This page is part of the web mail archives of SRFI 10 from before July 7th, 2015. The new archives for SRFI 10 contain all messages, not just those from before July 7th, 2015.
> 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.