6 The motivation behind this foreign function interface(FFI)
7 specification is to make it possible to describe in Haskell <Emphasis>source
8 code</Emphasis> the interface to foreign functionality in a Haskell system
9 independent manner. It builds on experiences made with the previous
10 foreign function interfaces provided by GHC and Hugs.
14 The FFI specified in this document is not in the market of trying to
15 completely bridge the gap between the actual type of an external
16 function, and what is a <Emphasis>convenient</Emphasis> type for that function to the
17 Haskell programmer. That is the domain of tools like HaskellDirect or
18 Green Card, both of which are capable of generating Haskell code that
23 The FFI can be split up into two complementary halves; one half that
24 provides Haskell constructs for importing foreign functionality into
25 Haskell, the other which lets you expose Haskell functions to the
26 outside world. We start with the former, how to import external
27 functionality into Haskell.
32 <Sect1 id="sec-primitive">
33 <Title>Calling foreign functions
37 To bind a Haskell variable name and type to an external function, we
38 introduce a new construct: <Literal>foreign import</Literal>. It defines the type of a Haskell function together with the name of an external function that actually implements it. The syntax of <Literal>foreign import</Literal> construct is as follows:
47 | 'foreign' 'import' [callconv] [ext_fun] ['unsafe'] varid '::' prim_type
53 A <Literal>foreign import</Literal> declaration is only allowed as a toplevel
54 declaration. It consists of two parts, one giving the Haskell type
55 (<Literal>prim_type</Literal>), Haskell name (<Literal>varid</Literal>) and a flag indicating whether the
56 primitive is unsafe, the other giving details of the name of the
57 external function (<Literal>ext_fun</Literal>) and its calling interface
58 (<Literal>callconv</Literal>.)
62 Giving a Haskell name and type to an external entry point is clearly
63 an unsafe thing to do, as the external name will in most cases be
64 untyped. The onus is on the programmer using <Literal>foreign import</Literal> to
65 ensure that the Haskell type given correctly maps on to the
66 type of the external function. Section
67 <XRef LinkEnd="sec-mapping"> specifies the mapping from
68 Haskell types to external types.
71 <Sect2 id="sec-prim-name">
72 <Title>Giving the external function a Haskell name
76 The external function has to be given a Haskell name. The name
77 must be a Haskell <Literal>varid</Literal>, so the language rules regarding
78 variable names must be followed, i.e., it must start with a
79 lower case letter followed by a sequence of alphanumeric
80 (`in the Unicode sense') characters or '.
84 Notice that with Haskell 98, underscore ('_') is included in
85 the character class <Literal>small</Literal>.
93 varid : small ( small | large | udigit | ' )*
99 <Sect2 id="sec-prim-ext-name">
100 <Title>Naming the external function
104 The name of the external function consists of two parts,
105 one specifying its location, the other its name:
111 ext_fun : ext_loc ext_name
127 foreign import stdcall "Advapi32" "RegCloseKey" regCloseKey :: Addr -> IO ()
133 states that the external function named <Function>RegCloseKey</Function> at location
134 <Function>Advapi32</Function> should be bound to the Haskell name <Function>regCloseKey</Function>.
135 For a Win32 Haskell implementation that supports the loading of DLLs
136 on-the-fly, this declaration will most likely cause the run-time
137 system to load the <Filename>Advapi32.dll</Filename> DLL before looking up the
138 function <Function>RegCloseKey()</Function> therein to get at the function pointer
139 to use when invoking <Function>regCloseKey</Function>.
143 Compiled implementations may do something completely different, i.e.,
144 mangle "RegCloseKey" to convert it into an archive/import library
145 symbol, that's assumed to be in scope when linking. The details of
146 which are platform (and compiler command-line) dependent.
150 If the location part is left out, the name of the external function
151 specifies a symbol that is assumed to be in scope when linking.
155 The location part can either contain an absolute `address' (i.e.,
156 path) of the archive/DLL, or just its name, leaving it up to the
157 underlying system (system meaning both RTS/compiler and OS) to resolve
158 the name to its real location.
