, in the interests of efficiency. (Perhaps we should
1968 change that decision, at least for <Literal>Main</Literal>.)
1975 <Emphasis>There are no restrictions on the type in an instance
1976 <Emphasis>head</Emphasis>, except that at least one must not be a type variable</Emphasis>.
1977 The instance "head" is the bit after the "=>" in an instance decl. For
1978 example, these are OK:
1982 instance C Int a where ...
1984 instance D (Int, Int) where ...
1986 instance E [[a]] where ...
1990 Note that instance heads <Emphasis>may</Emphasis> contain repeated type variables.
1991 For example, this is OK:
1995 instance Stateful (ST s) (MutVar s) where ...
1999 The "at least one not a type variable" restriction is to ensure that
2000 context reduction terminates: each reduction step removes one type
2001 constructor. For example, the following would make the type checker
2002 loop if it wasn't excluded:
2006 instance C a => C a where ...
2010 There are two situations in which the rule is a bit of a pain. First,
2011 if one allows overlapping instance declarations then it's quite
2012 convenient to have a "default instance" declaration that applies if
2013 something more specific does not:
2022 Second, sometimes you might want to use the following to get the
2023 effect of a "class synonym":
2027 class (C1 a, C2 a, C3 a) => C a where { }
2029 instance (C1 a, C2 a, C3 a) => C a where { }
2033 This allows you to write shorter signatures:
2045 f :: (C1 a, C2 a, C3 a) => ...
2049 I'm on the lookout for a simple rule that preserves decidability while
2050 allowing these idioms. The experimental flag
2051 <Option>-fallow-undecidable-instances</Option><IndexTerm><Primary>-fallow-undecidable-instances
2052 option</Primary></IndexTerm> lifts this restriction, allowing all the types in an
2053 instance head to be type variables.
2060 <Emphasis>Unlike Haskell 1.4, instance heads may use type
2061 synonyms</Emphasis>. As always, using a type synonym is just shorthand for
2062 writing the RHS of the type synonym definition. For example:
2066 type Point = (Int,Int)
2067 instance C Point where ...
2068 instance C [Point] where ...
2072 is legal. However, if you added
2076 instance C (Int,Int) where ...
2080 as well, then the compiler will complain about the overlapping
2081 (actually, identical) instance declarations. As always, type synonyms
2082 must be fully applied. You cannot, for example, write:
2087 instance Monad P where ...
2091 This design decision is independent of all the others, and easily
2092 reversed, but it makes sense to me.
2099 <Emphasis>The types in an instance-declaration <Emphasis>context</Emphasis> must all
2100 be type variables</Emphasis>. Thus
2104 instance C a b => Eq (a,b) where ...
2112 instance C Int b => Foo b where ...
2116 is not OK. Again, the intent here is to make sure that context
2117 reduction terminates.
2119 Voluminous correspondence on the Haskell mailing list has convinced me
2120 that it's worth experimenting with a more liberal rule. If you use
2121 the flag <Option>-fallow-undecidable-instances</Option> can use arbitrary
2122 types in an instance context. Termination is ensured by having a
2123 fixed-depth recursion stack. If you exceed the stack depth you get a
2124 sort of backtrace, and the opportunity to increase the stack depth
2125 with <Option>-fcontext-stack</Option><Emphasis>N</Emphasis>.
2138 <Sect1 id="universal-quantification">
2139 <Title>Explicit universal quantification
2143 GHC now allows you to write explicitly quantified types. GHC's
2144 syntax for this now agrees with Hugs's, namely:
2150 forall a b. (Ord a, Eq b) => a -> b -> a
2156 The context is, of course, optional. You can't use <Literal>forall</Literal> as
2157 a type variable any more!
2161 Haskell type signatures are implicitly quantified. The <Literal>forall</Literal>
2162 allows us to say exactly what this means. For example:
2180 g :: forall b. (b -> b)
2186 The two are treated identically.
2190 <Title>Universally-quantified data type fields
2194 In a <Literal>data</Literal> or <Literal>newtype</Literal> declaration one can quantify
2195 the types of the constructor arguments. Here are several examples:
2201 data T a = T1 (forall b. b -> b -> b) a
2203 data MonadT m = MkMonad { return :: forall a. a -> m a,
2204 bind :: forall a b. m a -> (a -> m b) -> m b
2207 newtype Swizzle = MkSwizzle (Ord a => [a] -> [a])
2213 The constructors now have so-called <Emphasis>rank 2</Emphasis> polymorphic
2214 types, in which there is a for-all in the argument types.:
2220 T1 :: forall a. (forall b. b -> b -> b) -> a -> T a
2221 MkMonad :: forall m. (forall a. a -> m a)
2222 -> (forall a b. m a -> (a -> m b) -> m b)
2224 MkSwizzle :: (Ord a => [a] -> [a]) -> Swizzle
2230 Notice that you don't need to use a <Literal>forall</Literal> if there's an
2231 explicit context. For example in the first argument of the
2232 constructor <Function>MkSwizzle</Function>, an implicit "<Literal>forall a.</Literal>" is
2233 prefixed to the argument type. The implicit <Literal>forall</Literal>
2234 quantifies all type variables that are not already in scope, and are
2235 mentioned in the type quantified over.
2239 As for type signatures, implicit quantification happens for non-overloaded
2240 types too. So if you write this:
2243 data T a = MkT (Either a b) (b -> b)
2246 it's just as if you had written this:
2249 data T a = MkT (forall b. Either a b) (forall b. b -> b)
2252 That is, since the type variable <Literal>b</Literal> isn't in scope, it's
2253 implicitly universally quantified. (Arguably, it would be better
2254 to <Emphasis>require</Emphasis> explicit quantification on constructor arguments
2255 where that is what is wanted. Feedback welcomed.)
