[cffi-devel] Proposal for Array Type

[Rayiner Hashem]

A while ago, somebody pointed out that structures with array members don't 
compile properly on Verrazano. 
In looking at the issue, I realized that arrays are something of a special 
case in C-FFI, as they are morally 
type-specifiers in defcstruct declarations, but do not have underlying 
types. Therefore, I propose a new C-FFI
declaration:

(defcarray <foreign-type-name> <base-type> <length>)

eg:

int[5] -> (defcarray int-array :int 5)

An explicit nil for the <length> argument would be equivalent to:

(define-foreign-type int-array () ':pointer)

All array types would be passed as :pointer under the hood, just like 
structure types.

Philosophically, it might be undesirable to add array types in order to keep 
with the minimally-typed design
of C-FFI. However, arrays, from the point of view of C semantics, are much 
more similar to structures than
to pointers. Indeed, in the C machine model, a structure is little more than 
a hetrogenous array. Thus, the
treatment of arrays versus structures in the C-FFI API presents something of 
an inconsistency, as:

(defcstruct foo (a1 :int) (a2 :int))

(define-foreign-type bar () 'foo)

(defcstruct baz (q bar))

is perfectly legal code, while:

(defcarray foo :int 2)

(define-foreign-type bar () 'foo)

(defcstruct baz (q bar))

is unrepresentable. 

This issue makes C-FFI somewhat incongruous with C, because in C, a 
structure field as a name and a type,
with array's being incorporated into the type system, while in C-FFI, a 
structure field has a name, a type,
and an optional length, with no explicit array type. Moreover, since arrays 
are not types, the usual type
machinery that can be used on array types in C header files must be 
translated into other things in C-FFI
declaration files. 

Now, there is a somewhat selfish undertone to this proposal, as the lack of 
array types in C-FFI necessitates
a rather ugly special case in Verrazano's C-FFI backend. However, I believe 
that an array type would be
an improvement for hand-made C-FFI declarations as well, specifically for 
cases where an array type
is not specified directly in a structure, but used via typedef in a 
structure.

Sincerely,
Rayiner Hashem
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