type and type string option

I have this little Ocaml code that should print the value of an 
environment variable specified as an anonymous argument. If no argument

is given, the environment variable content to print is EDITOR.


open Core.Std

let spec =

let open Command.Spec in

    empty

    +> anon (maybe ("var_name" %: string))

let print_value variable ()=

   let getenv_value_of varname =

     try

       Sys.getenv varname

     with Not_found -> varname ^ " not defined"

   in match variable with

   | None -> print_endline (getenv_value_of "EDITOR")

   | Some name -> print_endline (getenv_value_of name)


let command = Command.basic

    ~summary:"Print the value of an environment variable"

    ~readme:(fun () -> "Print the value of an environment variable you 
provide or the EDITOR variable it you didn't provide any")

    spec

    print_value


let () = Command.run ~version:"1.0" ~build_info:"RWO" command


If I try to compile it with corebuild -quiet print_en.native


I have this error:


+ ocamlfind ocamlc -c -w A-4-33-40-41-42-43-34-44 -strict-sequence -g 
-bin-annot -short-paths -thread -package core -ppx 'ppx-jane -as-ppx' -o 
MrHide_env_variables.cmo MrHide_env_variables.ml
File "MrHide_env_variables.ml", line 13, characters 19-43:
Error: This expression has type string but an expression was expected of 
type
          string option
Command exited with code 2.
Hint: Recursive traversal of subdirectories was not enabled for this build,
   as the working directory does not look like an ocamlbuild project (no
   '_tags' or 'myocamlbuild.ml' file). If you have modules in 
subdirectories,
   you should add the option "-r" or create an empty '_tags' file.

   To enable recursive traversal for some subdirectories only, you can 
use the
   following '_tags' file:

       true: -traverse
       <dir1> or <dir2>: traverse

I can not see how to solve this.


cedlemo