05-09-2010 /prog/ Challenge

Write a procedure find-variable that takes as arguments a variable and a compile-time environment and returns the lexical address of the variable with mad respect to that environment.

The problem with this challenge is that the environment's structure is different for each implementation, on CLISP it's a dumb (recurive) vector(for easier acess from C), on SBCL it's a fancy structure, on CCL it's a structure as well (however, structures there are most certainly always implemented with vectors), and so on. Since I don't know the exact meaning of "lexical address of the variable with mad respect to that environment", I'll just show how I located a variable in 3 minutes within the environment object without much prior knowledge of the compiler internals, and instead just using my instincts and general lisp knowledge to inspect compiler(SBCL) internals:

CL-USER> (defmacro get-environment (&environment env) env)
GET-ENVIRONMENT
CL-USER> (get-environment)
#<NULL-LEXENV>
CL-USER> (let ((x 1)) (get-environment))
; in: LAMBDA NIL
;     (LET ((X 1))
;       (GET-ENVIRONMENT))
;
; caught STYLE-WARNING:
;   The variable X is defined but never used.
;
; compilation unit finished
;   caught 1 STYLE-WARNING condition
#S(SB-KERNEL:LEXENV
   :FUNS NIL
   :VARS ((X . #<SB-C::LAMBDA-VAR :%SOURCE-NAME X {24626819}>))
   :BLOCKS NIL
   :TAGS NIL
   :TYPE-RESTRICTIONS NIL
   :LAMBDA #<SB-C::CLAMBDA
             :%SOURCE-NAME SB-C::.ANONYMOUS.
             :%DEBUG-NAME (LET ((X 1))
                            )
             :KIND :ZOMBIE
             :TYPE #<SB-KERNEL:BUILT-IN-CLASSOID FUNCTION (read-only)>
             :WHERE-FROM :DEFINED
             :VARS (X) {24626949}>
   :CLEANUP NIL
   :HANDLED-CONDITIONS ((COMPILER-NOTE . MUFFLE-WARNING))
   :DISABLED-PACKAGE-LOCKS NIL
   :%POLICY ((COMPILATION-SPEED . 1) (DEBUG . 1) (INHIBIT-WARNINGS . 1)
             (SAFETY . 1) (SPACE . 1) (SPEED . 1))
   :USER-DATA NIL)
CL-USER> (sb-c::lexenv-vars *)
((X . #<SB-C::LAMBDA-VAR :%SOURCE-NAME X {24626819}>))
CL-USER> (assoc 'x *)
(X . #<SB-C::LAMBDA-VAR :%SOURCE-NAME X {24626819}>)
CL-USER> (cdr *)
#<SB-C::LAMBDA-VAR :%SOURCE-NAME X {24626819}>
CL-USER> (describe *)
#<SB-C::LAMBDA-VAR :%SOURCE-NAME X {24626819}>
  [structure-object]

Slots with :INSTANCE allocation:
  NUMBER             = 4
  NUMBER             = 4
  %SOURCE-NAME       = X
  TYPE               = #<SB-KERNEL:NAMED-TYPE T>
  DEFINED-TYPE       = #<SB-KERNEL:NAMED-TYPE T>
  WHERE-FROM         = :ASSUMED
  REFS               = NIL
  EVER-USED          = T
  DYNAMIC-EXTENT     = NIL
  INFO               = NIL
  SETS               = NIL
  FLAGS              = 0
  HOME               = #<SB-C::CLAMBDA..
  ARG-INFO           = NIL
  SPECVAR            = NIL
  CONSTRAINTS        = NIL
  LAST-INITIAL-TYPE  = #<SB-KERNEL:NAMED-TYPE T>
  FOP-VALUE          = NIL
; No value
CL-USER>