Re: define-reader-ctor is merely an implementation hint

[Richard Kelsey <kelsey@xxxxxxxxxxxxxxxxxxx>]

   Date: Tue, 5 Oct 1999 18:42:45 GMT
   From: oleg@xxxxxxxxx

   > The only way to supply contructor functions in SRFI-10 is by doing
   > (define-reader-ctor <symbol> <proc>).

   No. The only requirement SRFI-10 mandates is that "There must be a way
   to declare an association between a symbolic tag and the corresponding
   constructor-procedure." SRFI-10 goes on to _suggest_ that an
   implementation may want to implement a define-reader-ctor function for
   that purpose.

This was not at all clear to me from reading SRFI-10.  I suggest
that you make it much more explicit.  You could say something like:

"An implementation of SRFI-10 must provide some method for
 associating symbolic tags with constructor procedures.  For
 example, it might provide a `define-reader-ctor' function for
 that purpose."

	   I admit SRFI-10's normative silence about precise ways to
   define reader-constructor associations may sound like a cop-out. Yet
   is is intentional: the purpose of SRFI-10 is to extend the _protocol_
   by which a Scheme system communicates with an external world. If my
   and your applications run by a SRFI-10-compliant system, and I send
   you a string
	   "#,(f32 1.0 2.0 3.0)"
   then you know what I am trying to say. What you're going to do about
   this -- reject or accept my request for constructing a value -- is
   another matter entirely.

No, I have no idea what you are trying to say, because I have no
idea what procedure `f32' refers to.

	   I don't think SRFI-10 is way out of the line in this
   respect. For example, SRFI-0 talks about feature identifiers -- yet it
   gives no details how an implementation happens to have these
   identifiers defined, and how a user may extend the system with more
   identifiers (if he can at all).

But SRFI-0 does say what each feature identifier means: it means that
the feature in the associated SRFI document is present.  If I see a
program like

 (cond-expand ((and srfi-1 srfi-17)
               (write 1))	
              (else))

then I know to look at the SRFI-1 and SRFI-10 documents to find out
what the program means.  If I see

  #,(f32 1.0 2.0 3.0)

in a program all I know is that the value will be the result returned
by applying some arbitrary function to `(1.0 2.0 3.0).  This doesn't
tell me very much about what the program means.

                                      -Richard Kelsey