162 An implementation is <Emphasis>expected</Emphasis> to be able to intelligently
163 transform the <Literal>ext_loc</Literal> location to fit platform-specific
164 practices for naming dynamic libraries. For instance, given the
171 foreign import "Foo" "foo" foo :: Int -> Int -> IO ()
177 an implementation should map <Filename>Foo</Filename> to <Filename>"Foo.dll"</Filename> on a Win32
178 platform, and <Filename>libFoo.so</Filename> on ELF platforms. If the lookup of the
179 dynamic library with this transformed location name should fail, the
180 implementation should then attempt to use the original name before
181 eventually giving up. As part of their documentation, implementations
182 of <Literal>foreign import</Literal> should specify the exact details of how
183 <Literal>ext_loc</Literal>s are transformed and resolved, including the list of
184 directories searched (and the order in which they are.)
188 In the case the Haskell name of the imported function is identical to
189 the external name, the <Literal>ext_fun</Literal> can be omitted. i.e.,
195 foreign import sin :: Double -> IO Double
207 foreign import "sin" sin :: Double -> IO Double
214 <Sect2 id="sec-cconv">
215 <Title>Calling conventions
219 The number of calling conventions supported is fixed:
225 callconv : ccall | stdcall
234 <Term><Literal>ccall</Literal></Term>
237 The 'default' calling convention on a platform, i.e., the one
238 used to do (C) function calls.
242 In the case of x86 platforms, the caller pushes function arguments
243 from right to left on the C stack before calling. The caller is
244 responsible for popping the arguments off of the C stack on return.
249 <Term><Literal>stdcall</Literal></Term>
252 A Win32 specific calling convention. The same as <Literal>ccall</Literal>, except
253 that the callee cleans up the C stack before returning.
257 The <Literal>stdcall</Literal> is a Microsoft Win32 specific wrinkle; it used
258 throughout the Win32 API, for instance. On platforms where
259 <Literal>stdcall</Literal> isn't meaningful, it should be treated as being equal
260 to <Literal>ccall</Literal>.
271 <Emphasis remap="bf">Some remarks:</Emphasis>
277 Interoperating well with external code is the name of the game here,
278 so the guiding principle when deciding on what calling conventions
279 to include in <Literal>callconv</Literal> is that there's a demonstrated need for
280 a particular calling convention. Should it emerge that the inclusion
281 of other calling conventions will generally improve the quality of
282 this Haskell FFI, they will be considered for future inclusion in
283 <Literal>callconv</Literal>.
289 Supporting <Literal>stdcall</Literal> (and perhaps other platform-specific calling
290 conventions) raises the issue of whether a Haskell FFI should allow
291 the user to write platform-specific Haskell code. The calling
292 convention is clearly an integral part of an external function's
293 interface, so if the one used differs from the standard one specified
294 by the platform's ABI <Emphasis>and</Emphasis> that convention is used by a
295 non-trivial amount of external functions, the view of the FFI authors
296 is that a Haskell FFI should support it.
302 For <Literal>foreign import</Literal> (and other <Literal>foreign</Literal> declarations),
303 supplying the calling convention is optional. If it isn't supplied,
304 it is treated as if <Literal>ccall</Literal> was specified. Users are encouraged
305 to leave out the specification of the calling convention, if possible.
315 <Sect2 id="sec-prim-types">
316 <Title>External function types
320 The range of types that can be passed as arguments to an external
321 function is restricted (as are the range of results coming back):
327 prim_type : IO prim_result
329 | prim_arg '->' prim_type
340 If you associate a non-IO type with an external function, you
341 have the same 'proof obligations' as when you make use of
342 <Function>IOExts.unsafePerformIO</Function> in your Haskell programs.
348 The external function is strict in all its arguments.
354 <Emphasis>GHC only:</Emphasis> The GHC FFI implementation provides one extension
355 to <Literal>prim_type</Literal>:
360 | unsafe_arr_ty '->' prim_type
362 unsafe_arr_ty : ByteArray a
363 | MutableByteArray i s a
367 GHC permits the passing of its byte array primitive types
368 to external functions. There's some restrictions on when
369 they can be used; see Section <XRef LinkEnd="sec-arguments">
379 Section <XRef LinkEnd="sec-results"> defines
380 <Literal>prim_result</Literal>; Section <XRef LinkEnd="sec-arguments">
381 defines <Literal>prim_arg</Literal>.
