Re: Feature request: Expose `ellipsis?' from psyntax.ss

["Marc Nieper-Wißkirchen" <marc@nieper-wisskirchen.de>]

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Hi Mark,

     > Let's assume we are writing a macro that reimplements syntax (or some
     > variation thereof) and which has to check whether identifiers are
     > ellipses. For example, the following could be given:
     >
     > (with-ellipsis e
     >   (my-syntax a e)
     >
     > Now, this could be a result of a macro expansion and e could carry
     > different marks than with-syntax or my-syntax. This is why I have
    been
     > thinking that one also needs the lexical context of my-syntax and not
     > only the context of e.

    I don't see what problem would be caused by 'e' carrying different marks
    than 'my-syntax'.

    As far as I can tell, in the end, the two instances of 'e' above will
    effectively be compared to one another using 'bound-identifier=?'.  They
    must have the same name and the same marks to match.  The marks on
    'my-syntax' are irrelevant here.


I have been thinking of the scope in which $sc-ellipsis is bound by 
`with-syntax'. If `my-syntax' is within the scope of `with-ellipsis', 
the binding of $sc-ellipsis introduced by this `with-syntax' will be 
relevant; if `my-syntax' is not in the lexical scope of `with-ellipsis', 
the binding should be irrelevant; thus my thought that we need the 
lexical information of my-syntax as well.

    Operationally, when (with-ellipsis e (my-syntax a e)) is expanded, 'e'
    will be added to the macro expansion environment as the innermost
    binding of the ellipsis identifier, and then (my-syntax a e) will be
    expanded within that new expansion environment.  That is the expansion
    environment that will be consulted by the 'ellipsis-identifier?'
    predicate to find the current ellipsis identifier, which is compared
    with its argument (after stripping its anti-mark) using
    'bound-identifier=?'.


Aha, so maybe I have misunderstood the scope of `with-syntax'. Please 
consider the following example:

(define-syntax foo
   (lambda (stx)
     (with-ellipsis e
       (syntax-case stx ()
         ((_ x e) (bar #'(x e)))))))

(eval-when (expand)
   (define (bar x*)
     (syntax-case x* ()
       ((x ...) ---))))

I would have thought that the `...' identifier in `bar' is recognized as 
an ellipsis, but from what you are saying it seems that the binding 
`with-syntax' is dynamic with respect to macro expansion (like syntax 
parameters). Is this really what we want? This way, as soon we use 
`with-ellipsis', we can never call third-party macro transformer helpers 
(like `bar') because they may rely on the fact that the ellipsis `...' 
still has its usual meaning. And circumventing the problem with 
(with-ellipsis e --- (with-ellipsis ... ---)) would also not work 
because the inner `...' is not `bound-identifier=?' to the the global 
ellipsis `...'.

Therefore I think, we want `with-ellipsis' to be lexically scoped (in 
the macro transformer code).

     > Thanks for the explanation. I have been toying with my own
     > implementation of the syntax-case system. In my implementation the
     > (shared) lexical environments are part of the wraps (so the
     > identifiers are in some way self-contained).

    Interesting.  Are locally-bound macro transformers included in those
    lexical environments?  If so, how do you implement 'letrec-syntax'?


My environments are lists of ribs where each rib corresponds to a 
lexical frame. Given the form

(letrec-syntax ((var init)) body ...)

I create a new rib that contains the binding of var to init and add a 
wrap around each expression in body ... that contains the new rib but no 
new marks. When the body is examined by the expander, the wraps are 
gradually pushed down (like in the original description of `syntax-case' 
by Dybvig and Hieb) so that eventually the environments stored with the 
identifiers in body gain another rib.

     > Will ellipsis? also work outside of macros? Say, what would be the
     > result of the following (run-time) code?
     >
     > (with-syntax e
     >   (ellipsis? #'e)

    No, this is an error.  Like 'syntax-local-binding', the
    'ellipsis-identifier?' predicate must be called within the dynamic
    extent of a macro transformer call by the macro expander.


Is this related to the question above of whether `with-syntax' has 
lexical or dynamic scope? In the former case I don't see a theoretical 
reason why it has to be restricted to the dynamic extent of a macro 
transformer call.

     > P.S.: By the way, the module (system syntax) and in particular the
     > procedure syntax-local-binding has already helped me a lot because I
     > needed to attach extra information to symbols and Guile doesn't (yet)
     > support Chez's define-property (well, this would be another feature
     > request).

    Hmm.  Can you tell me more specifically how you are using
    'syntax-local-binding' to accomplish this?  As the Guile manual warns,
    those interfaces are subject to change in future versions of Guile, and
    therefore it is best to avoid them where possible.


What I have been implementing is a pattern matcher and rewriter as a 
macro in Guile that works much like syntax-case/syntax. Let's call it 
my-syntax-case/my-syntax. When `my-syntax' is given a template, it has 
to check whether an identifier appearing in the template is a 
"my-"pattern variable or not. For that, `my-syntax-case' introduces (via 
`let-syntax') lexical bindings of the identifiers that are used as 
pattern variables. The associated syntax transformer just outputs an 
error (namely that the pattern variable is used outside of `my-syntax'). 
However, I also attach a custom property (with `make-object-property`) 
to this syntax transformer that holds information about the match and 
the nesting depth of the pattern variable. In order to retrieve this 
information in `my-syntax', I use `syntax-local-binding' to get hold of 
the associated syntax transformer.

In Chez Scheme, I would have used `define-property' to define my custom 
property directly on the identifier standing for the pattern variable. I 
haven't found an equivalent feature in Guile. I don't know how to nicely 
code my-syntax-case/my-syntax in standard R6RS.

Best regards,

Marc

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