3.1 Solution for Exercise 3

(define-syntax-rule (iflet x e1 e2 e3)

(let ([tmp e1])

(if tmp

(let ([x tmp]) e2)

e3)))

The following version is wrong, because it puts e3 in the scope of x:

; WRONG

(define-syntax-rule (iflet x e1 e2 e3)

(let ([x e1])

(if x e2 e3)))

An alternative is to “freeze” e3 outside of the let-binding of x and then “thaw” it inside:

; OKAY

(define-syntax-rule (iflet x e1 e2 e3)

(let ([thunk3 (lambda () e3)])

(let ([x e1])

(if x e2 (thunk3)))))

Don’t be afraid to use lambda to control the environment and evaluation order of an expression. In code such as this, the Racket compiler can inline the call to thunk3; that lambda costs nothing at run time.