384 <Sect3 id="sec-arguments">
385 <Title>Argument types
389 The external function expects zero or more arguments. The set of legal
390 argument types is restricted to the following set:
396 prim_arg : ext_ty | new_ty | ForeignObj
398 new_ty : a Haskell newtype of a prim_arg.
400 ext_ty : int_ty | word_ty | float_ty
401 | Addr | Char | StablePtr a
404 int_ty : Int | Int8 | Int16 | Int32 | Int64
405 word_ty : Word8 | Word16 | Word32 | Word64
406 float_ty : Float | Double
417 <Literal>ext_ty</Literal> represent the set of basic types supported by
418 C-like languages, although the numeric types are explicitly sized.
420 The <Emphasis>stable pointer</Emphasis> <Literal>StablePtr</Literal> type looks out of place in
421 this list of C-like types, but it has a well-defined and simple
422 C mapping, see Section <XRef LinkEnd="sec-mapping">
430 <Literal>prim_arg</Literal> represent the set of permissible argument types. In
431 addition to <Literal>ext_ty</Literal>, <Literal>ForeignObj</Literal> is also included.
433 The <Literal>ForeignObj</Literal> type represent values that are pointers to some
434 external entity/object. It differs from the <Literal>Addr</Literal> type in that
435 <Literal>ForeignObj</Literal>s are <Emphasis>finalized</Emphasis>, i.e., once the garbage collector
436 determines that a <Literal>ForeignObj</Literal> is unreachable, it will invoke a
437 finalising procedure attached to the <Literal>ForeignObj</Literal> to notify the
438 outside world that we're through with using it.
445 Haskell <Literal>newtype</Literal>s that wrap up a <Literal>prim_arg</Literal> type can also
446 be passed to external functions.
452 Haskell type synonyms for any of the above can also be used
453 in <Literal>foreign import</Literal> declarations. Qualified names likewise,
454 i.e. <Literal>Word.Word32</Literal> is legal.
461 <Literal>foreign import</Literal> does not support the binding to external
462 constants/variables. A <Literal>foreign import</Literal> declaration that takes no
463 arguments represent a binding to a function with no arguments.
469 <Emphasis>GHC only:</Emphasis> GHC's implementation of the FFI provides
476 Support for passing heap allocated byte arrays to an external
481 | prim_arg '->' prim_type
482 | unsafe_arr_ty '->' prim_type
484 unsafe_arr_ty : ByteArray a
485 | MutableByteArray i s a
489 GHC's <Literal>ByteArray</Literal> and <Literal>MutableByteArray</Literal> primitive types are
490 (im)mutable chunks of memory allocated on the Haskell heap, and
491 pointers to these can be passed to <Literal>foreign import</Literal>ed external
492 functions provided they are marked as <Literal>unsafe</Literal>. Since it is
493 inherently unsafe to hand out references to objects in the Haskell
494 heap if the external call may cause a garbage collection to happen,
495 you have to annotate the <Literal>foreign import</Literal> declaration with
496 the attribute <Literal>unsafe</Literal>. By doing so, the user explicitly states
497 that the external function won't provoke a garbage collection,
498 so passing out heap references to the external function is allright.
505 Another GHC extension is the support for unboxed types:
509 prim_arg : ... | unboxed_h_ty
510 ext_ty : .... | unboxed_ext_ty
512 unboxed_ext_ty : Int# | Word# | Char#
513 | Float# | Double# | Addr#
515 unboxed_h_ty : MutableByteArray# | ForeignObj#
520 Clearly, if you want to be portable across Haskell systems, using
521 system-specific extensions such as this is not advisable; avoid
522 using them if you can. (Support for using unboxed types might
523 be withdrawn sometime in the future.)
538 <Sect3 id="sec-results">
543 An external function is permitted to return the following
550 prim_result : ext_ty | new_ext_ty | ()
552 new_ext_ty : a Haskell newtype of an ext_ty.
558 where <Literal>()</Literal> represents <Literal>void</Literal> / no result.
567 External functions cannot raise exceptions (IO exceptions or non-IO ones.)
568 It is the responsibility of the <Literal>foreign import</Literal> user to layer
569 any error handling on top of an external function.
