Re: weird symbols in sc-expand output (was Re: defining module from macro)

On Wed, Aug 23, 2006 at 06:08:47AM -0700, Dan wrote:
> Thanks a lot. That does seem to work.
> 
> I don't understand why in some cases #%quote can be
> typed in, and in other cases not. This is what mislead
> ne. E.g.

Well, first off, these values are totally unsupported items
that underpin SISC for implementation purposes only.  They exist
to provide an unforgeable value that only ever means what
the form really means, and can't be redefined.  The syntax-expander
expands code into these forms for generated code, so that
when for example an 'if' is needed to implement a core
Scheme macro despite the presence of redefinition by the user,
#%if can be emitted and guarantee that.


> 
> #;> (let ((x #%quote)) x)
> Error: invalid context for syntax-identifier quote.

During syntax expansion, psyntax is not only expanding forms,
but checking correctness.  In this case, it discovers that
what is a core syntactic form (quote) isn't being used as such, 
and emits an error.

> 
> but
> 
> #;> (eq? 'a #%quote)

> #f
> #;> (define x #%quote)
> #;> x
> #%quote

In both these cases, #%quote just happens to be in a spot which
doesn't get processed as deeply by psyntax and so the error isn't 
caught.  

All of this is totally unsupported, so there aren't any guarantees
about any specific behavior anyway.

	Scott

> 
> 
> --- "Scott G. Miller" <[email protected]>
> wrote:
> 
> > On Wed, Aug 23, 2006 at 04:50:20AM -0700, Dan wrote:
> > > Scott, please help me out. I just need to go from
> > the
> > > entities #%quote, #&lambda etc to the symbols
> > quote,
> > > lambda etc. Nothing complicated, just a
> > suggestion.
> > > symbol->string doesn't work on them, they can't be
> > > typed in... what can I do?
> > 
> > You could create a similar function, for example:
> > 
> > (define (syntoken->symbol token)
> >   (case token
> >     ((#%quote) 'quote)
> >     ((#%begin) 'begin)
> >     ((#%lambda) 'lambda)
> >     ((#%define) 'define)
> >     ((#%letrec) 'letrec)
> >     ((#%program) 'program)
> >     ((#%set!) 'set!)
> >     ((#%if) 'if)
> >     ((#%annotate) 'annotate)
> >     (else (error 'syntoken->symbol "Invalid syntax
> > token '~s'" token))))
> > 
> > Or use an association list.
> > 
> > Cheers, 
> >   Scott
> > 
> 
> 
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