2261 <Title>Construction </Title>
2264 You construct values of types <Literal>T1, MonadT, Swizzle</Literal> by applying
2265 the constructor to suitable values, just as usual. For example,
2271 (T1 (\xy->x) 3) :: T Int
2273 (MkSwizzle sort) :: Swizzle
2274 (MkSwizzle reverse) :: Swizzle
2281 MkMonad r b) :: MonadT Maybe
2287 The type of the argument can, as usual, be more general than the type
2288 required, as <Literal>(MkSwizzle reverse)</Literal> shows. (<Function>reverse</Function>
2289 does not need the <Literal>Ord</Literal> constraint.)
2295 <Title>Pattern matching</Title>
2298 When you use pattern matching, the bound variables may now have
2299 polymorphic types. For example:
2305 f :: T a -> a -> (a, Char)
2306 f (T1 f k) x = (f k x, f 'c' 'd')
2308 g :: (Ord a, Ord b) => Swizzle -> [a] -> (a -> b) -> [b]
2309 g (MkSwizzle s) xs f = s (map f (s xs))
2311 h :: MonadT m -> [m a] -> m [a]
2312 h m [] = return m []
2313 h m (x:xs) = bind m x $ \y ->
2314 bind m (h m xs) $ \ys ->
2321 In the function <Function>h</Function> we use the record selectors <Literal>return</Literal>
2322 and <Literal>bind</Literal> to extract the polymorphic bind and return functions
2323 from the <Literal>MonadT</Literal> data structure, rather than using pattern
2328 You cannot pattern-match against an argument that is polymorphic.
2332 newtype TIM s a = TIM (ST s (Maybe a))
2334 runTIM :: (forall s. TIM s a) -> Maybe a
2335 runTIM (TIM m) = runST m
2341 Here the pattern-match fails, because you can't pattern-match against
2342 an argument of type <Literal>(forall s. TIM s a)</Literal>. Instead you
2343 must bind the variable and pattern match in the right hand side:
2346 runTIM :: (forall s. TIM s a) -> Maybe a
2347 runTIM tm = case tm of { TIM m -> runST m }
2350 The <Literal>tm</Literal> on the right hand side is (invisibly) instantiated, like
2351 any polymorphic value at its occurrence site, and now you can pattern-match
2358 <Title>The partial-application restriction</Title>
2361 There is really only one way in which data structures with polymorphic
2362 components might surprise you: you must not partially apply them.
2363 For example, this is illegal:
2369 map MkSwizzle [sort, reverse]
2375 The restriction is this: <Emphasis>every subexpression of the program must
2376 have a type that has no for-alls, except that in a function
2377 application (f e1…en) the partial applications are not subject to
2378 this rule</Emphasis>. The restriction makes type inference feasible.
2382 In the illegal example, the sub-expression <Literal>MkSwizzle</Literal> has the
2383 polymorphic type <Literal>(Ord b => [b] -> [b]) -> Swizzle</Literal> and is not
2384 a sub-expression of an enclosing application. On the other hand, this
2391 map (T1 (\a b -> a)) [1,2,3]
2397 even though it involves a partial application of <Function>T1</Function>, because
2398 the sub-expression <Literal>T1 (\a b -> a)</Literal> has type <Literal>Int -> T
2405 <Title>Type signatures
2409 Once you have data constructors with universally-quantified fields, or
2410 constants such as <Constant>runST</Constant> that have rank-2 types, it isn't long
2411 before you discover that you need more! Consider:
2417 mkTs f x y = [T1 f x, T1 f y]
2423 <Function>mkTs</Function> is a fuction that constructs some values of type
2424 <Literal>T</Literal>, using some pieces passed to it. The trouble is that since
2425 <Literal>f</Literal> is a function argument, Haskell assumes that it is
2426 monomorphic, so we'll get a type error when applying <Function>T1</Function> to
2427 it. This is a rather silly example, but the problem really bites in
2428 practice. Lots of people trip over the fact that you can't make
2429 "wrappers functions" for <Constant>runST</Constant> for exactly the same reason.
2430 In short, it is impossible to build abstractions around functions with
2435 The solution is fairly clear. We provide the ability to give a rank-2
2436 type signature for <Emphasis>ordinary</Emphasis> functions (not only data
2437 constructors), thus:
2443 mkTs :: (forall b. b -> b -> b) -> a -> [T a]
2444 mkTs f x y = [T1 f x, T1 f y]
2450 This type signature tells the compiler to attribute <Literal>f</Literal> with
2451 the polymorphic type <Literal>(forall b. b -> b -> b)</Literal> when type
2452 checking the body of <Function>mkTs</Function>, so now the application of
2453 <Function>T1</Function> is fine.
2457 There are two restrictions:
2466 You can only define a rank 2 type, specified by the following
2471 rank2type ::= [forall tyvars .] [context =>] funty
2472 funty ::= ([forall tyvars .] [context =>] ty) -> funty
2474 ty ::= ...current Haskell monotype syntax...
2478 Informally, the universal quantification must all be right at the beginning,
2479 or at the top level of a function argument.
2486 There is a restriction on the definition of a function whose
2487 type signature is a rank-2 type: the polymorphic arguments must be
2488 matched on the left hand side of the "<Literal>=</Literal>" sign. You can't
2489 define <Function>mkTs</Function> like this:
2493 mkTs :: (forall b. b -> b -> b) -> a -> [T a]
2494 mkTs = \ f x y -> [T1 f x, T1 f y]
2499 The same partial-application rule applies to ordinary functions with
2500 rank-2 types as applied to data constructors.
