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clemtibs/boxes.rkt

Created March 13, 2016 08:11
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  1. @brianclements brianclements created this gist Mar 13, 2016.
    41 changes: 41 additions & 0 deletions boxes.rkt
    View file
    Original file line number Diff line number Diff line change
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    #lang racket

    (define a 'one)

    (printf "var 'a' is '~a'\n\n" a)

    (define (foo x)
    (printf "we call '(foo)' here and pass it '2'\n")
    (printf "(foo) sets 'x'\n")
    (printf "->arg 'x' is '~a'\n" x)
    (set! x 'changed)
    (printf "->arg 'x' in this closure now is '~a'\n\n" x))

    (foo 2)

    (printf "but var 'a' in top level is still '~a'\n\n" a)

    (define (foo2 x)
    (printf "we now pass var 'a' as arg 'x' in (foo2): 'x' is '~a'\n" x)
    (printf "(foo) sets 'x'\n")
    (set! x 'changed)
    (printf "->arg x now is '~a' in this closure as well...\n\n" x))

    (foo2 a)

    (printf "...but var 'a' is still '~a' because it's passed by value\n\n" a)

    (define b (box 'two))

    (printf "'b' is a box, it's value is: '~a'\n\n" (unbox b))


    (define (new-foo x)
    (printf "we now pass box 'b' as arg 'x' in (new-foo): 'x' is '~a'\n" (unbox x))
    (printf "(new-foo) sets 'x'\n")
    (set-box! x 'changed)
    (printf "->arg 'x' in this closure now is '~a'...\n\n" (unbox x)))

    (new-foo b)

    (printf "...but so is our top level box 'b'!: '~a'\n\n" (unbox b))