In the following code, (make-syntax-delta-introducer a b) and
(make-syntax-delta-introducer b a) have scopes in common (#(8979 module) and
#(8980 module m 0)), which are part of both inputs. The docs do not mention
that some scopes present in both `a` and `b` can be part of the resulting
introducer too. What is causing this behaviour?
#lang racket
(define-syntax (foo stx1)
(syntax-case stx1 ()
[(_ name)
#`(define-syntax (name stx2)
(let* ([a (datum->syntax (quote-syntax #,stx1) 'x)]
[b (datum->syntax (quote-syntax #,(syntax-local-introduce
stx1)) 'y)]
[delta-a-b (make-syntax-delta-introducer a b)]
[delta-b-a (make-syntax-delta-introducer b a)])
(display "a ") (displayln (hash-ref (syntax-debug-info a)
'context))
(display "b ") (displayln (hash-ref (syntax-debug-info b)
'context))
(display "delta a b ") (displayln (hash-ref (syntax-debug-info
(delta-a-b (datum->syntax #f 'x) 'add)) 'context))
(display "delta b a ") (displayln (hash-ref (syntax-debug-info
(delta-b-a (datum->syntax #f 'x) 'add)) 'context))
#'(void)))]))
(foo bar)
(bar)
=>
a (#(8979 module) #(8980 module m 0) #(8993 use-site))
b (#(8979 module) #(8980 module m 0) #(8992 macro))
delta a b (#(8979 module) #(8980 module m 0) #(8993 use-site))
delta b a (#(8979 module) #(8980 module m 0) #(8992 macro))
It seems that `make-syntax-delta-introducer` reliably gives an introducer
encapsulating the set of scopes `a - b` only if `b` is the empty set. Based on
this, I wrote the following function, which seems reliably produce an
introducer for `a - b`:
(define (scopes-delta a b)
(let* ([+a (make-syntax-delta-introducer (datum->syntax a 'first)
(datum->syntax #f 'none))]
[+b (make-syntax-delta-introducer (datum->syntax b 'second)
(datum->syntax #f 'none))]
[a-b (+b (+a (datum->syntax #f 'none)
'add)
'remove)])
(make-syntax-delta-introducer a-b
(datum->syntax #f 'none))))
Georges Dupéron
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