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SRFI-10 intent: new notation and _some_ guidelines of its interpretation




> I am no longer at all sure that the intent of SRFI-10 is to provide
> a mechanism for read-time evaluation.
> One reading of SRFI-10 and the ensuing discussion is that SRFI-10
> is not proposing an extension of Scheme but rather a convention for
> future SRFIs (footnote 1).  What is being proposed is that the
> following rule be added to the grammar for external representations
> (see section 7.1.2 in R5RS):
>  <compound datum>  -->  #,(<symbol> <datum>*)
> and that future SRFIs that contain new read syntax for values use
> this syntax with an appropriate symbol.

You said it very well. SRFI-10'th intent is indeed to extend the
grammar for external representations; SRFI-10 indeed does not aim to
specify all the details (where tag-symbols are stored, how they come
into being, etc). SRFI-10 however resolves to provide #,()
interpretation guidelines -- an interface so to speak.

It appears that comparison with the Reader Algorithm of CL illustrates
very well what the latter phrase means:

<blockquote cite="http://www.harlequin.com/education/books/HyperSpec/Body/sec_2-2.html";>
4. If x is a terminating or non-terminating macro character then its
associated reader macro function is called with two arguments, the
input stream and x.

      The reader macro function may read characters from the input
stream; if it does, it will see those characters following the macro
character. The Lisp reader may be invoked recursively from the reader
macro function.

      The reader macro function must not have any side effects other
than on the input stream; because of backtracking and restarting of
the read operation, front ends to the Lisp reader (e.g., ``editors''
and ``rubout handlers'') may cause the reader macro function to be
called repeatedly during the reading of a single expression in which x
only appears once.

      The reader macro function may return zero values or one
value. If one value is returned, then that value is returned as the
result of the read operation; the algorithm is done. If zero values
are returned, then step 1 is re-entered.
</blockquote>

	The quote above defines the interface of a reader macro function,
and specifies what it can or cannot do. SRFI-10 aims to do the same
with regards to reader-constructors. The latter differ from CL macro
functions in many respects. For example, Lisp reader relies on a macro
dispatch function to read and parse its arguments if necessary. The
stream following a macro-character does not even have to follow basic
Lisp syntax. For example, one can easily write a CL reader macro
function that reads through a few lines of Python or Haskell code and
converts them to an appropriate Lisp object; Python code relies
extensively on the use of indentation, which is ignored in Lisp code.

	In contrast, when a SRFI-10 reader-constructor is called, the
corresponding #,() form (and all its arguments if any) has already
been read. Therefore, all arguments of a reader constructor must
follow the Scheme syntax. Informally, a #,() form should "look" like
Scheme code, even if we have no idea what a constructor-symbol
means. Since the #,() form is already read, it makes no sense for the
reader constructor to advance or backup in reader's stream. The
constructor generally does not have any idea what the current position
in the stream is. A particular Scheme reader may choose to call reader
constructors after it has read the stream entirely. As a #,() form is
completely read when the constructor is called, we can relax the CL
prohibition on side-effects in the constructor. Unlike CL reader macro
function, a reader-constructor cannot be called repeatedly during
reading of a single, non-nested #,() form. Besides, prohibition on
side effects is very difficult to reinforce. Lifting this prohibition
does not constitute encouragement of side-effecting
reader-constructors however.

	SRFI-10 mentions a few other differences between CL reader
macros and the proposed #,() form. BTW, one of my posts on
comp.lang.scheme indicated that these reader functions implement
different evaluation order rules -- normal vs. applicative.


It appears that read-time-ctor causes most of the controversy. This is
unfortunate as read-time-ctor is actually an implementation detail. It
appears that it make sense to clarify the following statement even
further:

>  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.


For example,

The exact nature of association between symbolic tags and
constructor procedures is unspecified. For example, an implementation
of SRFI-10 may:

	- offer a _fixed_ set of specific symbolic tags and
associated constructor procedures;
	- provide a set of symbolic tags and associated constructor
procedures. A user however may enable or disable some of them at
application's start-up time, via configurational files,
command-line options, environment variables and other similar means;
	- allow a user to add new constructor procedures and
associated tags at application's start-up time, by specifying
extension modules;
	- permit a user to add new constructor procedures at run-time.
For example, an implementation may provide a procedure
	define-reader-ctor SYMBOL PROC
for that purpose. 
	- an implementation may allow creation of new associations of
symbolic tags and existing constructor procedures at read-time, via a
dedicated reader-constructor, e.g.,
	'#,(define-reader-ctor 'f32 f32vector)

A user should be aware that define-reader-ctor function if provided
adds new associations at application's run-time. This function
therefore cannot affect reading of the source code containing that
define-reader-ctor. The last option is free from that
limitation. However, it essentially modifies the very reader while it
is processing the code. A special care must be taken to consider
the proper sequencing of side effects.

	A SRFI-10 reference implementation provides define-reader-ctor
function. The examples below assume that particular implementation
choice.