4.2 Exercise 4.2

define is a special form, but it is syntactically identical to a function application. If Louis makes eval check whether the form matches that of a function application before assignments, it will treat assignments as calls to a function named define, which is not the correct behavior.

If, as suggested, we change the language so that all function applications are denoted by a list whose first element is the symbol call, followed by the function name and all the arguments, we would have to rewrite the detection for function application as follows:

(define (application? exp) (tagged-list? exp 'call))

(define (operator exp) (cadr exp))

(define (operands exp) (cddr exp))

And then eval would look more like this:

(define (eval exp env)

(cond ((self-evaluating? exp) exp)

((variable? exp) (lookup-variable-value exp env))

((quoted? exp) (text-of-quotation exp))

((application? exp)

(apply (eval (operator exp) env)

(list-of-values (operands exp) env)))

((assignment? exp) (eval-assignment exp env))

((definition? exp) (eval-definition exp env))

((if? exp) (eval-if exp env))

((lambda? exp)

(make-procedure (lambda-parameters exp)

(lambda-body exp)

env))

((begin? exp)

(eval-sequence (begin-actions exp) e nv))

((cond? exp) (eval (cond->if exp) env))

(else

(error "Unknown expression type -- EVAL" exp))))