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Re: Countering the counter-proposal



   Date: Mon, 4 Oct 1999 20:50:52 GMT
   Reply-to: oleg@xxxxxxxxx
   From: oleg@xxxxxxxxx

   > Using `#,' in a program has two effects: it causes side effects to
   > occur at read time and it ensures that a form is evaluated only once.
   > The first has questionable utility and semantics.  The second is
   > already available in R5RS as `delay' and `force'. 

   I have come across a counter-example to the last statement.

Not too hard.  I don't know what I was thinking.  Please ignore
my alternative proposal.

The fact that my solution to the problem doesn't work does not
mean that SRFI-10 does.  You did not respond to the problem that
I was trying to solve.

The reader is used in two different ways: it is used to read
the original program and it is used when that program calls
`read'.  But how can a program modify the reader that was used
to read it?  The only way is to break the program in two parts,
the first of which runs before the second is read.  There is
no way to do this in Scheme without using either `load' or `eval'.
It is true that my solution uses `eval' (or `load'), but so does
yours.

   BTW, nowhere does SRFI-10 tell that the constructor
   is a Scheme function; it can be written in any language as long as the
   reader is able to invoke it passing appropriate parameters. For
   example, one can write a Scheme->foo _compiler_ in ML and implement
   reader-constructors as ML functions.

I don't understand.  The only way to supply contructor functions
in SRFI-10 is by doing

 (define-reader-ctor <symbol> <proc>).

How can <proc> be other than a Scheme function?

	   The biggest problem with the above eval-solution  is that it
   assumes that the form read from a file is an expression to be
   evaluated. Suppose the file contains a string
	   "(1 2 #,(f32 1.0 2.0))"
   How can you pass the result of reading of this string to eval?

It is easy to give a parameterized reader for use in a program.
Nothing so involved as read-time evaluation is required.  For
example:

 (make-reader)   -> <reader>
 (extend-reader <reader> <symbol> <proc>) -> <reader>

A <reader> is function of one argument, an input port, that
reads and returns the next s-expression from the port.

 (<reader> <input-port>) -> <s-expression>

`Make-reader' returns a reader that parses the R5RS external
syntax.  `Extend-reader' returns a new reader that is the same
as its first argument except that it uses <proc> as a constructor
when it finds #,(<symbol> . <args>) in the input, as is done
in SRFI-10.

This provides the same functionality as SRFI-10 for reading
data values during execution.  It has the added advantages
of being modular (a program may have multiple readers) and
being easy to implement in R5RS.

It doesn't solve the problem of using #, in a program, but
neither does SRFI-10.
                                  -Richard Kelsey