[plt-dev] Default syntax printer

At Mon, 10 Aug 2009 18:45:54 -0400, Carl Eastlund wrote:
> On Mon, Aug 10, 2009 at 6:02 PM, Matthew Flatt<mflatt at cs.utah.edu> wrote:
> > At Wed, 5 Aug 2009 15:16:57 -0400, Carl Eastlund wrote:
> >> Is there any chance we can change the default printer to show the
> >> syntax->datum of syntax objects?  Instead of #<syntax> in error
> >> messages, it would be much more helpful to see something like #'(foo
> >> x) or #<syntax:(foo x)>.  Even the error printer, which cuts off at a
> >> certain point with "...", would be preferable to the current total
> >> opacity.
> >
> > Yes, I should have done that a long time ago.
> >
> > v4.2.1.6 adds a `print-syntax-width' parameter that defaults to 32,
> > which means that up to 32 characters are used for printing a syntax
> > object's datum form within `#<syntax>':
> >
> >  > #'(+ 1 2)
> >  #<syntax::3 (+ 1 2)>
> >
> >  > (syntax-e #'(+ 1 2))
> >  (#<syntax::24 +> #<syntax::26 1> #<syntax::28 2>)
> >
> >  > #'((lambda (x) (x x)) (lambda (x) (x x)))
> >  #<syntax::34 ((lambda (x) (x x)) (lambda (...>
> >
> > I don't know whether 32 is the right default. I think there are
> > probably times when I want to see a compact printed form for syntax
> > objects (especially when there are several in a list, or something like
> > that), and I bet that 32 is usually enough characters to tell me what
> > syntax object I'm looking at. Then again, +inf.0 seems like a more
> > natural default. For now, I've gone with the more conservative default
> > (with respect to previous versions), but after trying it out, let me
> > know if you think some else would be better.
> 
> I like this move, but I'd prefer a different interface if we're going
> to have customizable printing.  I propose a 'current-syntax-printer'
> parameter.  If someone wants essentially the old behavior, they can
> set it to void; for full printing they could use write or
> pretty-print; the default could be the error printer (has a built-in
> width and uses "...").  I can't imagine using print-syntax-width to
> set a different number; if I ever used it, I'd set it to either 0 or
> +inf.0.  Custom printing provides more interesting flexibility.

The `pretty-print' library already supports custom printing for all
values, not just syntax objects, right? Is there something special
about syntax objects that merits more general built-in treatment?