FFI with ABI instead of C declaration

Maurício CA mauricio.antunes at gmail.com
Mon Mar 14 20:50:08 CET 2011

>> I wonder: why, in Haskell and also in other languages like ML, FFI is
>> done with C type and code as references? Since, ultimately, compiled
>> code goes to, say, ELF files or (in Windows) DLLs, shouldn't foreign
>> interfaces be consistent with those?
> Having an ELF file, a function name, and knowledge about the parameters
> for the function, is still not sufficient information to invoke it.
> I still need to know what calling convention is being used.

Is it possible to list the "set of sets" of suficient/consistent
specifications one needs to list for each parameter for a function call?

Types like those we have today (CInt, CSize, forall a. Ptr a etc.) are
examples of such sets, but my naive guess is that we could have weaker
ones. For instance, one doesn't need to know wheather a parameter is
a pointer or an integer of the same size, as long as it's not actually
used as a pointer, as opaque types sometimes do not identify themselves
as pointers, and that doesn't prevent us from using them.  Suppose we
have C functions below.

  type_name give_me_opaque (void);

  void use_opaque (type_name);

We could be able to say to Haskell (or other language) "foreign binder"
that we have a type named MyOpaque and that:

  Properties of MyOpaque: may be copied.

  foreign import give_me_opaque :: IO MyOpaque

  foreign import use_opaque :: MyOpaque -> IO ()

MyOpaque would be assigned a type whose only property is to have a value
(that you can copy and nothing else). The FFI tool would only have to
verify (from a binary library header) that 'use_opaque' uses a parameter
the same size of what 'give_me_opaque' returns.

Other properties one could list for a type: be usable as an int32 (or
int16, uint24 etc.), be usable as a pointer etc.  A foreign function
binding would have to list enough properties such that a set of suficient
and consistent specifications could be obtained by crossing those
properties with the information available in binary library headers.

Does that make sense?

Thanks for your toughts,

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