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

lib/dune

@@ -0,0 +1,20 @@
+(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

@@ -0,0 +1,17 @@
+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
+