Am Mi., 6. März 2024 um 20:11 Uhr schrieb Sergei Egorov <e...@acm.org>:
> > I don’t think that this is formally correct. SRFI 251 derives its > semantics from R7RS, and no longer relaxes R7RS requirement for body-level > ‘begins’ to be either ‘definition begins’ (i.e. contain only definitions) > or ‘expression begins’ (i.e. contain only expressions). > This would be incompatible with R6RS, in which body-level begin has splicing semantics and can contain any kind of form. The same seems to be true for R7RS's first form of begin (4.2.3.). Or have I misunderstood you? > I believe this puts your second definition of ‘define-number’ into > undefined territory: if expanded on body level, it yields a ‘begin’ with > 'it is an error' behavior, as in R7RS. To make the second ‘define-number’ > work as intended, it should be changed to something like this: > > (define-syntax define-number > (syntax-rules () > ((_ id val) > (begin > (define tmp val) > (define-values () (begin (unless (number? tmp) (error "boom!")) > (values))) > (define id tmp))))) > > As we have discussed earlier at some point, perhaps it makes sense to > supplement this SRFI with a derived form that makes such code easier to > read, e.g. > > (define! (unless (number? tmp) (error "boom!"))) > > > On Wednesday, March 6, 2024 10:18:19 AM (-05:00), Marc Nieper-Wißkirchen > wrote: > > A problem with SRFI 251's approach is that macros can gain unwanted spooky > actions at a distance. > > Consider the following expression E: > > (let ((x 1)) > (define (f) x) > (define-number y 42) > (define x 2) > (f)) > > Here, define-number is defined through > > (define-syntax define-number > (syntax-rules () > ((_ id val) (define id val)))) > > Evaluating E gives 2. > > Now the author of define-number decides to change its definition to > > (define-syntax define-number > (syntax-rules () > ((_ id val) > (begin > (define tmp val) > (unless (number? tmp) (error "boom!")) > (define id tmp))))) > > This is possible with SRFI 251. However, suddenly evaluating E gives 1. > > R6RS and R7RS solve this problem by making E together with the second > version of define-number an error. > > SRFI 245 doesn't have this problem. > > Am Mi., 6. März 2024 um 02:55 Uhr schrieb Arthur A. Gleckler < > s...@speechcode.com>: > >> I've just published draft #3 of SRFI 251 >> <https://srfi.schemers.org/srfi-251/>. It was submitted by Sergei >> Egorov, author of the SRFI. >> >> Here are Sergei's comments on the draft: >> >> Here is a third draft with an extra example (and test), demonstrating the >> 'staged' nature of define-syntax in stfi-251: macros defined in a >> definition group can be used in downstream expressions and definition >> groups. >> >> Here is the commit summary: >> >> - "Staged" define-syntax example added >> - Drop trailing whitespace. >> - Publish third draft. >> >> Here's the diff: >> >> >> https://github.com/scheme-requests-for-implementation/srfi-251/compare/draft-2..draft-3 >> >> Regards, >> >> SRFI Editor >> > >
