[racket] Understanding local-expand with definition contexts

That turns out to be tricky. I've enclosed an implementation, but it
works only for units that have no imports.

At Mon, 05 Jan 2015 17:21:37 +0000, Spencer Florence wrote:
> What I'm trying to do is capture all lifts inside a unit body, instead of
> having them propagating to the top level.
> 
> If I understand what you mean by "add internal-definition context to the
> bindings listed in `exports`", I can't do that because I need to capture
> any lifts inside the unit body, but a `(local-expand/capture-lifts #'(unit
> ...))` would put them outside. But maybe I miss understand you.
> On Mon Jan 05 2015 at 12:10:53 PM Matthew Flatt <mflatt at cs.utah.edu> wrote:
> 
> > I'm not really clear on what you're trying to do. One way to explain
> > more might be to explain how `test` is meant to differ from `begin`. Or
> > maybe you can say why it doesn't work to add the internal-definition
> > context to the bindings listed in `exports` (i.e., to bring everything
> > into the definition context).
> >
> > At Mon, 05 Jan 2015 16:44:10 +0000, Spencer Florence wrote:
> > > Is there any way to have the `test` macro work more like `begin`?
> > >
> > > What I am trying to accomplish is something like:
> > >
> > > (define-syntax (make-my-unit stx)
> > >    (syntax-parse stx
> > >       [(e ...)
> > >        (with-syntax ([body (local-expand/capture-lifts stx <something>)])
> > >          #'(unit (imports ...) (exports ...) body))]))
> > >
> > > Where the defines in #'(e ...) are visible to `unit` so that they can be
> > > used for exports.
> > >
> > > --spf
> > > On Mon Jan 05 2015 at 11:32:46 AM Matthew Flatt <mflatt at cs.utah.edu>
> > wrote:
> > >
> > > > I think it's more a question of what a definition context is supposed
> > > > to be, rather than how `syntax-local-bind-syntaxes` works.
> > > >
> > > > When you create a new definition context, the context's bindings are
> > > > visible only to expressions that are also in that context. The `test`
> > > > form here creates a new definition context in much the same way as
> > > > `(let () ...)`:
> > > >
> > > >  (let ()
> > > >    (let () (define x 1))
> > > >    x)
> > > >
> > > > In other words, the final `x` really is out of the scope of the
> > > > definition of `x`.
> > > >
> > > > At Mon, 05 Jan 2015 16:26:30 +0000, Spencer Florence wrote:
> > > > > Progress is a new error! I don't think I understand how
> > > > > syntax-local-bind-syntaxes is supposed to work. I extended the
> > previous
> > > > > program:
> > > > >
> > > > > #lang racket
> > > > > (require (for-syntax syntax/parse))
> > > > > (define-syntax (test stx)
> > > > >   (syntax-parse stx
> > > > >     [(_ e)
> > > > >      (define ctx
> > > > >        (if (list? (syntax-local-context))
> > > > >            (cons (gensym) (syntax-local-context))
> > > > >            (list (gensym))))
> > > > >      (define def-ctx (syntax-local-make-definition-context))
> > > > >      (define expd (local-expand #'e ctx (list #'define-values)
> > def-ctx))
> > > > >      (define ids (syntax-parse expd [(def (id) _) (list #'id)]))
> > > > >      (syntax-local-bind-syntaxes ids #f def-ctx)
> > > > >      (internal-definition-context-seal def-ctx)
> > > > >      expd]))
> > > > > (let ()
> > > > >   (test (define x 1))
> > > > >   x)
> > > > >
> > > > > And now I receive the error:  `x: unbound identifier in module in: x`
> > > > > Looking at the docs for `syntax-local-make-definition-context` it
> > seems
> > > > > like I need to provide it with the parent definition-context, but
> > I'm not
> > > > > sure how to get a hold of that.
> > > > >
> > > > > --spf
> > > > >
> > > > > On Mon Jan 05 2015 at 10:00:53 AM Matthew Flatt <mflatt at cs.utah.edu>
> > > > wrote:
> > > > >
> > > > > > The error message is intended for "end users" and turns out to be
> > > > > > misleading for the implementor of an internal-definition context.
> > The
> > > > > > documentation for `define-values` has essentially the same
> > problem: it
> > > > > > describes how `define-values` should work in an internal-definition
> > > > > > context, but it doesn't say how the form interacts with
> > `local-expand`.
> > > > > >
> > > > > > A `define-values` form will only expand in a module or top-level
> > > > > > context. To implement an internal-definition context, you must
> > expand
> > > > > > only far enough to see `define-values` form; in other words, supply
> > > > > > `#'define-values` in the stop list. Then, a partially expanded
> > > > > > `define-values` form must be recognized and handled explicitly,
> > with
> > > > > > tools like `syntax-local-bind-syntaxes` or re-writing to
> > > > > > `letrec-values`, as appropriate for the definition context.
> > > > > >
> > > > > > At Mon, 05 Jan 2015 14:49:03 +0000, Spencer Florence wrote:
> > > > > > > Hey all,
> > > > > > >
> > > > > > > I'm trying to use 'local-expand', however it seems to think its
> > > > never in
> > > > > > a
> > > > > > > definition context. For example:
> > > > > > >
> > > > > > > (require (for-syntax syntax/parse))
> > > > > > > (define-syntax (test stx)
> > > > > > >   (syntax-parse stx
> > > > > > >     [(_ e)
> > > > > > >      (define ctx
> > > > > > >        (if (list? (syntax-local-context))
> > > > > > >            (cons (gensym) (syntax-local-context))
> > > > > > >            (list (gensym))))
> > > > > > >      (local-expand
> > > > > > >       #'e ctx null
> > > > > > >       ;; result is the same with this uncommented
> > > > > > >       #;(syntax-local-make-definition-context))]))
> > > > > > > (let ()
> > > > > > >   (test (define x 1))
> > > > > > >   x)
> > > > > > >
> > > > > > > errors with a "define-values: not in a definition context in:
> > > > > > > (define-values (x) 1)"
> > > > > > >
> > > > > > > Can anyone provide any insight into what is going on?
> > > > > > >
> > > > > > > --spf
> > > > > > > ____________________
> > > > > > >   Racket Users list:
> > > > > > >   http://lists.racket-lang.org/users
> > > > > >
> > > >
> >
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