575 Only external types (<Literal>ext_ty</Literal>) can be passed back, i.e., returning
576 <Literal>ForeignObj</Literal>s is not supported/allowed.
582 Haskell newtypes that wrap up <Literal>ext_ty</Literal> are also permitted.
594 <Sect2 id="sec-mapping">
599 For the FFI to be of any practical use, the properties and sizes of
600 the various types that can be communicated between the Haskell world
601 and the outside, needs to be precisely defined. We do this by
602 presenting a mapping to C, as it is commonly used and most other
603 languages define a mapping to it. Table
604 <XRef LinkEnd="sec-mapping-table">
605 defines the mapping between Haskell and C types.
610 <Table id="sec-mapping-table">
611 <Title>Mapping of Haskell types to C types</Title>
614 <ColSpec Align="Left" Colsep="0">
615 <ColSpec Align="Left" Colsep="0">
616 <ColSpec Align="Left" Colsep="0">
617 <ColSpec Align="Left" Colsep="0">
618 <ColSpec Align="Left" Colsep="0">
619 <ColSpec Align="Left" Colsep="0">
622 <Entry>Haskell type </Entry>
623 <Entry> C type </Entry>
624 <Entry> requirement </Entry>
625 <Entry> range (9) </Entry>
631 <Literal>Char</Literal> </Entry>
632 <Entry> <Literal>HsChar</Literal> </Entry>
633 <Entry> unspec. integral type </Entry>
634 <Entry> <Literal>HS_CHAR_MIN</Literal> .. <Literal>HS_CHAR_MAX</Literal></Entry>
638 <Literal>Int</Literal> </Entry>
639 <Entry> <Literal>HsInt</Literal> </Entry>
640 <Entry> signed integral of unspec. size(4) </Entry>
641 <Entry> <Literal>HS_INT_MIN</Literal> ..
642 <Literal>HS_INT_MAX</Literal></Entry>
646 <Literal>Int8</Literal> (2) </Entry>
647 <Entry> <Literal>HsInt8</Literal> </Entry>
648 <Entry> 8 bit signed integral </Entry>
649 <Entry> <Literal>HS_INT8_MIN</Literal>
651 <Literal>HS_INT8_MAX</Literal></Entry>
655 <Literal>Int16</Literal> (2) </Entry>
656 <Entry> <Literal>HsInt16</Literal> </Entry>
657 <Entry> 16 bit signed integral </Entry>
658 <Entry> <Literal>HS_INT16_MIN</Literal>
659 .. <Literal>HS_INT16_MAX</Literal></Entry>
663 <Literal>Int32</Literal> (2) </Entry>
664 <Entry> <Literal>HsInt32</Literal> </Entry>
665 <Entry> 32 bit signed integral </Entry>
666 <Entry> <Literal>HS_INT32_MIN</Literal> ..
667 <Literal>HS_INT32_MAX</Literal></Entry>
671 <Literal>Int64</Literal> (2,3) </Entry>
672 <Entry> <Literal>HsInt64</Literal> </Entry>
673 <Entry> 64 bit signed integral (3) </Entry>
674 <Entry> <Literal>HS_INT64_MIN</Literal> ..
675 <Literal>HS_INT64_MAX</Literal></Entry>
679 <Literal>Word8</Literal> (2) </Entry>
680 <Entry> <Literal>HsWord8</Literal> </Entry>
681 <Entry> 8 bit unsigned integral </Entry>
682 <Entry> <Literal>0</Literal> ..
683 <Entry> <Literal>HS_WORD8_MAX</Literal></Entry>
687 <Literal>Word16</Literal> (2) </Entry>
688 <Entry> <Literal>HsWord16</Literal> </Entry>
689 <Entry> 16 bit unsigned integral </Entry>
690 <Entry> <Literal>0</Literal> ..
691 <Entry> <Literal>HS_WORD16_MAX</Literal></Entry>
695 <Literal>Word32</Literal> (2) </Entry>
696 <Entry> <Literal>HsWord32</Literal> </Entry>
697 <Entry> 32 bit unsigned integral </Entry>
698 <Entry> <Literal>0</Literal> ..
