I’m working my way through the implementation of racket/generic, and this
jumped out at me as a little odd. Apparently, in the #:defaults and
#:fast-defaults clauses of define-generics, there is support for an
undocumented #:dispatch clause. Using it looks a like this:
(define-generics fooable
(foo fooable)
#:defaults
([list? #:dispatch empty?
(define (foo fooable)
(displayln "the empty list was foo'd"))]))
As far I can tell, all this does is introduce an extra predicate that is
checked in addition to the other one. As far as I can tell, this could just be
done with (and/c list? empty?), so this feels quite redundant. More bizarre, if
the #:dispatch clause is omitted, then it defaults to the same as the first
predicate!
This means that the predicate is, when used as documented, always applied twice
for no reason I can discern. Any authors of racket/generic that could provide
some insight on what this is and why it exists?
Alexis
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