Racket has many macros that introduce new ids (which is not hygienic), such as struct.

this may sound really dumb and I ought to know better, but why is a macro that introduces ids non-hygenic? It seems OK to me to have a macro that expands to, say, a define. By my lights, that’s introducing an identifier. But perhaps I’m not looking at the situation in the right way.

In the define example the identifier is supplied by the user, and that — and only that — gets introduced.

(let ([foo-bar 42]) (struct foo (bar)) (+ foo-bar 1))

Here the struct macro introduces a foo-bar that shadows the one apparent in the code.

thanks, that helps

I think everyone would agree that it would be horribly tedious if you had to supply all the accessor names and the predicate when you invoke struct.

agreed

And hopefully it’s not surprising because people know struct.

but to my mind, all those things are still functions of the data I gave to struct; it’s as though there are lots of format-ids being done, not just a simple define

But strictly speaking, it’s “surprising” in the way that non-hygienic macros can be surprising in bad ways.

your let example is really illuminating

Yes, it definitely helps that the identifiers are created using a prefix that you supply.

it shows how there’s a kind of “polluting” going on by struct that wouldn’t normally occur

I mean there are many places on the continuum of “more vs. less ceremony” or “explicit vs. implicit”, and there are always trade-offs.

As usual, the right point is, “it depends”. :smile:

I think that goes for judicious use of non-hygienic macros, too.

Also “docs” :wink: