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Internal defines

From: Dirk Herrmann
Subject: Internal defines
Date: Sun, 09 Nov 2003 23:40:26 +0100
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.3) Gecko/20030312

Hi folks,

Guile currently chokes on the following form:

 (defmacro a forms
   (cons 'define forms))
 (let ((c identity) (x #t))
   (define (a x y) (and x y))
   (a (c x) (c x))))

According to R5RS, the body of a form like lambda, let, let* etc. looks like follows:

<body> --> <definition>* <sequence>
<sequence> --> <command>* <expression>
<command> --> <expression>
<definition> --> (define <variable> <expression>)
     | (define (<variable> <def formals>) <body>)
     | (begin <definition>*)

That is, it starts with zero or more definitions followed by one or more expressions. The definition forms must start with the literal symbol 'define' or 'begin', which may enclose further definitions or begin forms. In any case, there is no mentioning
of macro expansion here.

In other words, the examle code above should return #t, because the expression (a (c x) (c x)) refers to the internal define rather than to the outer macro. Guile, however, has up to now performed macro expansion before checking whether the
form is a definition.  Thus, the example above returns an error, since Guile
interprets (a (c x) (c x)) as a macro application, in this case a definition. The body then (according to Guile's interpretation) consists only of definitions.

The above example also shows that it is no good practice to define macros that expand into definitions. Messing with 'define' always means to play around with one of scheme's most basic syntactical structures. The optargs implementation of define* is an example for a macro that expands to a 'define' form. According to R5RS and as demonstrated by the above example, such macros are confusing, and Guile's current implementation breaks code that already has a meaning with respect
to R5RS.

I am about to submit a patch that fixes Guile's behaviour, such that the example above would work. This means, that there is not macro expansion performed prior to detecting the internal defines. I want, however, to check with you before doing so, since it is likely to break existing code. In the long term, just to mention it early, I would also like to fix Guile's code such that it also handles top-level
defines correctly, since there a similar problem applies.

Best regards
Dirk Herrmann

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