ExtLib OptParse (part 2)

I’ve already wrote something about OptParse last month. Today I discovered how to create a new option (that is not a string, int or bool) and validate it within the arg parser.

So suppose we want to write an application that can output both txt and html and we want the user to specify the format with command line option. One way would be to use a StdOpt.str_option - eventually with a default option - and to retrive it in application code with OptParse.Opt.get.

However this is not satisfactory as we are mixing the application code with command line parsing. A better way is to create a new type of option with Opt.value_option .

This is the concept :

type out_t = Txt | Html
module Options = struct
  open OptParse
  exception Format
  let out_option ?default ?(metavar = "<txt|html>") () =
    let corce = function
      |"txt" -> Txt
      |"html" -> Html
      | _ -> raise Format
    in
    let error _ s = Printf.sprintf "%s format not supported" s in
    Opt.value_option metavar default corce error

  let output = out_option ~default:Txt ()

  let description = "This is an example"
  let options = OptParser.make ~description:description ()

  open OptParser
  add options ~short_name:'o' ~long_name:"out" ~help:"Output type" output;
end

Note that the function Opt.value_option get a default value, a metavar - that is the sting associated with the option in the help ( -o<txt|html>, --out=<txt|html> Output type ), a corce function, that is, a function that transforms a string in the desired type, and an error function that is used by the parser to give a meaningful error is the option is not correctly validated.

For example :

$./test.native -oooo
usage: test.native [options]

test.native: option '-o': ooo format not supported

Now when we use this new option in the application code with OptParse.Opt.get and we can be certain that it was correctly validated.