2513 <Title>Type synonyms and hoisting
2517 GHC also allows you to write a <Literal>forall</Literal> in a type synonym, thus:
2519 type Discard a = forall b. a -> b -> a
2524 However, it is often convenient to use these sort of synonyms at the right hand
2525 end of an arrow, thus:
2527 type Discard a = forall b. a -> b -> a
2529 g :: Int -> Discard Int
2532 Simply expanding the type synonym would give
2534 g :: Int -> (forall b. Int -> b -> Int)
2536 but GHC "hoists" the <Literal>forall</Literal> to give the isomorphic type
2538 g :: forall b. Int -> Int -> b -> Int
2540 In general, the rule is this: <Emphasis>to determine the type specified by any explicit
2541 user-written type (e.g. in a type signature), GHC expands type synonyms and then repeatedly
2542 performs the transformation:</Emphasis>
2544 <Emphasis>type1</Emphasis> -> forall a. <Emphasis>type2</Emphasis>
2546 forall a. <Emphasis>type1</Emphasis> -> <Emphasis>type2</Emphasis>
2548 (In fact, GHC tries to retain as much synonym information as possible for use in
2549 error messages, but that is a usability issue.) This rule applies, of course, whether
2550 or not the <Literal>forall</Literal> comes from a synonym. For example, here is another
2551 valid way to write <Literal>g</Literal>'s type signature:
2553 g :: Int -> Int -> forall b. b -> Int
2560 <Sect1 id="existential-quantification">
2561 <Title>Existentially quantified data constructors
2565 The idea of using existential quantification in data type declarations
2566 was suggested by Laufer (I believe, thought doubtless someone will
2567 correct me), and implemented in Hope+. It's been in Lennart
2568 Augustsson's <Command>hbc</Command> Haskell compiler for several years, and
2569 proved very useful. Here's the idea. Consider the declaration:
2575 data Foo = forall a. MkFoo a (a -> Bool)
2582 The data type <Literal>Foo</Literal> has two constructors with types:
2588 MkFoo :: forall a. a -> (a -> Bool) -> Foo
2595 Notice that the type variable <Literal>a</Literal> in the type of <Function>MkFoo</Function>
2596 does not appear in the data type itself, which is plain <Literal>Foo</Literal>.
2597 For example, the following expression is fine:
2603 [MkFoo 3 even, MkFoo 'c' isUpper] :: [Foo]
2609 Here, <Literal>(MkFoo 3 even)</Literal> packages an integer with a function
2610 <Function>even</Function> that maps an integer to <Literal>Bool</Literal>; and <Function>MkFoo 'c'
2611 isUpper</Function> packages a character with a compatible function. These
2612 two things are each of type <Literal>Foo</Literal> and can be put in a list.
2616 What can we do with a value of type <Literal>Foo</Literal>?. In particular,
2617 what happens when we pattern-match on <Function>MkFoo</Function>?
2623 f (MkFoo val fn) = ???
2629 Since all we know about <Literal>val</Literal> and <Function>fn</Function> is that they
2630 are compatible, the only (useful) thing we can do with them is to
2631 apply <Function>fn</Function> to <Literal>val</Literal> to get a boolean. For example:
2638 f (MkFoo val fn) = fn val
2644 What this allows us to do is to package heterogenous values
2645 together with a bunch of functions that manipulate them, and then treat
2646 that collection of packages in a uniform manner. You can express
2647 quite a bit of object-oriented-like programming this way.
2650 <Sect2 id="existential">
2651 <Title>Why existential?
2655 What has this to do with <Emphasis>existential</Emphasis> quantification?
2656 Simply that <Function>MkFoo</Function> has the (nearly) isomorphic type
2662 MkFoo :: (exists a . (a, a -> Bool)) -> Foo
2668 But Haskell programmers can safely think of the ordinary
2669 <Emphasis>universally</Emphasis> quantified type given above, thereby avoiding
2670 adding a new existential quantification construct.
2676 <Title>Type classes</Title>
2679 An easy extension (implemented in <Command>hbc</Command>) is to allow
2680 arbitrary contexts before the constructor. For example:
2686 data Baz = forall a. Eq a => Baz1 a a
2687 | forall b. Show b => Baz2 b (b -> b)
2693 The two constructors have the types you'd expect:
2699 Baz1 :: forall a. Eq a => a -> a -> Baz
2700 Baz2 :: forall b. Show b => b -> (b -> b) -> Baz
2706 But when pattern matching on <Function>Baz1</Function> the matched values can be compared
2707 for equality, and when pattern matching on <Function>Baz2</Function> the first matched
2708 value can be converted to a string (as well as applying the function to it).
2709 So this program is legal:
2716 f (Baz1 p q) | p == q = "Yes"
2718 f (Baz1 v fn) = show (fn v)
2724 Operationally, in a dictionary-passing implementation, the
2725 constructors <Function>Baz1</Function> and <Function>Baz2</Function> must store the
2726 dictionaries for <Literal>Eq</Literal> and <Literal>Show</Literal> respectively, and
2727 extract it on pattern matching.
2731 Notice the way that the syntax fits smoothly with that used for
2732 universal quantification earlier.
2738 <Title>Restrictions</Title>
2741 There are several restrictions on the ways in which existentially-quantified
2742 constructors can be use.
2751 When pattern matching, each pattern match introduces a new,
2752 distinct, type for each existential type variable. These types cannot
2753 be unified with any other type, nor can they escape from the scope of
2754 the pattern match. For example, these fragments are incorrect:
2762 Here, the type bound by <Function>MkFoo</Function> "escapes", because <Literal>a</Literal>
2763 is the result of <Function>f1</Function>. One way to see why this is wrong is to
2764 ask what type <Function>f1</Function> has:
2768 f1 :: Foo -> a -- Weird!
2772 What is this "<Literal>a</Literal>" in the result type? Clearly we don't mean
2777 f1 :: forall a. Foo -> a -- Wrong!
