1.7 Implementation extensions

Here is a list of implementation extensions which may be non–compliant with R6RS.

1. According to R6RS: if the parent of a record type has a custom protocol function, the record type itself must have a custom protocol function; that is:

   (import (rnrs))
   
   (define-record-type alpha
     (fields a)
     (protocol (lambda (A) ---)))
   
   ;; this is good
   (define-record-type beta
     (parent alpha)
     (fields b)
     (protocol (lambda (A B) ---)))
   
   ;; this is bad according to R6RS!!!
   (define-record-type gamma
     (parent alpha)
     (fields b))
   

   This restriction is violated by Vicare, which allows the following:

   (import (rnrs))
   
   (define-record-type alpha
     (fields a)
     (protocol (lambda (A) ---)))
   
   ;; this is good in Vicare!!!
   (define-record-type beta
     (parent alpha)
     (fields b))
   

   Vicare just expects the parent protocol function to have the same interface of a default protocol function.

2. If vicare is run without the option --strict-r6rs: the syntax define-record-type automatically generates unsafe field accessor and mutator syntaxes; in the following example:

   (define-record-type color
     (fields (mutable red)
             (mutable green)
             (mutable blue)))
   

   in addition to the safe accessor and mutator definitions for:

   color-red           color-red-set!
   color-green         color-green-set!
   color-blue          color-blue-set!
   

   unsafe accessors and mutator definitions for:

   $color-red          $color-red-set!
   $color-green        $color-green-set!
   $color-blue         $color-blue-set!
   

   are generated with code like:

   (define-syntax $color-red
     (syntax-rules ()
       ((_ x)
        ($struct-ref x 0))))
   
   (define-syntax $color-red-set!
     (syntax-rules ()
       ((_ x v)
        ($struct-set! x 0 v))))
   

   Notice that the name of the unsafe accessors and mutators is always built from the field name, not from the custom names given in the definition; for example:

   (import (rnrs))
   
   (define-record-type color
     (fields (mutable red   the-red   set-the-red!)
             (mutable green the-green set-the-green!)
             (mutable blue  the-blue  set-the-blue!)))
   
   (define X
     (make-color 1 2 3))
   
   (define Y
     (make-color 1 2 3))
   
   (set-the-red!   X 10)
   (set-the-green! X 20)
   (set-the-blue!  X 30)
   (list (the-red   X)
         (the-green X)
         (the-blue  X)))
   ⇒ (10 20 30)
   
   ($color-red-set!   Y 10)
   ($color-green-set! Y 20)
   ($color-blue-set!  Y 30)
   (list ($color-red   Y)
         ($color-green Y)
         ($color-blue  Y)))
   ⇒ (10 20 30)
   

3. R6RS records can be automatically finalised whenever the garbage collector reclaims them. It is possible to register a destructor function in the R6RS record–type descriptor; whenever a record is instantiated and its record–type descriptor has a destructor function: the record is registered in an internal guardian. The guardian has an associated post garbage collection hook which applies the destructor to the record. iklib records final for details.