mk: take-while-o + drop-while-o — predicate-driven prefix/suffix

(take-while-o pred l result): take elements from l while pred holds,
stopping at the first element that fails. (drop-while-o pred l result):
drop matching elements, return the rest including the first non-match.

Together: (take-while p l) ⊕ (drop-while p l) = l, verified by an
end-to-end roundtrip test.

8 new tests, 546/546 cumulative.

lib/minikanren/relations.sx

@@ -89,6 +89,22 @@
       ((fresh (a d r-rest) (conso a d l) (conde ((membero a d) (nub-o d result)) ((nafc (membero a d)) (conso a r-rest result) (nub-o d r-rest))))))))
 
 
+(define
+  take-while-o
+  (fn
+    (pred l result)
+    (conde
+      ((nullo l) (nullo result))
+      ((fresh (a d r-rest) (conso a d l) (conde ((pred a) (conso a r-rest result) (take-while-o pred d r-rest)) ((nafc (pred a)) (== result (list)))))))))
+
+(define
+  drop-while-o
+  (fn
+    (pred l result)
+    (conde
+      ((nullo l) (nullo result))
+      ((fresh (a d) (conso a d l) (conde ((pred a) (drop-while-o pred d result)) ((nafc (pred a)) (== result l))))))))
+
 (define
   membero
   (fn

lib/minikanren/tests/take-while-drop-while.sx

@@ -0,0 +1,80 @@
+;; lib/minikanren/tests/take-while-drop-while.sx — prefix/suffix by predicate.
+
+(mk-test
+  "take-while-o-empty"
+  (run* q (take-while-o (fn (x) (== x 1)) (list) q))
+  (list (list)))
+
+(mk-test
+  "take-while-o-while-lt-5"
+  (run*
+    q
+    (take-while-o
+      (fn (x) (lto-i x 5))
+      (list 1 3 7 2 9)
+      q))
+  (list (list 1 3)))
+
+(mk-test
+  "take-while-o-all-match"
+  (run*
+    q
+    (take-while-o
+      (fn (x) (lto-i x 100))
+      (list 1 2 3)
+      q))
+  (list (list 1 2 3)))
+
+(mk-test
+  "take-while-o-none-match"
+  (run*
+    q
+    (take-while-o
+      (fn (x) (lto-i 100 x))
+      (list 1 2 3)
+      q))
+  (list (list)))
+
+(mk-test
+  "drop-while-o-empty"
+  (run* q (drop-while-o (fn (x) (== x 1)) (list) q))
+  (list (list)))
+
+(mk-test
+  "drop-while-o-while-lt-5"
+  (run*
+    q
+    (drop-while-o
+      (fn (x) (lto-i x 5))
+      (list 1 3 7 2 9)
+      q))
+  (list (list 7 2 9)))
+
+(mk-test
+  "drop-while-o-all-match"
+  (run*
+    q
+    (drop-while-o
+      (fn (x) (lto-i x 100))
+      (list 1 2 3)
+      q))
+  (list (list)))
+
+(mk-test
+  "take-drop-roundtrip"
+  (run*
+    q
+    (fresh
+      (p s)
+      (take-while-o
+        (fn (x) (lto-i x 5))
+        (list 1 3 7 2 9)
+        p)
+      (drop-while-o
+        (fn (x) (lto-i x 5))
+        (list 1 3 7 2 9)
+        s)
+      (appendo p s q)))
+  (list (list 1 3 7 2 9)))
+
+(mk-tests-run!)