2781 The original program is just plain wrong. Here's another sort of error
2785 f2 (Baz1 a b) (Baz1 p q) = a==q
2789 It's ok to say <Literal>a==b</Literal> or <Literal>p==q</Literal>, but
2790 <Literal>a==q</Literal> is wrong because it equates the two distinct types arising
2791 from the two <Function>Baz1</Function> constructors.
2799 You can't pattern-match on an existentially quantified
2800 constructor in a <Literal>let</Literal> or <Literal>where</Literal> group of
2801 bindings. So this is illegal:
2805 f3 x = a==b where { Baz1 a b = x }
2809 You can only pattern-match
2810 on an existentially-quantified constructor in a <Literal>case</Literal> expression or
2811 in the patterns of a function definition.
2813 The reason for this restriction is really an implementation one.
2814 Type-checking binding groups is already a nightmare without
2815 existentials complicating the picture. Also an existential pattern
2816 binding at the top level of a module doesn't make sense, because it's
2817 not clear how to prevent the existentially-quantified type "escaping".
2818 So for now, there's a simple-to-state restriction. We'll see how
2826 You can't use existential quantification for <Literal>newtype</Literal>
2827 declarations. So this is illegal:
2831 newtype T = forall a. Ord a => MkT a
2835 Reason: a value of type <Literal>T</Literal> must be represented as a pair
2836 of a dictionary for <Literal>Ord t</Literal> and a value of type <Literal>t</Literal>.
2837 That contradicts the idea that <Literal>newtype</Literal> should have no
2838 concrete representation. You can get just the same efficiency and effect
2839 by using <Literal>data</Literal> instead of <Literal>newtype</Literal>. If there is no
2840 overloading involved, then there is more of a case for allowing
2841 an existentially-quantified <Literal>newtype</Literal>, because the <Literal>data</Literal>
2842 because the <Literal>data</Literal> version does carry an implementation cost,
2843 but single-field existentially quantified constructors aren't much
2844 use. So the simple restriction (no existential stuff on <Literal>newtype</Literal>)
2845 stands, unless there are convincing reasons to change it.
2853 You can't use <Literal>deriving</Literal> to define instances of a
2854 data type with existentially quantified data constructors.
2856 Reason: in most cases it would not make sense. For example:#
2859 data T = forall a. MkT [a] deriving( Eq )
2862 To derive <Literal>Eq</Literal> in the standard way we would need to have equality
2863 between the single component of two <Function>MkT</Function> constructors:
2867 (MkT a) == (MkT b) = ???
2870 But <VarName>a</VarName> and <VarName>b</VarName> have distinct types, and so can't be compared.
2871 It's just about possible to imagine examples in which the derived instance
2872 would make sense, but it seems altogether simpler simply to prohibit such
2873 declarations. Define your own instances!
2885 <Sect1 id="sec-assertions">
2887 <IndexTerm><Primary>Assertions</Primary></IndexTerm>
2891 If you want to make use of assertions in your standard Haskell code, you
2892 could define a function like the following:
2898 assert :: Bool -> a -> a
2899 assert False x = error "assertion failed!"
2906 which works, but gives you back a less than useful error message --
2907 an assertion failed, but which and where?
2911 One way out is to define an extended <Function>assert</Function> function which also
2912 takes a descriptive string to include in the error message and
2913 perhaps combine this with the use of a pre-processor which inserts
2914 the source location where <Function>assert</Function> was used.
2918 Ghc offers a helping hand here, doing all of this for you. For every
2919 use of <Function>assert</Function> in the user's source:
2925 kelvinToC :: Double -> Double
2926 kelvinToC k = assert (k >= 0.0) (k+273.15)
2932 Ghc will rewrite this to also include the source location where the
2939 assert pred val ==> assertError "Main.hs|15" pred val
2945 The rewrite is only performed by the compiler when it spots
2946 applications of <Function>Exception.assert</Function>, so you can still define and
2947 use your own versions of <Function>assert</Function>, should you so wish. If not,
2948 import <Literal>Exception</Literal> to make use <Function>assert</Function> in your code.
2952 To have the compiler ignore uses of assert, use the compiler option
2953 <Option>-fignore-asserts</Option>. <IndexTerm><Primary>-fignore-asserts option</Primary></IndexTerm> That is,
2954 expressions of the form <Literal>assert pred e</Literal> will be rewritten to <Literal>e</Literal>.
2958 Assertion failures can be caught, see the documentation for the
2959 <literal>Exception</literal> library (<xref linkend="sec-Exception">)
2965 <Sect1 id="scoped-type-variables">
2966 <Title>Scoped Type Variables
2970 A <Emphasis>pattern type signature</Emphasis> can introduce a <Emphasis>scoped type
2971 variable</Emphasis>. For example
2977 f (xs::[a]) = ys ++ ys
2986 The pattern <Literal>(xs::[a])</Literal> includes a type signature for <VarName>xs</VarName>.
2987 This brings the type variable <Literal>a</Literal> into scope; it scopes over
2988 all the patterns and right hand sides for this equation for <Function>f</Function>.
2989 In particular, it is in scope at the type signature for <VarName>y</VarName>.
2993 At ordinary type signatures, such as that for <VarName>ys</VarName>, any type variables
2994 mentioned in the type signature <Emphasis>that are not in scope</Emphasis> are
2995 implicitly universally quantified. (If there are no type variables in
2996 scope, all type variables mentioned in the signature are universally
2997 quantified, which is just as in Haskell 98.) In this case, since <VarName>a</VarName>
2998 is in scope, it is not universally quantified, so the type of <VarName>ys</VarName> is
2999 the same as that of <VarName>xs</VarName>. In Haskell 98 it is not possible to declare
3000 a type for <VarName>ys</VarName>; a major benefit of scoped type variables is that
3001 it becomes possible to do so.
