4.52 Exercise 4.52

An if-fail expression takes two arguments. The first is an expression that is always going to be evaluated. If it successfully evaluates to a value v, then the result of the if-fail expression as a whole is v. However, if evaluation fails, then the result of the entire expression will be the result of evaluating the second argument. If the evaluation of the alternative fails, then the evaluation of the entire expression fails.

First, the boilerplate:

(define (if-fail? exp) (tagged-list? exp 'if-fail))

(define (if-fail-value exp) (cadr exp))

(define (if-fail-alternative exp) (caddr exp))

Implementing if-fail is a little tricker than most of the special forms we’ve written so far, because it requires constructing a new failure continuation and adding it to the right place (alongside a few other failure continuations being juggled around).

The one tricky bit is that the new failure continuation needs to be a thunk – if you forget this thunk wrapping, then you will always evaluate the alternative case. If the main evaluation succeeds, you will get very confusing results.

This is probably easier to understand by looking at the code:

(define (analyze-if-fail exp)

(let* ((vproc (analyze (if-fail-value exp)))

(aproc (analyze (if-fail-alternative exp))))

(lambda (env succeed fail)

(vproc env

; If the evaluation of the predicate succeeds

(lambda (val fail2)

(succeed val fail2))

; If the evaluation of the predicate fails

(lambda ()

(aproc env

(lambda (val fail3)

(succeed val fail3))

fail))))))