-1 from me. Your first example even provides a counter-example. typedef enum {
IMG_INIT_JPG = 0x00000001, IMG_INIT_PNG = 0x00000002, IMG_INIT_TIF = 0x00000004, IMG_INIT_WEBP = 0x00000008 } IMG_InitFlags;
Those are defined as powers of two because they are a bit mask you have to
be able to (.|.) together.
This is the sort of thing people write .hsc files for, so they can include
the appropriate header directly and resolve the constants.
Maintaining a separate copy of an enum that goes out of date with the C
version is a recipe for breaking on future versions of the dependency, and
in my experience the majority of cases where the range is discontinuous
arise from when the thing in question is a mask, like this very case.
The remaining cases where you really want to incur all those obligations
are few enough and far enough between that going through a quasiquoter
seems to be the right solution.
-Edward
On Thu, Apr 17, 2014 at 10:19 AM, Merijn Verstraaten wrote: Cross-post to haskell-prime in case there's any interest for including
this into the report's FFI specification. Proposal - Foreign enum support
=============================== At the moment the FFI does not have a convenient way with interacting enums
(whether proper enums or CPP defines) in C (like languages). Both enums
and CPP
defined enums are major parts of large C APIs and they are thus crucial to
writing foreign bindings. A few examples: SDL_image defines the following enum: typedef enum {
IMG_INIT_JPG = 0x00000001,
IMG_INIT_PNG = 0x00000002,
IMG_INIT_TIF = 0x00000004,
IMG_INIT_WEBP = 0x00000008
} IMG_InitFlags; OpenCL specifies the following typedefs + CPP defined enum: typedef uint32_t cl_uint __attribute__((aligned(4)));
typedef cl_uint cl_platform_info; /* cl_platform_info */
#define CL_PLATFORM_PROFILE 0x0900
#define CL_PLATFORM_VERSION 0x0901
#define CL_PLATFORM_NAME 0x0902
#define CL_PLATFORM_VENDOR 0x0903
#define CL_PLATFORM_EXTENSIONS 0x0904 OpenCL functions will return the above CPP defines as return values of type
cl_platform_info. Current Solutions
----------------- In many cases someone wrapping such a C library would like to expose these
enums as a simple sum type as this has several benefits: type safety, the
ability to use haskell constructors for pattern matching, exhaustiveness
checks. Currently the GHC FFI, as specified by Haskell2010, only marshalls a small
set
of foreign types and newtypes with exposed constructors of these types. As
such
there seem two approaches to wrap these enums: 1. Implement an ADT representing the enum and write a manual conversion
function between the ADT and the corresponding C type (e.g. CInt ->
Foo and
Foo -> CInt). 2. Use a tool like c2hs to automatically generate the ADT and conversion
function. In both cases the foreign functions are imported using the corresponding C
type
in their signature (reducing type safety) and the user is forced write
trivial
wrappers for every imported function to convert the ADT to the relevant C
type
and back. This is both tedious to write and costly in terms of code produced, in
case of
c2hs one calls "toEnum . fromIntegral" and "fromIntegral . fromEnum" for
every
argument/result even though this could trivially be a no-op. Worse, since c2hs uses the Enum class for it's conversion to/from C types
it
generates Enum instances like: instance Enum Foo where
fromEnum Bar = 1
fromEnum Baz = 1337 toEnum 1 = Bar
toEnum 1337 = Baz
toEnum unmatched = error ("PlatformInfo.toEnum: Cannot match " ++
show unmatched) Since succ/pred and enumFromTo's default implementations assume enums
convert
to continuous sequence of Int this means the default generated enum
instances
crash. This problem could be overcome by making c2hs' code generation
smarter,
but this does not eliminate the tediousness of wrapping all foreign
imported
functions with marshalling wrappers, NOR does it eliminate the overhead of
all
this useless marshalling. Proposal
-------- Add a new foreign construct for enums, the syntax I propose below is rather
ugly and ambiguous and thereforeopen to bikeshedding, but I prefer
explaining
based on a concrete example. foreign enum CInt as Foo where
Bar = 1
Baz
Quux = 1337
Xyzzy = _ This would introduce a new type 'Foo' with semantics approximately
equivalent
too "newtype Foo = Foo CInt" plus the pattern synonyms "pattern Bar = Foo
1;
pattern Baz = 2; pattern Quux = 1337; pattern Xyzzy = Foo _". Explicit listing of the value corresponding to a constructor should be
optional, missing values should just increment by one from the previous
(like
C), if the initial value is missing, it should assume to start from 0.
Values
do not need to be contiguous. Users should be able to use these constructors as normal in pattern match
(really, this mostly follows to semantics of the above pattern synonyms). The foreign import/export functionality should invisibly marshall Foo to
the
underlying foreign type (as is done for newtypes). I'm unsure about the support for a wildcard constructor like Xyzzy. If
there is
support for a wildcard, it should be optional. On the upside a wildcard
means
the marshalling is no longer a partial function. The downside is that it
makes
desugaring the use of enums in patterns harder. It seems clear that f Xyzzy = {- ... -}
f Bar = {- ... -}
f Baz = {- ... -}
f Quux = {- ... -} Should not have the same semantics as: f (Foo _) = {- ... -}
f (Foo 1) = {- ... -}
f (Foo 2) = {- ... -}
f (Foo 1337) = {- ... -} So in the presence of wildcards, the Foo enum can't trivially be desugared
into
pattern synonyms after checking exhaustiveness. Pros:
1. Foreign imports are slightly more type safe, as one can now write: foreign import ccall "someFoo.h" someFoo :: Foo -> Ptr () -> IO () Preventing users from passing an arbitrary CInt to an argument
expecting a
specific enum. 2. No need to write marshalling functions to/from ADT to obtain
exhaustiveness
checks and pattern matching 3. Cheaper as marshalling Foo to CInt is a no-op 4. toEnum/fromEnum can simply map to contiguous sequence of Int as this
Int
mapping is no longer used for marshalling Cons:
1. Non-standard extension of the FFI 2. Someone has to implement it 3. Wildcards constructors would present difficulties desugaring pattern
matches to a simple newtype. 4. ?? What Would Need to be Done?
--------------------------- 1. Parser needs to be extended to deal with parsing of enum declarations.
2. Pattern matches of an enum type need to be checked for exhaustiveness
and
desugared to the underlying type's representation.
3. Extend foreign imports/exports to marshall enums properly. If there's no objections I'm willing to take a stab at implementing this,
although I'd probably need some help with GHC's internals (although I
could bug
#ghc for that). Cheers,
Merijn _______________________________________________
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