mk: enumerate-i / enumerate-from-i — 501/501 milestone

(enumerate-i l result): result is l with each element paired with its
0-based index. (enumerate-from-i n l result): same but starts at n.

  (enumerate-i (list :a :b :c) q) -> (((0 :a) (1 :b) (2 :c)))

5 new tests, 501/501 cumulative.

lib/minikanren/intarith.sx

@@ -117,3 +117,13 @@
     (conde
       ((nullo l) (== n 0))
       ((fresh (a d n-1) (conso a d l) (lengtho-i d n-1) (pluso-i 1 n-1 n))))))
+
+(define
+  enumerate-from-i
+  (fn
+    (start l result)
+    (conde
+      ((nullo l) (nullo result))
+      ((fresh (a d r-rest start-prime) (conso a d l) (conso (list start a) r-rest result) (pluso-i 1 start start-prime) (enumerate-from-i start-prime d r-rest))))))
+
+(define enumerate-i (fn (l result) (enumerate-from-i 0 l result)))

lib/minikanren/tests/enumerate.sx

@@ -0,0 +1,31 @@
+;; lib/minikanren/tests/enumerate.sx — index-each-element relation.
+
+(mk-test
+  "enumerate-i-empty"
+  (run* q (enumerate-i (list) q))
+  (list (list)))
+
+(mk-test
+  "enumerate-i-three"
+  (run* q (enumerate-i (list :a :b :c) q))
+  (list
+    (list (list 0 :a) (list 1 :b) (list 2 :c))))
+
+(mk-test
+  "enumerate-i-strings"
+  (run* q (enumerate-i (list "x" "y" "z") q))
+  (list
+    (list (list 0 "x") (list 1 "y") (list 2 "z"))))
+
+(mk-test
+  "enumerate-from-i-100"
+  (run* q (enumerate-from-i 100 (list :x :y :z) q))
+  (list
+    (list (list 100 :x) (list 101 :y) (list 102 :z))))
+
+(mk-test
+  "enumerate-from-i-singleton"
+  (run* q (enumerate-from-i 0 (list :only) q))
+  (list (list (list 0 :only))))
+
+(mk-tests-run!)