699 <Literal>HS_WORD32_MAX</Literal></Entry>
703 <Literal>Word64</Literal> (2,3) </Entry>
704 <Entry> <Literal>HsWord64</Literal> </Entry>
705 <Entry> 64 bit unsigned integral (3) </Entry>
706 <Entry> <Literal>0</Literal> ..
707 <Literal>HS_WORD64_MAX</Literal></Entry>
711 <Literal>Float</Literal> </Entry>
712 <Entry> <Literal>HsFloat</Literal> </Entry>
713 <Entry> floating point of unspec. size (5) </Entry>
714 <Entry> (10) </Entry>
718 <Literal>Double</Literal> </Entry>
719 <Entry> <Literal>HsDouble</Literal> </Entry>
720 <Entry> floating point of unspec. size (5) </Entry>
721 <Entry> (10) </Entry>
725 <Literal>Bool</Literal> </Entry>
726 <Entry> <Literal>HsBool</Literal> </Entry>
727 <Entry> unspec. integral type </Entry>
728 <Entry> (11) </Entry>
732 <Literal>Addr</Literal> </Entry>
733 <Entry> <Literal>HsAddr</Literal> </Entry>
734 <Entry> void* (6) </Entry>
739 <Literal>ForeignObj</Literal> </Entry>
740 <Entry> <Literal>HsForeignObj</Literal> </Entry>
741 <Entry> void* (7) </Entry>
746 <Literal>StablePtr</Literal> </Entry>
747 <Entry> <Literal>HsStablePtr</Literal> </Entry>
748 <Entry> void* (8) </Entry>
764 <Emphasis remap="bf">Some remarks:</Emphasis>
770 A Haskell system that implements the FFI will supply a header file
771 <Filename>HsFFI.h</Filename> that includes target platform specific definitions
772 for the above types and values.
778 The sized numeric types <Literal>Hs{Int,Word}{8,16,32,64}</Literal> have
779 a 1-1 mapping to ISO C 99's <Literal>{,u}int{8,16,32,64}_t</Literal>. For systems
780 that doesn't support this revision of ISO C, a best-fit mapping
781 onto the supported C types is provided.
787 An implementation which does not support 64 bit integral types
788 on the C side should implement <Literal>Hs{Int,Word}64</Literal> as a struct. In
789 this case the bounds <Constant>HS_INT64_{MIN,MAX}</Constant> and <Constant>HS_WORD64_MAX</Constant>
796 A valid Haskell representation of <Literal>Int</Literal> has to be equal to or
797 wider than 30 bits. The <Literal>HsInt</Literal> synonym is guaranteed to map
798 onto a C type that satisifies Haskell's requirement for <Literal>Int</Literal>.
804 It is guaranteed that <Literal>Hs{Float,Double}</Literal> are one of C's
805 floating-point types <Literal>float</Literal>/<Literal>double</Literal>/<Literal>long double</Literal>.
811 It is guaranteed that <Literal>HsAddr</Literal> is of the same size as <Literal>void*</Literal>, so
812 any other pointer type can be converted to and from HsAddr without any
813 loss of information (K&R, Appendix A6.8).
819 Foreign objects are handled like <Literal>Addr</Literal> by the FFI, so there
820 is again the guarantee that <Literal>HsForeignObj</Literal> is the same as
821 <Literal>void*</Literal>. The separate name is meant as a reminder that there is
822 a finalizer attached to the object pointed to.
828 Stable pointers are passed as addresses by the FFI, but this is
829 only because a <Literal>void*</Literal> is used as a generic container in most
830 APIs, not because they are real addresses. To make this special
831 case clear, a separate C type is used here.
837 The bounds are preprocessor macros, so they can be used in
838 <Literal>#if</Literal> and for array bounds.
