minor code improvements, added ninth and tenth

svn: r8364

collects/scheme/list.ss

@@ -1,14 +1,6 @@
+#lang scheme/base
 
-(module list scheme/base
+(provide first second third fourth fifth sixth seventh eighth ninth tenth
-
-  (provide first
-           second
-           third
-           fourth
-           fifth
-           sixth
-           seventh
-           eighth
          last
 
          rest
@@ -18,10 +10,9 @@
          empty?)
 
 (define (first x)
-    (unless (and (pair? x) 
+  (if (and (pair? x) (list? x))
-                 (list? x))
+    (car x)
-      (raise-type-error 'first "non-empty list" x))
+    (raise-type-error 'first "non-empty list" x)))
-    (car x))
 
 (define-syntax define-lgetter
   (syntax-rules ()
@@ -31,7 +22,8 @@
          (let loop ([l l0] [pos npos])
            (if (pair? l)
              (if (eq? pos 1) (car l) (loop (cdr l) (sub1 pos)))
-                   (raise-type-error 'name (format "list with ~a or more items" npos) l0)))
+             (raise-type-error
+              'name (format "list with ~a or more items" npos) l0)))
          (raise-type-error 'name "list" l0)))]))
 (define-lgetter second  2)
 (define-lgetter third   3)
@@ -40,22 +32,22 @@
 (define-lgetter sixth   6)
 (define-lgetter seventh 7)
 (define-lgetter eighth  8)
+(define-lgetter ninth   9)
+(define-lgetter tenth   10)
 
-  (define (last x)
+(define (last l)
-    (unless (and (pair? x)
+  (if (and (pair? l) (list? l))
-                 (list? x))
+    (let loop ([l l])
-      (raise-type-error 'last "non-empty list" x))
+      (if (pair? (cdr l))
-    (let loop ([x x])
+        (loop (cdr l))
-      (if (pair? (cdr x))
+        (car l)))
-          (loop (cdr x))
+    (raise-type-error 'last "non-empty list" l)))
-          (car x))))
 
-  (define (rest x)
+(define (rest l)
-    (unless (and (pair? x)
+  (if (and (pair? l) (list? l))
-                 (list? x))
+    (cdr l)
-      (raise-type-error 'rest "non-empty list" x))
+    (raise-type-error 'rest "non-empty list" l)))
-    (cdr x))
 
-  (define cons? (lambda (x) (pair? x)))
+(define cons? (lambda (l) (pair? l)))
-  (define empty? (lambda (x) (null? x)))
+(define empty? (lambda (l) (null? l)))
-  (define empty '()))
+(define empty '())

collects/scribblings/reference/pairs.scrbl

@@ -461,6 +461,10 @@ Like @scheme[assoc], but finds an element using the predicate
 
 @defproc[(eighth [lst list?]) any]{Returns the eighth element of the list.}
 
+@defproc[(ninth [lst list?]) any]{Returns the ninth element of the list.}
+
+@defproc[(tenth [lst list?]) any]{Returns the tenth element of the list.}
+
 @defproc[(last [lst list?]) any]{Returns the last element of the list.}
 
 @; ----------------------------------------