fix `slicef-after`

sugar/list.rkt

@@ -35,27 +35,28 @@
   ((list? procedure?) (boolean?) . ->* . (listof list?))
   (unless (list? xs)
     (raise-argument-error 'slicef-at "list?" xs))
+  (unless (procedure? pred)
+    (raise-argument-error 'slicef-at "procedure?" pred))
   (let loop ([xs xs][acc empty])
     (match xs
       [(== empty) (reverse acc)]
-      [(cons (? pred first) rest)
-       (define-values (not-pred-xs tail) (splitf-at rest (negate pred)))
-       (loop tail (cons (cons first not-pred-xs) acc))]
-      [rest
-       (define-values (not-pred-xs tail) (splitf-at rest (negate pred)))
+      [(list* (? pred pred-x) (? (negate pred) not-pred-xs) ... tail)
+       (loop tail (cons (cons pred-x not-pred-xs) acc))]
+      [(list* (? (negate pred) not-pred-xs) ... tail)
        (loop tail (if force? acc (cons not-pred-xs acc)))])))
 
-(define+provide+safe (slicef-after xs pred)
-  (list? procedure? . -> . (listof list?))
+(define+provide+safe (slicef-after xs pred [force? #f])
+  ((list? procedure?) (boolean?) . ->* . (listof list?))
   (unless (list? xs)
     (raise-argument-error 'slicef-after "list?" xs))
+  (unless (procedure? pred)
+    (raise-argument-error 'slicef-after "procedure?" pred))
   (let loop ([xs xs][acc empty])
-    (if (empty? xs)
-        (reverse acc)
-        (match/values (splitf-at xs (negate pred))
-                      [(not-pred-xs (cons first-pred-x other-pred-xs))
-                       (loop other-pred-xs (cons (append not-pred-xs (list first-pred-x)) acc))]
-                      [(not-pred-xs _) not-pred-xs]))))
+    (match xs
+      [(== empty) (reverse acc)]
+      [(list* (? (negate pred) not-pred-xs) ... (? pred pred-x) tail)
+       (loop tail (cons (append not-pred-xs (list pred-x)) acc))]
+      [tail (loop empty (if force? acc (cons tail acc)))])))
 
 (define+provide+safe (slice-at xs len [force? #f])
   ((list? exact-nonnegative-integer?) (boolean?) . ->* . (listof list?))

sugar/scribblings/list.scrbl

@@ -74,7 +74,9 @@ Divide @racket[_lst] into sublists that are homogeneously @racket[_pred] or not
 [pred procedure?]
 [force? boolean? #f])
 (listof list?)]
-Divide @racket[_lst] into sublists starting with elements matching @racket[_pred]. The first element of the first sublist may not match @racket[_pred]. Or, if you really & truly want only the sublists starting with an element matching @racket[_pred], set @racket[_force?] to @racket[#t].
+Divide @racket[_lst] into sublists, each starting with an element matching @racket[_pred]. The first element of the first sublist may not match @racket[_pred]. But if you really & truly want only the sublists starting with an element matching @racket[_pred], set @racket[_force?] to @racket[#true].
+
+If none of the elements match @racket[_pred], there is no slice to be made, and the result is the whole input list.
 
 @examples[#:eval my-eval
 (slicef-at (range 5) even?)
@@ -86,9 +88,12 @@ Divide @racket[_lst] into sublists starting with elements matching @racket[_pred
 @defproc[
 (slicef-after
 [lst list?]
-[pred procedure?])
+[pred procedure?]
+[force? boolean? #f]))
 (listof list?)]
-Divide @racket[_lst] into sublists ending with elements matching @racket[_pred]. (Distinct from @racket[slicef-at], which gives you sublists that @italic{start} with elements matching @racket[_pred].) If none of the elements match @racket[_pred], there is no slice to be made, and the result is the whole input list.
+Divide @racket[_lst] into sublists, each ending with an element matching @racket[_pred]. The last element of the last sublist may not match @racket[_pred]. But if you really & truly want only the sublists ending with an element matching @racket[_pred], set @racket[_force?] to @racket[#true]. 
+
+If none of the elements match @racket[_pred], there is no slice to be made, and the result is the whole input list.
 
 @examples[#:eval my-eval
 (slicef-after '(1 2 2 1 2) even?)

sugar/test/main.rkt

@@ -198,6 +198,14 @@
  
  (check-equal? (slicef-at '(1 2 2 1 2) even?) '((1) (2) (2 1) (2)))
  (check-equal? (slicef-at '(1 2 2 1 2) even? #t) '((2) (2 1) (2)))
+
+ (check-equal? (slicef-after (range 5) even?) '((0) (1 2) (3 4)))
+ (check-equal? (slicef-after (range 5) odd?) '((0 1) (2 3) (4)))
+ (check-equal? (slicef-after (range 5) odd? #t) '((0 1) (2 3)))
+ (check-equal? (slicef-after (range 5) procedure?) '((0 1 2 3 4)))
+ 
+ (check-equal? (slicef-after '(2 1 2 2 1) even?) '((2) (1 2) (2) (1)))
+ (check-equal? (slicef-after '(2 1 2 2 1) even? #t) '((2) (1 2) (2)))
  
  (check-equal? (sublist (range 5) 0 0) '())
  (check-equal? (sublist (range 5) 0 1) '(0))