844 Floating-point limits are a little bit more complicated, so
845 preprocessor macros mirroring ISO C's <Filename>float.h</Filename> are provided:
848 HS_{FLOAT,DOUBLE}_RADIX
849 HS_{FLOAT,DOUBLE}_ROUNDS
850 HS_{FLOAT,DOUBLE}_EPSILON
851 HS_{FLOAT,DOUBLE}_DIG
852 HS_{FLOAT,DOUBLE}_MANT_DIG
853 HS_{FLOAT,DOUBLE}_MIN
854 HS_{FLOAT,DOUBLE}_MIN_EXP
855 HS_{FLOAT,DOUBLE}_MIN_10_EXP
856 HS_{FLOAT,DOUBLE}_MAX
857 HS_{FLOAT,DOUBLE}_MAX_EXP
858 HS_{FLOAT,DOUBLE}_MAX_10_EXP
866 It is guaranteed that Haskell's <Literal>False</Literal>/<Literal>True</Literal> map to
867 C's <Literal>0</Literal>/<Literal>1</Literal>, respectively, and vice versa. The mapping of
868 any other integral value to <Literal>Bool</Literal> is left unspecified.
874 To avoid name clashes, identifiers starting with <Literal>Hs</Literal> and
875 macros starting with <Literal>HS_</Literal> are reserved for the FFI.
881 <Emphasis>GHC only:</Emphasis> The GHC specific types <Literal>ByteArray</Literal> and
882 <Literal>MutableByteArray</Literal> both map to <Literal>char*</Literal>.
892 <Sect2 id="sec-prim-remarks">
893 <Title>Some <Literal>foreign import</Literal> wrinkles
902 By default, a <Literal>foreign import</Literal> function is <Emphasis>safe</Emphasis>. A safe
903 external function may cause a Haskell garbage collection as a result
904 of being called. This will typically happen when the imported
905 function end up calling Haskell functions that reside in the same
906 'Haskell world' (i.e., shares the same storage manager heap) -- see
907 Section <XRef LinkEnd="sec-entry"> for
908 details of how the FFI let's you call Haskell functions from the outside.
910 If the programmer can guarantee that the imported function won't
911 call back into Haskell, the <Literal>foreign import</Literal> can be marked as
912 'unsafe' (see Section <XRef LinkEnd="sec-primitive"> for details of
915 Unsafe calls are cheaper than safe ones, so distinguishing the two
916 classes of external calls may be worth your while if you're extra
917 conscious about performance.
924 A <Literal>foreign import</Literal>ed function should clearly not need to know that
925 it is being called from Haskell. One consequence of this is that the
926 lifetimes of the arguments that are passed from Haskell <Emphasis>must</Emphasis>
927 equal that of a normal C call. For instance, for the following decl,
931 foreign import "mumble" mumble :: ForeignObj -> IO ()
933 f :: Addr -> IO ()
935 fo <- makeForeignObj ptr myFinalizer
940 The <Literal>ForeignObj</Literal> must live across the call to <Function>mumble</Function> even if
941 it is not subsequently used/reachable. Why the insistence on this?
942 Consider what happens if <Function>mumble</Function> calls a function which calls back
943 into the Haskell world to execute a function, behind our back as it
944 were. This evaluation may possibly cause a garbage collection, with
945 the result that <Literal>fo</Literal> may end up being finalised.
947 By guaranteeing that <Literal>fo</Literal> will be considered live across the call
948 to <Function>mumble</Function>, the unfortunate situation where <Literal>fo</Literal> is finalised
949 (and hence the reference passed to <Function>mumble</Function> is suddenly no longer
964 <Sect1 id="sec-prim-dynamic">
965 <Title>Invoking external functions via a pointer
969 A <Literal>foreign import</Literal> declaration imports an external
970 function into Haskell. (The name of the external function
971 is statically known, but the loading/linking of it may very well
972 be delayed until run-time.) A <Literal>foreign import</Literal> declaration is then
973 (approximately) just a type cast of an external function with a
974 <Emphasis>statically known name</Emphasis>.
