+ (let ((loop (gensym)))
+ `(begin
+ (define (,loop)
+ (if ,condition
+ (begin ,@body (,loop))))
+ (,loop))))
+
+; cond
+
+(define (cond-predicate clause) (car clause))
+(define (cond-actions clause) (cdr clause))
+(define (cond-else-clause? clause)
+ (eq? (cond-predicate clause) 'else))
+
+(define (expand-clauses clauses)
+ (if (null? clauses)
+ (none)
+ (let ((first (car clauses))
+ (rest (cdr clauses)))
+ (if (cond-else-clause? first)
+ (if (null? rest)
+ `(begin ,@(cond-actions first))
+ (error "else clause isn't last in cond expression."))
+ `(if ,(cond-predicate first)
+ (begin ,@(cond-actions first))
+ ,(expand-clauses rest))))))
+
+(define-macro (cond . clauses)
+ (if (null? clauses)
+ (error "cond requires at least one clause.")
+ (expand-clauses clauses)))
+
+; and
+
+(define (expand-and-expressions expressions)
+ (let ((first (car expressions))
+ (rest (cdr expressions)))
+ (if (null? rest)
+ first
+ `(if ,first
+ ,(expand-and-expressions rest)
+ #f))))
+
+(define-macro (and . expressions)
+ (if (null? expressions)
+ #t
+ (expand-and-expressions expressions)))
+
+; or
+
+(define (expand-or-expressions expressions)
+ (if (null? expressions)
+ #f
+ (let ((first (car expressions))
+ (rest (cdr expressions))
+ (val (gensym)))
+ `(let ((,val ,first))
+ (if ,val
+ ,val
+ ,(expand-or-expressions rest))))))
+
+(define-macro (or . expressions)
+ (expand-or-expressions expressions))
+