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Enhancement to the syntax system?

From: Stefan Israelsson Tampe
Subject: Enhancement to the syntax system?
Date: Sat, 30 Jun 2012 17:22:52 +0200


I want to discuss some of my experiences using the current syntax system when making the racket match
matcher utilizing syntax-parse.

When doing this I tried to use another style of macro parsing then seen in guiles ice-9 match e.g. Alex Shinns matcher for example. What I mean here is that I tried to use #' #, #,@ and calling out to functions in stead of writing one more macro. This works but it has a caveat.
Hygiene is harder to maintain. e.g.  I kept on hitting this kind of code snippets

#'(let ((x v))
    #,(f rest #'x))

The problem with this code is hygiene, I need to make a gensym and use with-syntax to bound x to that gensym in order to be safe
at the macro expansion. Apart from this I liked the flow of the program and how natural it was to comprehend the code.

The downside to this style is that we looses safety with respect to hygiene bugs. So can we improve on this. I would say yes!
Consider introduceing #. and #.@ that works very much like #, and #,@ but with the distinction that #. creates a syntactic lambda e.g.
The construction of the syntax object translates to
#'(let ((x v)) #.(f rest #'x))


#'(let ((x v)) #,(lambda (env) (with-syntax-env env (f rest #'x)))

E.g. we inject a lambda inside the syntax structure to be parsed by the syntax expander later on and the syntax expander
will recognize the lambda and call it with the current environment of the expander. The lambda will in turn set the syntax env that
should be used by the #'x form in order match that x with the bounded x. I think that you get my intention.

The question is why something like this does not exists. Is it because of weaknesses making this procedure unsound or any other reasons. I do think such a feature would be important but on the other hand, someone should have thought about this before and rejected the idea so my question is simply why is this rejected any clues or pointers?


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