978 An extension of <Literal>foreign import</Literal> is the support for <Emphasis>dynamic</Emphasis> type
979 casts of external names/addresses:
988 | 'foreign' 'import' [callconv] 'dynamic' ['unsafe']
989 varid :: Addr -> (prim_args -> IO prim_result)
995 i.e., identical to a <Literal>foreign import</Literal> declaration, but for the
996 specification of <Literal>dynamic</Literal> instead of the name of an external
997 function. The presence of <Literal>dynamic</Literal> indicates that when an
998 application of <Literal>varid</Literal> is evaluated, the function pointed to by its
999 first argument will be invoked, passing it the rest of <Literal>varid</Literal>'s
1004 What are the uses of this? Native invocation of COM methods,
1007 Or the interfacing to any other software component technologies.
1010 Haskell libraries that want to be dressed up as C libs (and hence may have
1011 to support C callbacks), Haskell code that need to dynamically load
1017 <Sect1 id="sec-entry">
1018 <Title>Exposing Haskell functions
1022 So far we've provided the Haskell programmer with ways of importing
1023 external functions into the Haskell world. The other half of the FFI
1024 coin is how to expose Haskell functionality to the outside world. So,
1025 dual to the <Literal>foreign import</Literal> declaration is <Literal>foreign export</Literal>:
1034 | 'foreign' 'export' callconv [ext_name] varid :: prim_type
1040 A <Literal>foreign export</Literal> declaration tells the compiler to expose a
1041 locally defined Haskell function to the outside world, i.e., wrap
1042 it up behind a calling interface that's useable from C. It is only
1043 permitted at the toplevel, where you have to specify the type at
1044 which you want to export the function, along with the calling
1045 convention to use. For instance, the following export declaration:
1051 foreign export ccall "foo" bar :: Int -> Addr -> IO Double
1057 will cause a Haskell system to generate the following C callable
1064 HsDouble foo(HsInt arg1, HsAddr arg2);
1070 When invoked, it will call the Haskell function <Function>bar</Function>, passing
1071 it the two arguments that was passed to <Function>foo()</Function>.
1080 The range of types that can be passed as arguments and results
1081 is restricted, since <Literal>varid</Literal> has got a <Literal>prim_type</Literal>.
1087 It is not possible to directly export operator symbols.
1093 The type checker will verify that the type given for the
1094 <Literal>foreign export</Literal> declaration is compatible with the type given to
1095 function definition itself. The type in the <Literal>foreign export</Literal> may
1096 be less general than that of the function itself. For example,
1101 f :: Num a => a -> a
1102 foreign export ccall "fInt" f :: Int -> Int
1103 foreign export ccall "fFloat" f :: Float -> Float
1107 These declarations export two C-callable procedures <Literal>fInt</Literal> and
1108 <Literal>fFloat</Literal>, both of which are implemented by the (overloaded)
1109 Haskell function <Function>f</Function>.
1116 The <Literal>foreign export</Literal>ed IO action must catch all exceptions, as
1117 the FFI does not address how to signal Haskell exceptions to the
1126 <Sect2 id="sec-callback">
1127 <Title>Exposing Haskell function values
1131 The <Literal>foreign export</Literal> declaration gives the C programmer access to
1132 statically defined Haskell functions. It does not allow you to
1133 conveniently expose dynamically-created Haskell function values as C
1134 function pointers though. To permit this, the FFI supports
1135 <Emphasis>dynamic</Emphasis> <Literal>foreign export</Literal>s:
1144 | 'foreign' 'export' [callconv] 'dynamic' varid :: prim_type -> IO Addr
1150 A <Literal>foreign export dynamic</Literal> declaration declares a C function
1151 pointer <Emphasis>generator</Emphasis>. Given a Haskell function value of some restricted
1152 type, the generator wraps it up behind an externally callable interface,
1153 returning an <Literal>Addr</Literal> to an externally callable (C) function pointer.
1157 When that function pointer is eventually called, the corresponding
1158 Haskell function value is applied to the function pointer's arguments
1159 and evaluated, returning the result (if any) back to the caller.
1163 The mapping between the argument to a <Literal>foreign export dynamic</Literal>
1164 declaration and its corresponding C function pointer type, is as
1171 typedef cType[[Res]] (*Varid_FunPtr)
1172 (cType[[Ty_1]] ,.., cType[[Ty_n]]);
1178 where <Literal>cType[[]]</Literal> is the Haskell to C type mapping presented
1179 in Section <XRef LinkEnd="sec-mapping">.
