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

.gitignore

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+_build
+dune.lock

bin/dune

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+(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))
+;;

dune-project

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+(lang dune 3.20)
+
+(name pass_strategy_synthesis)
+
+(generate_opam_files true)
+
+(source
+ (github username/reponame))
+
+(authors "Author Name <author@example.com>")
+
+(maintainers "Maintainer Name <maintainer@example.com>")
+
+(license LICENSE)
+
+(documentation https://url/to/documentation)
+
+(package
+ (name pass_strategy_synthesis)
+ (synopsis "A short synopsis")
+ (description "A longer description")
+ (depends
+  (ocaml
+   (= 4.14.2))
+  GT
+  (OCanren
+   (>= 0.3.0~))
+  (OCanren-ppx
+   (>= 0.3.0~)))
+ (tags
+  ("add topics" "to describe" your project)))
+
+; See the complete stanza docs at https://dune.readthedocs.io/en/stable/reference/dune-project/index.html

lib/dune

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+(env
+ (_
+  (flags
+   (:standard -warn-error +5))))
+
+(library
+ (name lib1)
+ (modules lib)
+ (flags (-rectypes))
+ (libraries OCanren OCanren.tester)
+ (inline_tests)
+ (wrapped false)
+ (preprocess
+  (pps
+   OCanren-ppx.ppx_repr
+   OCanren-ppx.ppx_fresh
+   OCanren-ppx.ppx_distrib
+   GT.ppx
+   GT.ppx_all
+   ppx_inline_test)))

lib/lib.ml

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+open OCanren
+
+module Lam = struct
+  [%%distrib
+  type nonrec ('varname, 'self) t =
+    | V of 'varname
+    | App of 'self * 'self
+    | Abs of 'varname * 'self
+  [@@deriving gt ~options:{ show; fmt; gmap }]
+
+  type ground = (GT.string, ground) t]
+
+  let varX = !!"x"
+  let varY = !!"y"
+  let varF = !!"f"
+end
+

pass_strategy_synthesis.opam

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+# This file is generated by dune, edit dune-project instead
+opam-version: "2.0"
+synopsis: "A short synopsis"
+description: "A longer description"
+maintainer: ["Maintainer Name <maintainer@example.com>"]
+authors: ["Author Name <author@example.com>"]
+license: "LICENSE"
+tags: ["add topics" "to describe" "your" "project"]
+homepage: "https://github.com/username/reponame"
+doc: "https://url/to/documentation"
+bug-reports: "https://github.com/username/reponame/issues"
+depends: [
+  "dune" {>= "3.20"}
+  "ocaml" {= "4.14.2"}
+  "GT"
+  "OCanren" {>= "0.3.0~"}
+  "OCanren-ppx" {>= "0.3.0~"}
+  "odoc" {with-doc}
+]
+build: [
+  ["dune" "subst"] {dev}
+  [
+    "dune"
+    "build"
+    "-p"
+    name
+    "-j"
+    jobs
+    "@install"
+    "@runtest" {with-test}
+    "@doc" {with-doc}
+  ]
+]
+dev-repo: "git+https://github.com/username/reponame.git"
+x-maintenance-intent: ["(latest)"]

test/dune

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+(test
+ (name test_pass_strategy_synthesis))