Closure of lexical environment in Scheme closures

["Dr. Rich Artym" <rartym@...>]

Here's an example of a Scheme function capturing the state of its
environment at the point of closure, blocking further modification.

I've tried to replicate the conditions under which our example Lua
program showed sensitivity to environment change, although it's hard
to create exactly the same conditions in a purely functional program.

; ------------------------------------------------------------
; Demonstration of Scheme lexical scoping and function closure

; Gentle intro example without functions
(let* ((foo
            10)
       (bar
            65))
     foo)                       ; prints 10
)

; Defines and applies function incfoo1 using free variable foo
(let* ((foo                     ; define foo
            10)
       (incfoo1                 ; define incfoo1
            (lambda () (+ foo 0.1)))
       (bar                     ; define bar
            65))                ; bar not used (just for symmetry)
     (incfoo1))                 ; prints 10.1

; Defines and applies function incfoo1 using free variable foo
; but overrides (foo 10) with (foo 20) before function is used.
(let* ((foo
            10)
       (incfoo1
            (lambda () (+ foo 0.1)))
       (incfoo2
            (let* ((foo         ; Let's change foo (as in Lua)
                       20))     ; to try to make incfoo1 use 20.
                  incfoo1))     ; Will this closure be affected?
                  
       (bar
            65))
     (incfoo2))     ; No, prints 10.1 because incfoo1 is a closure
                    ; which protects it against the change in foo
; ----------------------------------------------------------------

That was a bit of a quickie.  I'll try to find a closer approximation.

Rich Artym.
-- 
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