3005 Scoped type variables are implemented in both GHC and Hugs. Where the
3006 implementations differ from the specification below, those differences
3011 So much for the basic idea. Here are the details.
3015 <Title>Scope and implicit quantification</Title>
3023 All the type variables mentioned in the patterns for a single
3024 function definition equation, that are not already in scope,
3025 are brought into scope by the patterns. We describe this set as
3026 the <Emphasis>type variables bound by the equation</Emphasis>.
3033 The type variables thus brought into scope may be mentioned
3034 in ordinary type signatures or pattern type signatures anywhere within
3042 In ordinary type signatures, any type variable mentioned in the
3043 signature that is in scope is <Emphasis>not</Emphasis> universally quantified.
3050 Ordinary type signatures do not bring any new type variables
3051 into scope (except in the type signature itself!). So this is illegal:
3060 It's illegal because <VarName>a</VarName> is not in scope in the body of <Function>f</Function>,
3061 so the ordinary signature <Literal>x::a</Literal> is equivalent to <Literal>x::forall a.a</Literal>;
3062 and that is an incorrect typing.
3069 There is no implicit universal quantification on pattern type
3070 signatures, nor may one write an explicit <Literal>forall</Literal> type in a pattern
3071 type signature. The pattern type signature is a monotype.
3079 The type variables in the head of a <Literal>class</Literal> or <Literal>instance</Literal> declaration
3080 scope over the methods defined in the <Literal>where</Literal> part. For example:
3094 (Not implemented in Hugs yet, Dec 98).
3105 <Title>Polymorphism</Title>
3113 Pattern type signatures are completely orthogonal to ordinary, separate
3114 type signatures. The two can be used independently or together. There is
3115 no scoping associated with the names of the type variables in a separate type signature.
3120 f (xs::[b]) = reverse xs
3129 The function must be polymorphic in the type variables
3130 bound by all its equations. Operationally, the type variables bound
3131 by one equation must not:
3138 Be unified with a type (such as <Literal>Int</Literal>, or <Literal>[a]</Literal>).
3144 Be unified with a type variable free in the environment.
3150 Be unified with each other. (They may unify with the type variables
3151 bound by another equation for the same function, of course.)
3158 For example, the following all fail to type check:
3162 f (x::a) (y::b) = [x,y] -- a unifies with b
3164 g (x::a) = x + 1::Int -- a unifies with Int
3166 h x = let k (y::a) = [x,y] -- a is free in the
3167 in k x -- environment
3169 k (x::a) True = ... -- a unifies with Int
3170 k (x::Int) False = ...
3173 w (x::a) = x -- a unifies with [b]
3182 The pattern-bound type variable may, however, be constrained
3183 by the context of the principal type, thus:
3187 f (x::a) (y::a) = x+y*2
3191 gets the inferred type: <Literal>forall a. Num a => a -> a -> a</Literal>.
3202 <Title>Result type signatures</Title>
3210 The result type of a function can be given a signature,
3215 f (x::a) :: [a] = [x,x,x]
3219 The final <Literal>:: [a]</Literal> after all the patterns gives a signature to the
3220 result type. Sometimes this is the only way of naming the type variable
3225 f :: Int -> [a] -> [a]
3226 f n :: ([a] -> [a]) = let g (x::a, y::a) = (y,x)
3227 in \xs -> map g (reverse xs `zip` xs)
3239 Result type signatures are not yet implemented in Hugs.
3245 <Title>Pattern signatures on other constructs</Title>
3253 A pattern type signature can be on an arbitrary sub-pattern, not
3258 f ((x,y)::(a,b)) = (y,x) :: (b,a)
3267 Pattern type signatures, including the result part, can be used
3268 in lambda abstractions:
3272 (\ (x::a, y) :: a -> x)
3276 Type variables bound by these patterns must be polymorphic in
3277 the sense defined above.
3282 f1 (x::c) = f1 x -- ok
3283 f2 = \(x::c) -> f2 x -- not ok
3287 Here, <Function>f1</Function> is OK, but <Function>f2</Function> is not, because <VarName>c</VarName> gets unified
3288 with a type variable free in the environment, in this
3289 case, the type of <Function>f2</Function>, which is in the environment when
3290 the lambda abstraction is checked.
3297 Pattern type signatures, including the result part, can be used
3298 in <Literal>case</Literal> expressions:
3302 case e of { (x::a, y) :: a -> x }
3306 The pattern-bound type variables must, as usual,
3307 be polymorphic in the following sense: each case alternative,
3308 considered as a lambda abstraction, must be polymorphic.
3313 case (True,False) of { (x::a, y) -> x }
3317 Even though the context is that of a pair of booleans,
3318 the alternative itself is polymorphic. Of course, it is
3323 case (True,False) of { (x::Bool, y) -> x }
3332 To avoid ambiguity, the type after the “<Literal>::</Literal>” in a result
3333 pattern signature on a lambda or <Literal>case</Literal> must be atomic (i.e. a single
3334 token or a parenthesised type of some sort). To see why,
3335 consider how one would parse this:
3348 Pattern type signatures that bind new type variables
3349 may not be used in pattern bindings at all.
3354 f x = let (y, z::a) = x in ...
3358 But these are OK, because they do not bind fresh type variables:
3362 f1 x = let (y, z::Int) = x in ...
3363 f2 (x::(Int,a)) = let (y, z::a) = x in ...
3367 However a single variable is considered a degenerate function binding,
3368 rather than a degerate pattern binding, so this is permitted, even
3369 though it binds a type variable:
3373 f :: (b->b) = \(x::b) -> x
3382 Such degnerate function bindings do not fall under the monomorphism
3389 g :: a -> a -> Bool = \x y. x==y
3395 Here <Function>g</Function> has type <Literal>forall a. Eq a => a -> a -> Bool</Literal>, just as if
3396 <Function>g</Function> had a separate type signature. Lacking a type signature, <Function>g</Function>
3397 would get a monomorphic type.
