I am reading SICP which uses Scheme dialect of lisp. My question is why is there a need for sequence to expression conversion function as defined below which is used in conditional definitions but isn't used in the if expressions?
(define (sequence->exp seq)
(cond ((null? seq) seq)
((last-exp? seq) (first-exp seq))
(else (make-begin seq))))
(define (make-begin seq) (cons 'begin seq))
Following is the definition of if expression which doesn't require sequence to expression conversion:
(define (if? exp) (tagged-list? exp 'if))
(define (if-predicate exp) (cadr exp))
(define (if-consequent exp) (caddr exp))
(define (if-alternative exp)
(if (not (null? (cdddr exp)))
(cadddr exp)
'false))
(define (eval-if exp env)
(if (true? (eval (if-predicate exp) env))
(eval (if-consequent exp) env)
(eval (if-alternative exp) env)))
Following is the definition of conditional whose predicate does require sequence to expression conversion.
(define (cond? exp) (tagged-list? exp 'cond))
(define (cond-clauses exp) (cdr exp))
(define (cond-else-clause? clause)
(eq? (cond-predicate clause) 'else))
(define (cond-predicate clause) (car clause))
(define (cond-actions clause) (cdr clause))
(define (cond->if exp) (expand-clauses (cond-clauses exp)))
(define (expand-clauses clauses)
(if (null? clauses)
'false ; no else clause
(let ((first (car clauses))
(rest (cdr clauses)))
(if (cond-else-clause? first)
(if (null? rest)
(sequence->exp (cond-actions first))
(error "ELSE clause isn't last: COND->IF"
clauses))
(make-if (cond-predicate first)
(sequence->exp (cond-actions first))
(expand-clauses rest))))))