Expanded utility of define-c-struct

[patternspandemic]

Additions to resolve issues as explained in #614.

The new form of the define-c-struct macro adds two optional positional arguments:

;; . . .
;; compatible-tags => list of symbols representing additional C type declarations compatible with struct
;;    This is usually the name of a typedef alias, when it differs from the C struct tag.
;; as-typedef => set to #t when defining an anonymous typdef'd struct
;;    In this case, `struct` should be the name of the typedef alias.
(define-macro (define-c-struct struct #!optional (members '()) release-function compatible-tags as-typedef)

The procedure's interface could be made nicer with named optionals if backward compatibility is not a priority, otherwise specifying #f for release-function and compatible-tags will occur often when dealing with typedef'd structs.

Specifying a list of compatible tags allows for type compatibility when a struct is aliased by a typedef of a different name:

; For when the typedef alias name differs from the struct tag:
(c-declare "
typedef struct pnt {
  int x;
  int y;
} pnt_t;
")

; Include the compatible tag for specifying type compatibility
(define-c-struct pnt
  ((x . int) (y . int)) #f (pnt_t))

Specifying #t for as-typedef makes the macro operate on the typedef alias rather than struct type. This allows for the use of define-c-struct on typedef'd anonymous structs:

; For when the typedef aliases an anonymous struct
(c-declare "
typedef struct {
  int x;
  int y;
} pnt_t;
")

; Flag the macro to work on the alias type rather than a struct type
(define-c-struct pnt_t
  ((x . int) (y . int)) #f #f #t)

[fare]

Looks good to me, modulo improving comments and documentation.

[fare]

Looks excellent to me!

[patternspandemic]

Just have to make changes to the Guide and I'll remove draft status for review.

[patternspandemic]

Nearly there.. Kind of a brain dump. Misspellings and errors likely. Going to refine the Sample.

[vyzo]

This is excellent, i am going to merge it.

@patternspandemic sorry for the obscene delay, it fell though the cracks.