3403 <Title>Existentials</Title>
3411 Pattern type signatures can bind existential type variables.
3416 data T = forall a. MkT [a]
3419 f (MkT [t::a]) = MkT t3
3436 <Sect1 id="pragmas">
3441 GHC supports several pragmas, or instructions to the compiler placed
3442 in the source code. Pragmas don't affect the meaning of the program,
3443 but they might affect the efficiency of the generated code.
3446 <Sect2 id="inline-pragma">
3447 <Title>INLINE pragma
3449 <IndexTerm><Primary>INLINE pragma</Primary></IndexTerm>
3450 <IndexTerm><Primary>pragma, INLINE</Primary></IndexTerm></Title>
3453 GHC (with <Option>-O</Option>, as always) tries to inline (or “unfold”)
3454 functions/values that are “small enough,” thus avoiding the call
3455 overhead and possibly exposing other more-wonderful optimisations.
3459 You will probably see these unfoldings (in Core syntax) in your
3464 Normally, if GHC decides a function is “too expensive” to inline, it
3465 will not do so, nor will it export that unfolding for other modules to
3470 The sledgehammer you can bring to bear is the
3471 <Literal>INLINE</Literal><IndexTerm><Primary>INLINE pragma</Primary></IndexTerm> pragma, used thusly:
3474 key_function :: Int -> String -> (Bool, Double)
3476 #ifdef __GLASGOW_HASKELL__
3477 {-# INLINE key_function #-}
3481 (You don't need to do the C pre-processor carry-on unless you're going
3482 to stick the code through HBC—it doesn't like <Literal>INLINE</Literal> pragmas.)
3486 The major effect of an <Literal>INLINE</Literal> pragma is to declare a function's
3487 “cost” to be very low. The normal unfolding machinery will then be
3488 very keen to inline it.
3492 An <Literal>INLINE</Literal> pragma for a function can be put anywhere its type
3493 signature could be put.
3497 <Literal>INLINE</Literal> pragmas are a particularly good idea for the
3498 <Literal>then</Literal>/<Literal>return</Literal> (or <Literal>bind</Literal>/<Literal>unit</Literal>) functions in a monad.
3499 For example, in GHC's own <Literal>UniqueSupply</Literal> monad code, we have:
3502 #ifdef __GLASGOW_HASKELL__
3503 {-# INLINE thenUs #-}
3504 {-# INLINE returnUs #-}
3512 <Sect2 id="noinline-pragma">
3513 <Title>NOINLINE pragma
3517 <IndexTerm><Primary>NOINLINE pragma</Primary></IndexTerm>
3518 <IndexTerm><Primary>pragma, NOINLINE</Primary></IndexTerm>
3522 The <Literal>NOINLINE</Literal> pragma does exactly what you'd expect: it stops the
3523 named function from being inlined by the compiler. You shouldn't ever
3524 need to do this, unless you're very cautious about code size.
3529 <Sect2 id="specialize-pragma">
3530 <Title>SPECIALIZE pragma
3534 <IndexTerm><Primary>SPECIALIZE pragma</Primary></IndexTerm>
3535 <IndexTerm><Primary>pragma, SPECIALIZE</Primary></IndexTerm>
3536 <IndexTerm><Primary>overloading, death to</Primary></IndexTerm>
3540 (UK spelling also accepted.) For key overloaded functions, you can
3541 create extra versions (NB: more code space) specialised to particular
3542 types. Thus, if you have an overloaded function:
3548 hammeredLookup :: Ord key => [(key, value)] -> key -> value
3554 If it is heavily used on lists with <Literal>Widget</Literal> keys, you could
3555 specialise it as follows:
3558 {-# SPECIALIZE hammeredLookup :: [(Widget, value)] -> Widget -> value #-}
3564 To get very fancy, you can also specify a named function to use for
3565 the specialised value, by adding <Literal>= blah</Literal>, as in:
3568 {-# SPECIALIZE hammeredLookup :: ...as before... = blah #-}
3571 It's <Emphasis>Your Responsibility</Emphasis> to make sure that <Function>blah</Function> really
3572 behaves as a specialised version of <Function>hammeredLookup</Function>!!!
3576 NOTE: the <Literal>=blah</Literal> feature isn't implemented in GHC 4.xx.
3580 An example in which the <Literal>= blah</Literal> form will Win Big:
3583 toDouble :: Real a => a -> Double
3584 toDouble = fromRational . toRational
3586 {-# SPECIALIZE toDouble :: Int -> Double = i2d #-}
3587 i2d (I# i) = D# (int2Double# i) -- uses Glasgow prim-op directly
3590 The <Function>i2d</Function> function is virtually one machine instruction; the
3591 default conversion—via an intermediate <Literal>Rational</Literal>—is obscenely
3592 expensive by comparison.
3596 By using the US spelling, your <Literal>SPECIALIZE</Literal> pragma will work with
3597 HBC, too. Note that HBC doesn't support the <Literal>= blah</Literal> form.
3601 A <Literal>SPECIALIZE</Literal> pragma for a function can be put anywhere its type
3602 signature could be put.
