Environments

Environments in LIPS are first class objects that you can interact with. Scheme spec define procedure (interactive-environment) LIPS add also (current-environment).

(let ((x 10))
  (eval '(+ x x) (current-environment)))
;; ==> 20

Environment chain

Environments are created in a chain. In JavaScript there is a notion of scope chain the same, is in Scheme. But with environment object you can access that chain.

(let ((x 10))
  (let ((x 20))
    (print (eval '(+ x x) (current-environment)))
    (print (eval '(+ x x) (. (current-environment) '__parent__)))))
;; ==> 40
;; ==> 20

Changing environment without eval

In LIPS you can change the environment with let-env syntax:

(define env (let ((x 10) (y 20))
              (current-environment)))

(let-env env
  (+ x y))
;; ==> 30

Scheme Report Environments

Scheme standard provide environments for given version of the RⁿRS specification in a form of a function scheme-report-environment.

You can use this function in LIPS with version 5 and 7 to get R⁵RS or R⁷RS.

NOTE: that some of the functions from R⁵RS may have features of R⁷RS since some of them got additional arguments. RⁿRS is backward compatible.

You can use this function with eval or let-env:

(let ((env (scheme-report-environment 7)))
  (let-env env
     (display (+ 1 2))))
;; ==> 3
(let-env (scheme-report-environment 7)
  (display (--> "string" (toUpperCase))))
;; ==> Unbound variable `-->'
(let-env (scheme-report-environment 7)
   (write (vector-map + #(1 2 3) #(4 5 6 7))))
;; ==> #(5 7 9)
(let-env (scheme-report-environment 5)
   (write (vector-map + #(1 2 3) #(4 5 6 7))))
;; ==> Unbound variable `vector-map'

R⁵RS doesn't support vector-map that was added in version R⁷RS. The same both Scheme versions doesn't support LIPS extensions like --> macro.

Introspection

Since environments are JavaScript objects you can access its properties like __name__ or __env__.

(let ((x 10) (y 20))
  (write (Object.keys (. (current-environment) '__env__))))
;; ==> #("x" "y")

__env__ property is an object with the variables. Here it returns variables defined in let.

(let ((x 10) (y 20))
  (let ((env (current-environment)))
    (write env.__name__)))
;; ==> "let"

Here you can access name of the lexical environment.

Frames

In LIPS inspired by R programming language, there are two procedures parent.frame and parent.frames you can use them to get access to function call stack environments.

(define (foo)
  (define x 20)
  (bar))

(define (bar)
  (define x 30)
  (baz))

(define (baz)
   (for-each (lambda (env)
                (let-env env
                  (print x)))
     (parent.frames)))

(define x 10)
(foo)
;; ==> 10
;; ==> 20
;; ==> 30

You can mix lexical scope chain with frames:

(define (foo)
  (define x 10)
  (let ((y "world"))
    (bar)))

(define (bar)
  (define x 20)
  (let ((y "hello"))
    (baz)))

(define (baz)
   (for-each (lambda (env)
               (display env.__name__)
               (display " ==> ")
               (print (Object.keys env.__env__))
               (display env.__parent__.__name__)
               (display " ==> ")
               (print (Object.keys env.__parent__.__env__)))
     ;; car is top level environment
     (cdr (parent.frames))))
(foo)
;; ==> let ==> #(y)
;; ==> lambda ==> #(arguments parent.frame x)
;; ==> let ==> #(y)
;; ==> lambda ==> #(arguments parent.frame x)

Global environment

in lips.env is user global environment but real global environment where all functions and macros that are located (it's also a place where names from bootstrapping are saved) is lips.env.__parent__.

If you want to really overwrite builtin function.

(define-macro (define x)
  `(print ,x))

(define 10)
;; ==> 10
(unset! define)
(define foo 10)
(print foo)
;; ==> 10

If you execute this code in REPL or in a script it will only add define into interaction-environment. If you really want to overwrite define you can (not that you should):

(let-env lips.env.__parent__
  (define-macro (define x)
    `(print ,x)))

(define 10)
;; ==> 10
(unset! define)
(define foo 10)
;; ==> Unbound variable `define'

The only time you may want to use lips.env.__parent__ when you bootstrap the LIPS Scheme system.

(let-env lips.env.__parent__
  (load "<path or URL>/dist/std.xcb"))