first draft for Slate's ffi

[Paul Dufresne]

Well, this is just a simple text file, I was writing to put some order in 
the
mess going on in my mind about ffi. Clearly, this is still a mess. It would
need some more cleaning, but I hope one could get the idea from just
that, and be able to comment early in the design process.

Here I am trying to summarize my thinking about Slate's ffi to C

openDynLib
  enter:
    libname: name of lib
    symbLib: symbol that will identify it
    structs: dictionnary of structSymbole ->
      tuple of slateType of each member of the structure
    constants: dictionnary of constantSymbole -> slateType
    "note that slateType can be a simpleSlateType, or a structSymb"
  out:
    Boolean (was able to open it, by prelinking all)

closeDynLib
  enter:
    symbLib
  out:
    Nothing

Defining a ffi method:
  " ffiC is a special macro with paramaters "
paramName1!(slateType) paramName2!(slateType)... returns:!(slateType)
selector [
   " I see no reason to have a body here"
  ] 'ffiC symlib funcname!String

calling the method would looks like:
  ffiCallC selector param1 param2 param3 ...
  where params would be of the correct slateType

Under the hood
==============

opendDynLib:
  -call dlopen with libname, asking prelinking
  -save structs and constants in ffiCStore

closeDynLib:
  call dlclose

ffiC macro:
  -funcPtr = dlsym to find funcname in symlib (Symbol for the library)
  -generate ffi_type **atypes from all the params
    To do this, use a dictionnary of simpleSlateType -> ffi_type,
    if param is part of structs saved in ffiCStore, give the
    good ffi_type for that
    "note alse that each symbolType can
    have "Ptr" at the end, if so, add a pointer ffi_type"
  -call in ffi_prep_cif (cif, FFI_DEFAULT_ABI, number of params
    excluding returns: , returns: value, generated atypes ), after
    which cif (Call interface function) is initialized and ready
    be called
  -Save in ffiCStore
    funcInfo: dictionnary of  selector -> ( cif, atypes, list of
    slateType, returnLength of returns:, returns: , funcPtr )
    "The Slate list of ffi_type is usefull, to check when calling "

ffiCallC:
  "too fuzzy, need to be more cleanly designed"
  -use selector to get from ffiCStore: cif, atypes, list of slateType,
   returnLength of returns: , returns: and funcPtr
  -allocate void **avalues, that is, a list of pointers to
   each parameter allocated space
  -allocate a rvalue buffer big enough for returns:
  -call ffi_call (cif, funcPtr, rvalue, aavalues )
  -Translate back, each element of aavalues to it's slateType


ffiCStore is an Object holding infos on libraries, and
on each function of these libraries
  funcInfo: dictionnary of  selector -> ( cif, atypes, list of
    slateType, returnLength of returns:, returns: , funcPtr )
    "The Slate list of ffi_type is usefull, to check when calling "
  bufferLengths: dictionnary of slateType -> length of a ByteArrayBuffer
    needed to hold slateType
  ... need to think more

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