Okay - for posterity's sake, here's an updated version of Alex's code that
supports nested Loop/Accum, and doesn't leave any "syntactic residue" after
expansion. This is now what I set out to accomplish with my original set of
questions:

> #lang racket/base
>
> (require racket/stxparam
>          (for-syntax racket/base
>                      racket/set
>                      racket/syntax
>                      syntax/parse
>                      syntax/stx
>                      syntax/context))
>
> (define-syntax Loop
>   (lambda (stx)
>     (syntax-parse stx
>       [(Loop accum-id:id up-to-expr:expr body:expr ...+)
>        (let* ([defs (syntax-local-make-definition-context)]
>               [ctx (generate-expand-context)])
>          (syntax-local-bind-syntaxes
>           (list #'accum-id)
>           (local-transformer-expand
>            #'(let ([vars (mutable-set)])
>                (lambda ([stx #f])
>                  (if (syntax? stx)
>                      (syntax-parse stx
>                        [(accum-id x:id dx:expr)
>                         (set-add! vars (syntax-local-introduce #'x))
>                         #'(set! x (+ x dx))])
>                      vars))) 'expression null) defs)
>          (internal-definition-context-seal defs)
>          (with-syntax* ([(body ...)
>                          (stx-map
>                           (lambda (body)
>                             (with-syntax ([body body])
>                               (local-expand #'body ctx null defs)))
>                           #'(body ...))]
>                         [(init ...)
>                          (map
>                           (lambda (var)
>                             (with-syntax ([var (syntax-local-introduce
> var)]) #'(set! var 0)))
>                           (set->list
>                            ((syntax-local-value
>                              (internal-definition-context-apply defs
> #'accum-id)
>                              #f defs))))])
>            #'(let ([up-to up-to-expr])
>                (letrec
>                    ([loop
>                      (lambda (n)
>                        body ...
>                        (let ([n (add1 n)])
>                          (if (< n up-to)
>                              (loop n)
>                              (void))))])
>                  init ...
>                  (loop 0)))))]
>       [(Loop up-to-expr:expr body:expr ...+)
>        (with-syntax
>            ([Accum (datum->syntax stx 'Accum)])
>          #'(Loop Accum up-to-expr body ...))])))
>
> (let ([x (void)] [y (void)] [z (void)])
>   (Loop 2
>         (let ([dx 1] [dy 2] [dz 3])
>           (Accum x dx)
>           (Loop AccumInner 2
>                 (printf "x = ~v, y = ~v, z = ~v\n" x y z)
>                 (Accum y dy)
>                 (AccumInner z dz)))))
>

One last question - the change to using local-expand meant that the loop
body was no longer happy accepting define-values forms and raised "Not in a
definition context" errors. I can use let to accomplish the same thing
without any problems, but if someone could explain what changes would need
to be made to the local-expand call for that to work, I'd be grateful (for
my own understanding).


Thomas Dickerson

Brown University
Department of Computer Science
115 Waterman St, 4th Floor
Providence, RI 02912

802-458-0637

On Tue, Jun 23, 2015 at 11:17 AM, Thomas Dickerson <
thomas_dicker...@brown.edu> wrote:

