3.2 Solution for Exercise 7

When implementing a macro’s dynamic behavior with a function, an expression becomes a function; an expression to be evaluated in an extended environment becomes a function with arguments.

In andlet1, e2 is evaluated in the scope of the variable x. So the function for e2 has a single formal parameter: x.

(define-syntax-rule (andlet1 x e1 e2)

(andlet1-fun (lambda () e1) (lambda (x) e2)))

(define (andlet1-fun thunk1 fun2)

(let ([tmp (thunk1)])

(if tmp

(fun2 tmp)

#f)))

Of course, this technique breaks down if the variable’s scope includes multiple expressions handled separately by the macro and if the expressions use set! to mutate the variable.