[plt-scheme] Defining syntax parameters that act like ordinary symbols outside of syntax-parameterize

From: Matthias Felleisen (matthias at ccs.neu.edu)
Date: Mon Feb 22 08:14:02 EST 2010

#lang scheme

(require scheme/stxparam)

(define foo-symbol 'foo)
(define-syntax-parameter foo (make-rename-transformer #'foo-symbol))

(let ((foo 10))
  (+ foo 20))
-- Matthias

On Feb 22, 2010, at 4:35 AM, Remco Bras wrote:

> Hi,
> I'll respond to Ryan, Matthias and Robby here.
> First, I'll clarify what I wanted to achieve and in what context.
> I'm using the macro alambda, given below.
> (define-syntax-parameter self
>  ...) ;;To be filled in later
> (define-syntax alambda
>  (syntax-rules ()
>    [(alambda args body ...)
>     (letrec [(me (lambda args
>                    (syntax-parameterize [(self (make-rename-transformer #'me))]
> 		      body ...)))]
>       me)]))
> The question here is what self should be defined to.
> Both Ryan and Matthias (I apologize in advance if I misrepresent your position) appear to argue that
> self should be defined to raise a syntax error by default, unless within a syntax-parameterize as in the
> expansion of alambda.
> This is a reasonable point of view if self is to be considered an integral part of alambda's syntax, that is,
> a keyword that has a well-defined meaning only in alambda expressions.
> However, I see the role of alambda as binding the ordinary identifier self using letrec. 
> That is, I view self as just another identifier, that just happens to implicitly receive a certain value in alambda expressions.
> To me, (let ((self 10)) self) should evaluate to 10, without raising a syntax error.
> More generally, self should be free for any use at phase level 0.
> What I meant by "cluttering up the namespace" in my first message was "making a symbol unavailable for any uses outside of specific expressions".
> My attempt to alpha-rename self to a gensym was intended to redirect any non-alambda references to self to a specific symbol, which could then be bound
> as required by any programmer who wished to do so.
> As Matthias correctly remarked, this gensym is unbound by default in phase 0.
> Robby's suggestion seems to avoid that problem and looks like a reasonable compromise, if I understood it correctly.
> -Remco
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