1183 To make it all a bit more concrete, here's an example:
1189 foreign export dynamic mkCallback :: (Int -> IO Int) -> IO Addr
1191 foreign import registerCallback :: Addr -> IO ()
1193 exportCallback :: (Int -> IO Int) -> IO ()
1194 exportCallback f = do
1195 fx <- mkCallback f
1202 The <Literal>exportCallback</Literal> lets you register a Haskell function value as
1203 a callback function to some external library. The C type of the
1204 callback that the external library expects in <Literal>registerCallback()</Literal>,
1208 An FFI implementation is encouraged to generate the C typedef corresponding
1209 to a <Literal>foreign export dynamic</Literal> declaration, but isn't required
1219 typedef HsInt (*mkCallback_FunPtr) (HsInt arg1);
1225 Creating the view of a Haskell closure as a C function pointer entails
1226 registering the Haskell closure as a 'root' with the underlying
1227 Haskell storage system, so that it won't be garbage collected. The FFI
1228 implementation takes care of this, but when the outside world is
1229 through with using a C function pointer generated by a <Literal>foreign
1230 export dynamic</Literal> declaration, it needs to be explicitly freed. This is
1237 void freeHaskellFunctionPtr(void *ptr);
1243 In the event you need to free these function pointers from within
1244 Haskell, a standard 'foreign import'ed binding of the above C entry
1245 point is also provided,
1251 Foreign.freeHaskellFunctionPtr :: Addr -> IO ()
1258 <Sect2 id="sec-foreign-label">
1259 <Title>Code addresses
1263 The <Literal>foreign import</Literal> declaration allows us to invoke an external
1264 function by name from within the comforts of the Haskell world, while
1265 <Literal>foreign import dynamic</Literal> lets us invoke an external function by
1266 address. However, there's no way of getting at the code address of
1267 some particular external label though, which is at times useful,
1268 e.g. for the construction of method tables for, say, Haskell COM
1269 components. To support this, the FFI has got <Literal>foreign label</Literal>s:
1275 foreign label "freeAtLast" addrOf_freeAtLast :: Addr
1281 The meaning of this declaration is that <Literal>addrOf_freeAtLast</Literal> will now
1282 contain the address of the label <Literal>freeAtLast</Literal>.
1289 <Sect1 id="sec-changelog">
1290 <Title>Change history
1305 changed the C representation of <Literal>Haskell_ForeignObj</Literal> from
1306 <Literal>(long*)</Literal> to <Literal>(void*)</Literal> -- ANSI C guarantees that <Literal>(void*)</Literal>
1307 is the widest possible data pointer.
1313 Updated defnition of <Literal>varid</Literal> in Section
1314 <XRef LinkEnd="sec-prim-name"> to reflect Haskell98's.
1320 Replaced confusing uses of <Literal>stdcall</Literal> with <Literal>ccall</Literal>.
1337 Simplified the calling convention section, support for Pascal (and
1338 fastcall) calling conventions dropped.
1344 Clarified that the arguments to a safe <Literal>foreign import</Literal> must have
1345 lifetimes that equal that of a C function application.
1351 Outlawed the use of the (GHC specific) types <Literal>ByteArray</Literal>
1352 and <Literal>MutableByteArray</Literal> in safe <Literal>foreign import</Literal>s.
1358 Added a note that support for the use of unboxed types in
1359 <Literal>foreign import</Literal> may be withdrawn/deprecated sometime in the future.
1365 Simplified section which sketches a possible implementation.
1371 Use <Literal>Hs</Literal> as prefix for the typedefs for the primitive Haskell
1372 FFI types rather than the longer <Literal>Haskell_</Literal>.
1389 Leave out implementation section; of limited interest.
1395 Outlined the criteria used to decide on what calling
1396 conventions to support.
1402 Include <Literal>newtype</Literal>s that wrap primitive types in the list
1403 of types that can be both passed to and returned from external
1421 Updated the section on type mapping to integrate some comments
1422 from people on <ffi@haskell.org> (a fair chunk of the text
1423 in that section was contributed by Sven Panne.)
1429 <Function>freeHaskellFunctionPtr</Function> should belong to module <Literal>Foreign</Literal>, not <Literal>IOExts</Literal>.
1447 <Literal>Bool</Literal> is now an FFI-supported type (i.e., added it to
1448 <Literal>ext_ty</Literal>.)