mk: defrel — Prolog-style relation-definition macro

(defrel (NAME ARGS...) (CLAUSE1 ...) (CLAUSE2 ...) ...) expands to
(define NAME (fn (ARGS...) (conde (CLAUSE1 ...) (CLAUSE2 ...) ...))).

Mirrors Prolog's `name(Args) :- goals.` shape. Inherits the Zzz-on-each-
clause laziness from conde, so user relations defined via defrel
terminate on partial answers without needing manual delay. Tests
redefine membero / listo / pluso through defrel and verify equivalence.

3 new tests, 347/347 cumulative.

lib/minikanren/defrel.sx

@@ -0,0 +1,25 @@
+;; lib/minikanren/defrel.sx — Prolog-style defrel macro.
+;;
+;;   (defrel (NAME ARG1 ARG2 ...)
+;;     (CLAUSE1 ...)
+;;     (CLAUSE2 ...)
+;;     ...)
+;;
+;; expands to
+;;
+;;   (define NAME (fn (ARG1 ARG2 ...) (conde (CLAUSE1 ...) (CLAUSE2 ...))))
+;;
+;; This puts each clause's goals immediately after the head, mirroring
+;; Prolog's `name(Args) :- goals.` shape. Clauses are conde-conjoined
+;; goals — `Zzz`-wrapping is automatic via `conde`, so recursive
+;; relations terminate on partial answers.
+
+(defmacro
+  defrel
+  (head &rest clauses)
+  (let
+    ((name (first head)) (args (rest head)))
+    (list
+      (quote define)
+      name
+      (list (quote fn) args (cons (quote conde) clauses)))))

lib/minikanren/tests/defrel.sx

@@ -0,0 +1,40 @@
+;; lib/minikanren/tests/defrel.sx — Prolog-style relation definition macro.
+
+(defrel
+  (my-membero x l)
+  ((fresh (d) (conso x d l)))
+  ((fresh (a d) (conso a d l) (my-membero x d))))
+
+(mk-test
+  "defrel-defines-membero"
+  (run* q (my-membero q (list 1 2 3)))
+  (list 1 2 3))
+
+(defrel
+  (my-listo l)
+  ((nullo l))
+  ((fresh (a d) (conso a d l) (my-listo d))))
+
+(mk-test
+  "defrel-listo-bounded"
+  (run 3 q (my-listo q))
+  (list
+    (list)
+    (list (make-symbol "_.0"))
+    (list (make-symbol "_.0") (make-symbol "_.1"))))
+
+;; Multi-arg relation with arithmetic.
+
+(defrel
+  (my-pluso a b c)
+  ((== a :z) (== b c))
+  ((fresh (a-1 c-1) (== a (list :s a-1)) (== c (list :s c-1)) (my-pluso a-1 b c-1))))
+
+(mk-test
+  "defrel-pluso-2-3"
+  (run*
+    q
+    (my-pluso (list :s (list :s :z)) (list :s (list :s (list :s :z))) q))
+  (list (list :s (list :s (list :s (list :s (list :s :z)))))))
+
+(mk-tests-run!)

plans/minikanren-on-sx.md

@@ -173,6 +173,11 @@ _(none yet)_
 
 _Newest first._
 
+- **2026-05-08** — **defrel — Prolog-style relation definition macro**:
+  `(defrel (NAME ARGS...) (CLAUSE1 ...) (CLAUSE2 ...) ...)` expands to
+  `(define NAME (fn (ARGS...) (conde (CLAUSE1 ...) (CLAUSE2 ...) ...)))`.
+  Mirrors Prolog's clause syntax and inherits Zzz-via-conde so recursive
+  relations terminate. 3 new tests, 347/347 cumulative.
 - **2026-05-08** — **lasto / init-o**: classic head/tail-style list relations.
   lasto extracts the final element; init-o extracts everything-but-the-last.
   Together with appendo, the round-trip `init ⊕ (last) = l` holds. 8 new