Arc Forumnew | comments | leaders | submitlogin
4 points by rocketnia 238 days ago | link | parent

Right now, the only things ns.arc can import into local scopes are non-macro values. (Or rather, it can import macro values, but it determines what value they have well after the code it's in has been macroexpanded, so you'll just see them as (annotate 'mac ...) values, and calling them will be a function call that fails with an error.)

What you want are local macros (Common Lisp's `macrolet`, Racket's `let-syntax`). Suppose we name Arc's equivalent `w/mac`.

Then (w/html ...) could expand to this:

  (w/mac html tag-html
         head tag-head
         body tag-body
         ...
    ...)
To implement `w/mac` requires changes to the macroexpander in ac.rkt. I don't know how much this will make sense to you, but this is the approach for anyone who wants to take it: It requires a change to the way `ac-call` looks up the macro's value (`ac-macro?`) so that it can look it up from the local scope (the `env` parameter). This in turn means the local scope value `env` needs to be refactored so it's a hash table rather than a list, and then `w/mac` can look it up from that table. The `env` parameter isn't passed into macros, so either `w/mac` would need to be implemented as a special form or we'd need to come up with a way that macros can optionally declare that they want to receive an `env` parameter.

This would be something of a breaking change to Anarki because it means Anarki will now allow a local variable to shadow a global macro. That was the problem you encountered with (body) in the first place; some calls to local variables were expanding as macro calls now because the macro lookup ignored local variables altogether.

There might be some code out there of the form

  (let foo 1
    (bar foo))
where (bar ...) is a macro call that expands to (foo ...) for some global macro `foo`. This code would now fail.

For some time I've thought it would be worth it for the local macro support, but I'm too afraid to make the breaking changes.

Nevertheless, a while back, there was a consensus here that the way things worked already was a bug. See "Global macro names take precedence over local lexical variables" at [1]. I don't remember if this bug was ever fixed in Anarki; it doesn't seem to be fixed now. (Maybe there was another consensus to reverse it? It could happen.) It's the very same bug you encountered with (body) in the first place.

[1] https://sites.google.com/site/arclanguagewiki/arc-3_1/known-...