<Bindings> should have the form ((<variable1> <init1>) ...) where each <init> is an expression. It is an error for a <variable> to appear more than once in the list of variables.
The <init>s are evaluated in the current environment, the values of the <variable>s are saved, the results of the <init>s are assigned to the <variable>s, and the <body> is evaluated in the current environment before the <variable>s are restored to their saved values and the values returned by the last expression of <body> are returned as the values of the entire expression.
It is an error for the <variable>s to be unbound at the time their old values are saved.
If control leaves the dynamic scope of the <body> before the last expression of <body> has returned, then the current values of the <variables>s inside the scope are saved and the saved values from outside the scope are restored before control leaves the scope of the FLUID-LET. If control subsequently re-enters the scope of the <body>, then the current values of the <variable>s outside the scope are saved and the saved values from inside the scope are restored before execution continues inside <body>. In this case, it is the last saved values from outside the scope that will be restored when the <body> finally returns.
(define v 1) (define again #f) (define (test1) (display v) (fluid-let ((v 2)) (call-with-current-continuation (lambda (k) (set! again (lambda () (set! again #f) (k #t))))) (test2) (set! v 3)) (display v) (set! v 4) (if again (again))) (define (test2) (display v)) (test1) --> 12134
The implementation assumes that some top-level names defined by the R5RS are bound to their original values.
(define-syntax fluid-let (syntax-rules () ((_ ((v1 e1) ...) b1 b2 ...) (fluid-let "temps" () ((v1 e1) ...) b1 b2 ...)) ((_ "temps" (t ...) ((v1 e1) x ...) b1 b2 ...) (let ((temp e1)) (fluid-let "temps" ((temp e1 v1) t ...) (x ...) b1 b2 ...))) ((_ "temps" ((t e v) ...) () b1 b2 ...) (let-syntax ((swap! (syntax-rules () ((swap! a b) (let ((tmp a)) (set! a b) (set! b tmp)))))) (dynamic-wind (lambda () (swap! t v) ...) (lambda () b1 b2 ...) (lambda () (swap! t v) ...))))))
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