On 10/16/2017 12:38 PM, 'John Clements' via users-redirect wrote:
I’m in the process of trying to update the stepper to handle check-random, and
I’m somewhat baffled by the amount of difficulty I’m running into in and around
‘syntax-disarm’.
It occurs to me that it would probably be simpler just to walk over the
expanded syntax tree and reformulate it by disarming all syntax and rebuilding
the tree using
(datum->syntax (syntax-disarm old-stx inspector) rebuilt) new-stx old-stx
old-stx)
.. at every syntax wrapper, with recursive calls for every compound form. This
seems like an awfully big hammer, though; is it likely to bite me, or have
unexpected consequences? I would be sad if the stepper created security
vulnerabilities.
Many thanks for any advice.
IIRC, the usual pattern for processing fully-expanded code looks like this:
(begin-for-syntax
;; transform : Syntax[Form] -> Syntax[Form]
(define (transform stx)
(define disarmed-stx (syntax-disarm stx privileged-inspector))
(define transformed-stx (.... disarmed-stx ....))
(syntax-rearm transformed-stx stx)))
You might do
(syntax-arm _ privileged-inspector #t)
before, after, or instead of `syntax-rearm` if you want to protect the
integrity of the code your transformation generates too.
I think the pattern you're proposing would look like this:
(begin-for-syntax
;; transform : Syntax[Form] -> Syntax[Form]
;; Neither disarms nor re-arms.
(define (transform stx) ....)
;; transform-top : Syntax[Top-Level-Form] -> Syntax[Top-Level-Form]
(define (transform-top stx)
(define disarmed-stx
(recursively-disarm-form stx privileged-inspector))
(define transformed-stx (.... disarmed-stx ....))
(syntax-arm transformed-stx privileged-inspector #t)))
I think that's fine *if* `recursively-disarm-form` recurs along the
structure of fully-expanded Racket syntax. If it recurred naively
through syntax-objects, then you might disarm something inside of a
`quote-syntax` expression that shouldn't be disarmed. (And I didn't
realize that subtlety until I started typing up the wrong answer, so I
might be missing something else.)
Ryan
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