start with template code https://github.com/Kakadu/OCanren-basic-template

bin/dune

@@ -0,0 +1,13 @@
+(env
+ (_
+  (flags
+   (:standard -warn-error +5))))
+
+(executable
+ (public_name main)
+ (modules main)
+ (flags
+  (:standard -rectypes))
+ (libraries lib1)
+ (preprocess
+  (pps OCanren-ppx.ppx_repr OCanren-ppx.ppx_fresh GT.ppx GT.ppx_all)))

bin/main.ml

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+(* let () = print_endline "Hello, World!" *)
+open OCanren
+open Lib
+open Lam
+
+let rec substo l x a l' =
+  let open Lam in
+  conde
+    [ fresh y (l === v y) (y === x) (l' === a)
+    ; fresh
+        (m n m' n')
+        (l === app m n)
+        (l' === app m' n')
+        (substo m x a m')
+        (substo n x a n')
+    ; fresh
+        (v b)
+        (l === abs v b)
+        (conde [ x === v &&& (l' === l); fresh b' (l' === abs v b') (substo b x a b') ])
+    ]
+;;
+
+let rec evalo m n =
+  conde
+    [ fresh x (m === v x) (n === m)
+    ; fresh (x l) (m === abs x l) (n === m)
+    ; fresh
+        (f a f' a')
+        (m === app f a)
+        (evalo f f')
+        (evalo a a')
+        (conde
+           [ fresh (x l l') (f' === abs x l) (substo l x a' l') (evalo l' n)
+           ; fresh (p q) (f' === app p q) (n === app f' a')
+           ; fresh x (f' === v x) (n === app f' a')
+           ])
+    ]
+;;
+
+let a_la_quine q r s = ?&[ evalo (app q r) s; evalo (app r s) q; evalo (app s q) r ]
+
+open Tester
+
+let runL n = run_r Lam.reify (GT.show Lam.logic) n
+let run_exn eta = run_r Lam.prj_exn eta
+
+let _ =
+  run_exn (GT.show Lam.ground) 1 q qh (REPR (fun q -> substo (v varX) varX (v varY) q));
+  run_exn (GT.show Lam.ground) 1 q qh (REPR (fun q -> evalo (abs varX (v varX)) q));
+  run_exn (GT.show Lam.ground) 2 q qh (REPR (fun q -> evalo (abs varX (v varX)) q));
+  run_exn
+    (GT.show Lam.ground)
+    1
+    q
+    qh
+    (REPR (fun q -> evalo (app (abs varX (v varX)) (v varY)) q));
+  run_exn
+    (GT.show Lam.ground)
+    1
+    q
+    qh
+    (REPR (fun q -> evalo (app (abs varX (v varX)) q) (v varY)));
+  run_exn
+    (GT.show Lam.ground)
+    1
+    q
+    qh
+    (REPR (fun q -> evalo (app (abs varX q) (v varY)) (v varY)));
+  run_exn (GT.show Lam.ground) 1 q qh (REPR (fun q -> evalo (app (v varX) (v varX)) q));
+  run_exn (GT.show Lam.ground) 1 q qh (REPR (fun q -> evalo (v varX) q))
+;;
+
+let _withFree =
+  runL 1 q qh (REPR (fun q -> evalo (app q (v varX)) (v varX)));
+  runL 1 qr qrh (REPR (fun q r -> evalo (app r q) (v varX)));
+  runL 2 qrs qrsh (REPR (fun q r s -> a_la_quine q r s))
+;;