Have you considered a type class and new types ? :-) On Saturday, April 19, 2014, Merijn Verstraaten <[email protected]> wrote:
> After some thinking it'd be possible to implement this using a quasiquoter > and pattern synonyms, however you'd lose the ability to get exhaustiveness > checks for the pattern synonyms. Would people be equally opposed to the > idea of defining a "closed" set of synonyms? That is a user defines a set > of synonyms 'Foo', 'Bar', 'Baz' and a function defined on all three is > considered exhaustive regardless of the underlying type. Otherwise using > synonyms as abstract constructors for such enum types will result in rather > confusing warning of functions not being exhaustively defined. > > Cheers, > Merijn > > On Apr 17, 2014, at 18:06 , John Lato wrote: > > I wouldn't be keen on adding such a specific feature to the language > either. Another concern is that this proposal doesn't seem to address a > very common use case for C enums, bit vectors. IMHO any FFI proposal for > working with C enums should take that idiom into account. > On Apr 17, 2014 7:19 AM, "Merijn Verstraaten" <[email protected]> > 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 > >
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