3607 <Sect2 id="specialize-instance-pragma">
3608 <Title>SPECIALIZE instance pragma
3612 <IndexTerm><Primary>SPECIALIZE pragma</Primary></IndexTerm>
3613 <IndexTerm><Primary>overloading, death to</Primary></IndexTerm>
3614 Same idea, except for instance declarations. For example:
3617 instance (Eq a) => Eq (Foo a) where { ... usual stuff ... }
3619 {-# SPECIALIZE instance Eq (Foo [(Int, Bar)] #-}
3622 Compatible with HBC, by the way.
3627 <Sect2 id="line-pragma">
3632 <IndexTerm><Primary>LINE pragma</Primary></IndexTerm>
3633 <IndexTerm><Primary>pragma, LINE</Primary></IndexTerm>
3637 This pragma is similar to C's <Literal>#line</Literal> pragma, and is mainly for use in
3638 automatically generated Haskell code. It lets you specify the line
3639 number and filename of the original code; for example
3645 {-# LINE 42 "Foo.vhs" #-}
3651 if you'd generated the current file from something called <Filename>Foo.vhs</Filename>
3652 and this line corresponds to line 42 in the original. GHC will adjust
3653 its error messages to refer to the line/file named in the <Literal>LINE</Literal>
3660 <Title>RULES pragma</Title>
3663 The RULES pragma lets you specify rewrite rules. It is described in
3664 <XRef LinkEnd="rewrite-rules">.
3671 <Sect1 id="rewrite-rules">
3672 <Title>Rewrite rules
3674 <IndexTerm><Primary>RULES pagma</Primary></IndexTerm>
3675 <IndexTerm><Primary>pragma, RULES</Primary></IndexTerm>
3676 <IndexTerm><Primary>rewrite rules</Primary></IndexTerm></Title>
3679 The programmer can specify rewrite rules as part of the source program
3680 (in a pragma). GHC applies these rewrite rules wherever it can.
3688 "map/map" forall f g xs. map f (map g xs) = map (f.g) xs
3695 <Title>Syntax</Title>
3698 From a syntactic point of view:
3704 Each rule has a name, enclosed in double quotes. The name itself has
3705 no significance at all. It is only used when reporting how many times the rule fired.
3711 There may be zero or more rules in a <Literal>RULES</Literal> pragma.
3717 Layout applies in a <Literal>RULES</Literal> pragma. Currently no new indentation level
3718 is set, so you must lay out your rules starting in the same column as the
3719 enclosing definitions.
3725 Each variable mentioned in a rule must either be in scope (e.g. <Function>map</Function>),
3726 or bound by the <Literal>forall</Literal> (e.g. <Function>f</Function>, <Function>g</Function>, <Function>xs</Function>). The variables bound by
3727 the <Literal>forall</Literal> are called the <Emphasis>pattern</Emphasis> variables. They are separated
3728 by spaces, just like in a type <Literal>forall</Literal>.
3734 A pattern variable may optionally have a type signature.
3735 If the type of the pattern variable is polymorphic, it <Emphasis>must</Emphasis> have a type signature.
3736 For example, here is the <Literal>foldr/build</Literal> rule:
3739 "fold/build" forall k z (g::forall b. (a->b->b) -> b -> b) .
3740 foldr k z (build g) = g k z
3743 Since <Function>g</Function> has a polymorphic type, it must have a type signature.
3750 The left hand side of a rule must consist of a top-level variable applied
3751 to arbitrary expressions. For example, this is <Emphasis>not</Emphasis> OK:
3754 "wrong1" forall e1 e2. case True of { True -> e1; False -> e2 } = e1
3755 "wrong2" forall f. f True = True
3758 In <Literal>"wrong1"</Literal>, the LHS is not an application; in <Literal>"wrong2"</Literal>, the LHS has a pattern variable
3765 A rule does not need to be in the same module as (any of) the
3766 variables it mentions, though of course they need to be in scope.
3772 Rules are automatically exported from a module, just as instance declarations are.
3783 <Title>Semantics</Title>
3786 From a semantic point of view:
3792 Rules are only applied if you use the <Option>-O</Option> flag.
3798 Rules are regarded as left-to-right rewrite rules.
3799 When GHC finds an expression that is a substitution instance of the LHS
3800 of a rule, it replaces the expression by the (appropriately-substituted) RHS.
3801 By "a substitution instance" we mean that the LHS can be made equal to the
3802 expression by substituting for the pattern variables.
3809 The LHS and RHS of a rule are typechecked, and must have the
3817 GHC makes absolutely no attempt to verify that the LHS and RHS
3818 of a rule have the same meaning. That is undecideable in general, and
3819 infeasible in most interesting cases. The responsibility is entirely the programmer's!
3826 GHC makes no attempt to make sure that the rules are confluent or
3827 terminating. For example:
3830 "loop" forall x,y. f x y = f y x
3833 This rule will cause the compiler to go into an infinite loop.
3840 If more than one rule matches a call, GHC will choose one arbitrarily to apply.
3846 GHC currently uses a very simple, syntactic, matching algorithm
3847 for matching a rule LHS with an expression. It seeks a substitution
3848 which makes the LHS and expression syntactically equal modulo alpha
3849 conversion. The pattern (rule), but not the expression, is eta-expanded if
3850 necessary. (Eta-expanding the epression can lead to laziness bugs.)
3851 But not beta conversion (that's called higher-order matching).
3855 Matching is carried out on GHC's intermediate language, which includes
3856 type abstractions and applications. So a rule only matches if the
3857 types match too. See <XRef LinkEnd="rule-spec"> below.
3863 GHC keeps trying to apply the rules as it optimises the program.
3864 For example, consider:
3873 The expression <Literal>s (t xs)</Literal> does not match the rule <Literal>"map/map"</Literal>, but GHC
3874 will substitute for <VarName>s</VarName> and <VarName>t</VarName>, giving an expression which does match.
3875 If <VarName>s</VarName> or <VarName>t</VarName> was (a) used more than once, and (b) large or a redex, then it would
3876 not be substituted, and the rule would not fire.
3883 In the earlier phases of compilation, GHC inlines <Emphasis>nothing
3884 that appears on the LHS of a rule</Emphasis>, because once you have substituted
3885 for something you can't match against it (given the simple minded
3886 matching). So if you write the rule
3889 "map/map" forall f,g. map f . map g = map (f.g)
3892 this <Emphasis>won't</Emphasis> match the expression <Literal>map f (map g xs)</Literal>.
