type expr = Foo [@@deriving show { with_path = false }]

let expr_of_parsetree : Parsetree.expression -> expr = function _ -> Foo

let parsetree_of_expr : expr -> Parsetree.expression =
  let expression (pexp_desc : Parsetree.expression_desc) : Parsetree.expression
      =
    {
      pexp_desc;
      pexp_loc = Location.none;
      pexp_loc_stack = [];
      pexp_attributes = [];
    }
  in
  function Foo -> expression (Pexp_constant (Pconst_char '!'))