[racket] Test to disambiguate x-expression attributes vs. content?

There is also an XML representation and it uses structs for some nodes in the representation. But as Carl says, once they are S-expressions, it is easy to use regular list functionality for traversals and construction. See 2e -- Matthias



On Jun 11, 2013, at 1:08 PM, Matthew Butterick wrote:

> To ask an abstract question: is there a particular reason that X-expressions are defined in Racket as a special kind of S-expression, rather than a struct or primitive datatype?
> 
> 
> On Tue, Jun 11, 2013 at 6:46 AM, Matthias Felleisen <matthias at ccs.neu.edu> wrote:
> 
> You may have been speaking abstractly but the point is that it is a mostly accurate summary of Xexpr and I use just this form in HtDP/2e to process X-expressions. -- Matthias
> 
> 
> 
> On Jun 10, 2013, at 3:27 PM, Jay McCarthy wrote:
> 
> > I wasn't referencing the docs. I'm just talking abstractly. The point
> > is that you have to consider the attribute list as a totally different
> > kind of thing. The thing you cite can be directly turned into a match
> > pattern, though:
> >
> > (match xe
> > [(? string? s) ....]
> > [(list (? symbol? tag) (list (list (? symbol? k) (? string? v)) ...) xe ...)
> >  ....]
> > [(cons (? symbol? tag) (list xe ...))
> >  ...]
> > etc
> >
> > On Mon, Jun 10, 2013 at 1:20 PM, Matthew Butterick
> > <mb.list.acct at gmail.com> wrote:
> >> Aha, the xexpr-drop-empty-attributes is probably best, since it will produce
> >> consistent forms. Thanks.
> >>
> >> BTW what part of the docs are you citing for (list* Symbol (option
> >> AttributeList) XexprList)? The only definition of X-expressions I'm aware of
> >> is here.
> >>
> >>
> >> On Mon, Jun 10, 2013 at 11:58 AM, Jay McCarthy <jay.mccarthy at gmail.com>
> >> wrote:
> >>>
> >>> The type of Xexpr is...
> >>> | AtomicContent
> >>> | (list* Symbol (option AttributeList) XexprList)
> >>>
> >>> It sounds like you have one function F that you would like to give the
> >>> type "Xexpr -> Value", but you are having a problem when you apply it
> >>> to the AttributeList. You should have such a problem, because an
> >>> AttributeList is not an Xexpr, so your types are wrong.
> >>>
> >>> In other words, if you want to recursively process an Xexpr, then you
> >>> need two functions, one for attribute lists and one for xexpr lists.
> >>>
> >>> Finally, I find it convenient when automatically processing Xexprs to
> >>> ensure that xexpr-drop-empty-attributes is #f so that there is always
> >>> an attribute list and do:
> >>>
> >>> (match xe
> >>> [(list* tag attrs content) ....])
> >>>
> >>>
> >>>
> >>> On Mon, Jun 10, 2013 at 12:40 PM, Matthew Butterick
> >>> <mb.list.acct at gmail.com> wrote:
> >>>> Consider an x-expression that represents an XML tag, so it has the
> >>>> general
> >>>> form
> >>>>
> >>>> '(tag ((attr-name "attr-value") ... ) content ...)
> >>>>
> >>>> The attributes or content might be omitted, of course.
> >>>>
> >>>> When I recursively process these expressions, I start by decomposing the
> >>>> x-expression into components:
> >>>>
> >>>> tag
> >>>> ((attr-name "attr-value") ... )
> >>>> (content ...)
> >>>>
> >>>> The problem that surfaces in a recursive-processing context is that the
> >>>> list
> >>>> of content might have the same form as an attribute expression, e.g.,
> >>>>
> >>>> ((field-1 "value-1") (field-2 "value-2"))
> >>>> ((p "some text") (p "some more text"))
> >>>>
> >>>> Is there a test I could use to reliably disambiguate between these two
> >>>> cases, short of creating a new struct for x-expressions? Seems like I'm
> >>>> overlooking some middle ground.
> >>>>
> >>>>
> >>>>
> >>>> ____________________
> >>>>  Racket Users list:
> >>>>  http://lists.racket-lang.org/users
> >>>>
> >>>
> >>>
> >>>
> >>> --
> >>> Jay McCarthy <jay at cs.byu.edu>
> >>> Assistant Professor / Brigham Young University
> >>> http://faculty.cs.byu.edu/~jay
> >>>
> >>> "The glory of God is Intelligence" - D&C 93
> >>
> >>
> >>
> >> ____________________
> >>  Racket Users list:
> >>  http://lists.racket-lang.org/users
> >>
> >
> >
> >
> > --
> > Jay McCarthy <jay at cs.byu.edu>
> > Assistant Professor / Brigham Young University
> > http://faculty.cs.byu.edu/~jay
> >
> > "The glory of God is Intelligence" - D&C 93
> > ____________________
> >  Racket Users list:
> >  http://lists.racket-lang.org/users
> 
> 
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