3893 It will only match something written with explicit use of ".".
3894 Well, not quite. It <Emphasis>will</Emphasis> match the expression
3900 where <Function>wibble</Function> is defined:
3903 wibble f g = map f . map g
3906 because <Function>wibble</Function> will be inlined (it's small).
3908 Later on in compilation, GHC starts inlining even things on the
3909 LHS of rules, but still leaves the rules enabled. This inlining
3910 policy is controlled by the per-simplification-pass flag <Option>-finline-phase</Option><Emphasis>n</Emphasis>.
3917 All rules are implicitly exported from the module, and are therefore
3918 in force in any module that imports the module that defined the rule, directly
3919 or indirectly. (That is, if A imports B, which imports C, then C's rules are
3920 in force when compiling A.) The situation is very similar to that for instance
3932 <Title>List fusion</Title>
3935 The RULES mechanism is used to implement fusion (deforestation) of common list functions.
3936 If a "good consumer" consumes an intermediate list constructed by a "good producer", the
3937 intermediate list should be eliminated entirely.
3941 The following are good producers:
3953 Enumerations of <Literal>Int</Literal> and <Literal>Char</Literal> (e.g. <Literal>['a'..'z']</Literal>).
3959 Explicit lists (e.g. <Literal>[True, False]</Literal>)
3965 The cons constructor (e.g <Literal>3:4:[]</Literal>)
3971 <Function>++</Function>
3977 <Function>map</Function>
3983 <Function>filter</Function>
3989 <Function>iterate</Function>, <Function>repeat</Function>
3995 <Function>zip</Function>, <Function>zipWith</Function>
4004 The following are good consumers:
4016 <Function>array</Function> (on its second argument)
4022 <Function>length</Function>
4028 <Function>++</Function> (on its first argument)
4034 <Function>map</Function>
4040 <Function>filter</Function>
4046 <Function>concat</Function>
4052 <Function>unzip</Function>, <Function>unzip2</Function>, <Function>unzip3</Function>, <Function>unzip4</Function>
4058 <Function>zip</Function>, <Function>zipWith</Function> (but on one argument only; if both are good producers, <Function>zip</Function>
4059 will fuse with one but not the other)
4065 <Function>partition</Function>
4071 <Function>head</Function>
4077 <Function>and</Function>, <Function>or</Function>, <Function>any</Function>, <Function>all</Function>
4083 <Function>sequence_</Function>
4089 <Function>msum</Function>
4095 <Function>sortBy</Function>
4104 So, for example, the following should generate no intermediate lists:
4107 array (1,10) [(i,i*i) | i <- map (+ 1) [0..9]]
4113 This list could readily be extended; if there are Prelude functions that you use
4114 a lot which are not included, please tell us.
4118 If you want to write your own good consumers or producers, look at the
4119 Prelude definitions of the above functions to see how to do so.
4124 <Sect2 id="rule-spec">
4125 <Title>Specialisation
4129 Rewrite rules can be used to get the same effect as a feature
4130 present in earlier version of GHC:
4133 {-# SPECIALIZE fromIntegral :: Int8 -> Int16 = int8ToInt16 #-}
4136 This told GHC to use <Function>int8ToInt16</Function> instead of <Function>fromIntegral</Function> whenever
4137 the latter was called with type <Literal>Int8 -> Int16</Literal>. That is, rather than
4138 specialising the original definition of <Function>fromIntegral</Function> the programmer is
4139 promising that it is safe to use <Function>int8ToInt16</Function> instead.
4143 This feature is no longer in GHC. But rewrite rules let you do the
4148 "fromIntegral/Int8/Int16" fromIntegral = int8ToInt16
4152 This slightly odd-looking rule instructs GHC to replace <Function>fromIntegral</Function>
4153 by <Function>int8ToInt16</Function> <Emphasis>whenever the types match</Emphasis>. Speaking more operationally,
4154 GHC adds the type and dictionary applications to get the typed rule
4157 forall (d1::Integral Int8) (d2::Num Int16) .
4158 fromIntegral Int8 Int16 d1 d2 = int8ToInt16
4162 this rule does not need to be in the same file as fromIntegral,
4163 unlike the <Literal>SPECIALISE</Literal> pragmas which currently do (so that they
4164 have an original definition available to specialise).
4170 <Title>Controlling what's going on</Title>
4178 Use <Option>-ddump-rules</Option> to see what transformation rules GHC is using.
4184 Use <Option>-ddump-simpl-stats</Option> to see what rules are being fired.
4185 If you add <Option>-dppr-debug</Option> you get a more detailed listing.
4191 The defintion of (say) <Function>build</Function> in <FileName>PrelBase.lhs</FileName> looks llike this:
4194 build :: forall a. (forall b. (a -> b -> b) -> b -> b) -> [a]
4195 {-# INLINE build #-}
4199 Notice the <Literal>INLINE</Literal>! That prevents <Literal>(:)</Literal> from being inlined when compiling
4200 <Literal>PrelBase</Literal>, so that an importing module will “see” the <Literal>(:)</Literal>, and can
4201 match it on the LHS of a rule. <Literal>INLINE</Literal> prevents any inlining happening
4202 in the RHS of the <Literal>INLINE</Literal> thing. I regret the delicacy of this.
4209 In <Filename>ghc/lib/std/PrelBase.lhs</Filename> look at the rules for <Function>map</Function> to
4210 see how to write rules that will do fusion and yet give an efficient
4211 program even if fusion doesn't happen. More rules in <Filename>PrelList.lhs</Filename>.
4224 ;;; Local Variables: ***
4226 ;;; sgml-parent-document: ("users_guide.sgml" "book" "chapter" "sect1") ***
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