> On Tuesday, June 23, 2015 at 8:30:32 AM UTC-4, Matthew Flatt wrote:
> > Providing #f as the third argument to `local-expand` means that
> >
> >  (+ i j)
> >
> > is expanded only as far as exposing the primitive function-call form,
> also known as `#%plain-app`:
> >
> >  (#%plain-app + i j)
> >
> > When `i` is encountered later, there's no binding in the compile-time
> > environment, because the internal-definition context has been
> > discarded as the `my-def-stx` macro returned.
> >
> > Probably you want to replace the `#f` with `null` (which means "expand
> > without stopping").
> Got it - this makes sense, thanks.
>
> > Another possibility is to expand to a
> > `letrec-syntaxes+values` form to bind `i` and `j`.
>
> This is actually what I'm trying to avoid - see the earlier note on why I
> need a solution that won't leave behind empty (let-values () ...) forms (or
> any other Racket-specific forms) in the output syntax.
>
> > At Tue, 23 Jun 2015 02:13:19 -0400, Thomas Dickerson wrote:
> > > I seem to be missing some key ingredient here. The following really
> simply
> > > test-case, using let-syntaxes, works as expected:
> > >
> > > > (define-syntax my-def-stx
> > > >   (lambda (stx)
> > > >     (syntax-case stx (my-def-stx)
> > > >       [(my-def-stx (id ...) rhs expr)
> > > >        #'(let-syntaxes ([(id ...) rhs]) expr)])))
> > > >
> > > > (my-def-stx
> > > >  (i j) (values (lambda (stx) #'3) (lambda (stx) #'4))
> > > >  (+ i j))
> > > >
> > > Trying the same thing, but with this:
> > >
> > > > (require (for-syntax syntax/context))
> > > > (define-syntax my-def-stx
> > > >   (lambda (stx)
> > > >     (syntax-case stx (my-def-stx)
> > > >       [(my-def-stx (id ...) rhs expr)
> > > >        (let* ([intdef (syntax-local-make-definition-context)]
> > > >              [ctx (generate-expand-context)])
> > > >            (syntax-local-bind-syntaxes (syntax->list #'(id ...))
> > > > (local-transformer-expand #'rhs 'expression null) intdef)
> > > >            (internal-definition-context-seal intdef)
> > > >            (local-expand #'expr ctx #f intdef))])))
> > > >
> > > > (my-def-stx
> > > >  (i j) (values (lambda (stx) #'3) (lambda (stx) #'4))
> > > >  (+ i j))
> > > >
> > > instead, gives me "i: undefined; cannot reference an identifier before
> its
> > > definition".
> > >
> > > Thomas Dickerson
> > >
> > > Brown University
> > > Department of Computer Science
> > > 115 Waterman St, 4th Floor
> > > Providence, RI 02912
> > >
> > > 802-458-0637
> > >
> > > On Mon, Jun 22, 2015 at 11:39 PM, Matthew Flatt <mfl...@cs.utah.edu>
> wrote:
> > >
> > > > Probably you don't want to work with namespaces, which are intended
> more
> > > > for run-time reflection.
> > > >
> > > > For an example of turning internal definitions into
> > > > 'letrec-syntaxes+values', try the implementation of 'racket/block'.
> > > >
> > > > > On Jun 23, 2015, at 10:06 AM, Thomas Dickerson <
> > > > thomas_dicker...@brown.edu> wrote:
> > > > >
> > > > > Update / addendum: I was also able to fairly trivially extend
> Alex's
> > > > code using let-syntax to have the more hygienic behavior for where
> Accum
> > > > is/is-not visible, but let-syntax ends up leaving an additional
> layer of
> > > > empty (let-values() ...) behind.
> > > > >
> > > > > The documentation for let-syntax makes the following claim - "The
> > > > evaluation of each trans-expr is parameterized to set
> current-namespace to
> > > > a namespace that shares bindings and variables with the namespace
> being
> > > > used to expand the let-syntax form, except that its base phase is one
> > > > greater." which seems very much related to what I'd like to
> accomplish (and
> > > > Shriram's suggestion that I mentioned in the original post);
> however, I see
> > > > no evidence of this actually taking place in the Racket source-code,
> it
> > > > just appears to desugar, with very few bells and whistles, to
> > > > letrec-syntaxes+values which evidently is implemented in the C code.
> > > > Digging into syntax-parameterize shows that, unsurprisingly, this is
> also
> > > > how syntax-parameterize gets desugared, with a few more bells and
> whistles.
> > > > >
> > > > > So I guess at this point the remainder of my question boils down to
> > > > this: can someone offer any insight into the process of turning
> macros
> > > > which are introduced by let-syntax or equivalent into macros which
> are
> > > > introduced implicitly by an internal-definition-context argument to
