I know in principle but on occasion I fail to understand the implications.
Let me think aloud. I don't have to be perfectly accurate, maybe just about
right. Hygiene here means that every symbol there e.g. arguments my macro
receives carry their "environment" with them. There exists some oracle
which can tell when two symbols refer to the same thing probably by
checking environments somehow. Since I just typed that *foo-impl* there in
the template it must be getting some fresh tag or "environment" attached to
it to avoid capturing something with the same name defined at the macro
call site, right? Ok. How the *define* before *foo-impl *is special then?
We both know the "define" I mean. Or is the newly attached "environment" is
in fact not empty and comes enriched with a bunch of Racket stuff? How do I
reason when its safe to just type a name and when it isn't? In fact, here.
I just defined a foo-impl outside. If I now remove the macro-defined
foo-impl the code will still work correctly and grab the outer definition.
So define *inside* a template refers to the usual define, but *foo-impl*
doesn't? Why?
(define (foo-impl op a b) (op a b))
(define-simple-macro (define-foo (name:id formal:id ...) body:expr ...)
... same ...
(define-foo (bar op a b) (op a b))
(define-foo (baz op a b) (op a b))
(bar + 1 2)
;; => 3
On Thursday, 4 April 2019 21:02:58 UTC+1, Ben Greenman wrote:
>
> Racket's macros are hygienic. They'll gensym for you.
>
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