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3 points by gaah 6138 days ago | link | parent

I think you want this:

  arc> (= a 3)
  3
  arc> (= b 5)
  5
  arc> (let a (+ a 4)
         (+ a b))
  12
  arc> (with (a 2 b (+ b 2))
         (+ a b))
  9
  arc> (with (a 6 b 2)
         (prn a) (prn b)
         (= b a) (= a 'foo)
         (prn a) b)
  6
  2
  'foo
  6
  arc> a
  3
  arc> b
  5
The title of your post is misleading. I think when people saw "How to implement a no-side-effects macro" they read that as "How to implement macros with no side effects" rather than "macro that hides side effects from the surrounding scope".

You asked a simple question about lexical scoping. You got complex and confusing answers about macro hygiene. It is not that Arc has no easy answer to your lexical-scoping question, it is just that your simple question looks like a much harder one about macro hygiene.



1 point by lacker 6138 days ago | link

Sorry I was unclear - I was indeed actually trying to ask a hard question.

I don't know a priori what variables this code will try to access. I could possibly figure that out with a macro, but some of the details are still unclear to me. I also don't want side effects if annotate, def, mac, or anything like that was run from the no-side-effect'd code.

At any rate, I think just wrapping in a let is insufficient, even if you do know all the variable names. E.g. you can break out of the sandbox with scdr:

> (= a '(1 2))

(1 2)

> (let a a (scdr a '(3)))

(3)

> a

(1 3)

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