> > > > local-expand, or by explicit namespace manipulation as hinted at in
> the
> > > > let-syntax documentation (but which I can't turn up anywhere).
> > > > >
> > > > >> On Monday, June 22, 2015 at 5:35:50 PM UTC-4, Thomas Dickerson
> wrote:
> > > > >> Thanks for the effort that went into figuring that out! A couple
> first
> > > > thoughts on the implementation:
> > > > >> To first order, this is exactly what I want to be able to do (and
> it
> > > > elegantly avoids local-expand by reasoning about expansion order);
> however,
> > > > as a general pattern, it's a bit unhygienic in that Accum is now in
> the
> > > > global namespace, which precludes using it as a child macro for other
> > > > constructs, and prevents specific instantiations of Loop from
> requesting a
> > > > different name for Accum to cooperate in nested loops.
> > > > >>
> > > > >> I also noticed that (syntax-parameterize) isn't totally
> transparent in
> > > > the expanded syntax - it leaves a bunch of empty (let-values () ...)
> > > > behind, which is fine for Racket, but problematic in my target
> use-case of
> > > > emitting something which ultimately needs to be an entirely different
> > > > language. This is why my experiments have all involved local-expand
> - I
> > > > want to introduce expand-time names without introducing wrapping
> > > > let-syntaxes (or similar) forms around my body code.
> > > > >>
> > > > >>
> > > > >>> On Saturday, June 20, 2015 at 2:24:40 PM UTC-4, Alex Knauth
> wrote:
> > > > >>>> On Jun 19, 2015, at 7:44 PM, Thomas Dickerson <
> > > > thomas_dicker...@brown.edu> wrote:
> > > > >>>>
> > > > >>>> I was intending for that example to have the variables be
> defined
> > > > outside the macro, but being able to create the (set! ...) forms
> outside
> > > > should mean I could also hypothetically create let/define-values
> forms.
> > > > This is why I originally specified being able to effect generation
> of both
> > > > prologue + epilogue code.
> > > > >>>
> > > > >>> I figured this out for code that will be expanded after the body:
> > > > >>>
> > > > >>> #lang racket/base
> > > > >>>
> > > > >>> (require racket/stxparam
> > > > >>>         (for-syntax racket/base
> > > > >>>                     syntax/parse
> > > > >>>                     racket/set
> > > > >>>                     ))
> > > > >>>
> > > > >>> ;; (syntax-parameter-value vars) : (U #f (MutableSetof
> Identifier))
> > > > >>> (define-syntax-parameter vars #f)
> > > > >>>
> > > > >>> (define-syntax Accum
> > > > >>>  (lambda (stx)
> > > > >>>    (syntax-parse stx
> > > > >>>      [(Accum x:id dx:expr)
> > > > >>>       (define vs (syntax-parameter-value #'vars))
> > > > >>>       (unless (set-mutable? vs)
> > > > >>>         (raise-syntax-error #f "cannot be used outside Loop"
> stx))
> > > > >>>       (set-add! vs (syntax-local-introduce #'x))
> > > > >>>       #'(set! x (+ x dx))])))
> > > > >>>
> > > > >>> (define-syntax set-accums-zero!
> > > > >>>  (syntax-parser
> > > > >>>    [(set-accums-zero!)
> > > > >>>     #:with [x ...]
> > > > >>>     (map syntax-local-introduce (set->list
> (syntax-parameter-value
> > > > #'vars)))
> > > > >>>     #'(begin (set! x 0) ...)]))
> > > > >>>
> > > > >>> (define-syntax Loop
> > > > >>>  (syntax-parser
> > > > >>>    [(Loop up-to-expr:expr body:expr ...+)
> > > > >>>     #'(syntax-parameterize ([vars (mutable-set)])
> > > > >>>         (let ([up-to up-to-expr])
> > > > >>>           (letrec
> > > > >>>               ([loop
> > > > >>>                 (lambda (n)
> > > > >>>                   body
> > > > >>>                   ...
> > > > >>>                   (if (< n up-to)
> > > > >>>                       (loop (add1 n))
> > > > >>>                       (void)))])
> > > > >>>             (set-accums-zero!)
> > > > >>>             (loop 0))))]))
> > > > >>>
> > > > >>> (let ([x "x"] [y "y"] [z "z"])
> > > > >>>  (Loop 10
> > > > >>>        (begin
> > > > >>>          (define-values
> > > > >>>            [dx dy dz]
> > > > >>>            (values 1 2 3))
> > > > >>>          (printf "x = ~v, y = ~v, z = ~v\n" x y z)
> > > > >>>          (Accum x dx)
> > > > >>>          (Accum y dy)
> > > > >>>          (Accum z dz))))
> > > > >>>
> > > > >>> To generate a let outside of that you’d probably need to do some
> kind
> > > > of local-expand, but I’m not sure.
> > > > >
> > > > > --
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