lisp - Is that possible I lexical binding special var in other package?

Question

I have a very weird requirement that lexically binding a special variable of another package.

In file A.lisp

(defpackage A
  (:use #:CL)
  (:export #:a-test))
(in-package A)
(declaim (special *a-sp-v*))
(defun a-test (&key (v *a-sp-v*))
  (print v))

in file B.lisp

(defpackage B
  (:use #:CL #:A))
(in-package B)

(defun b-test ()
  (let ((*a-sp-v* 1))  ; cannot let a-test know
    (a-test)))

I keep receiving the error The variable A::*A-SP-V* is unbound. in my REPL.

Are there some methods I can bind *a-sp-v* in the package B? Let a-test function running with special *a-sp-v* no matter what package it is in?

question from:https://stackoverflow.com/questions/65948149/is-that-possible-i-lexical-binding-special-var-in-other-package

Answer 1

Your code is fine (I would use defvar instead of declaim special though). You can bind any symbol, it does not matter which package it resides in.

You just need to make sure you bind the symbol you need to bind and not something else. E.g., since you are not exporting a::*a-sp-v* from a, your b-test binds b::*a-sp-v* rather than a::*a-sp-v*. If you replace *a-sp-v* with a::*a-sp-v* in b-test, it should work. Alternatively, you can add #:*a-sp-v* to the :export section in (defpackage A ...)