I see that the documentation --- at least a reasonable reading of that
prose --- doesn't match the implementation.
The implementation treats the two arguments as having no scopes in
common if the second is not a subset of the first. Since the scopes on
`a` and `b` are not subsets of each other
(make-syntax-delta-introducer a b)
counts the full set of `a` scopes as the "delta", while
(make-syntax-delta-introducer b a)
counts the full set of `b` scopes as the "delta".
The implemented behavior turned out to be useful for implementing
macros that work with identifiers from different modules at once. The
behavior is certainly confusing absent a use case, and it's not
documented correctly, so I'll fix the documentation.
At Sun, 31 Jul 2016 14:11:17 -0700 (PDT), Dupéron Georges wrote:
> 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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