How to pass flags to dune?

I am trying to write a compiler for a simple C inspired language. My dune file looks like this

(ocamllex scanner)     
(ocamlyacc mylanguageparse) 
(executable
 (name mylanguage)         
 (libraries
  llvm
  llvm.analysis
 )
 (link_flags (-linkall))
)

I want to be able to pass in a “-a” as an argument to give the option of just printing my ast, so in top level compiler mylanguage.ml file I do

type action = Ast 

let () =
  let action = ref Compile in
  let set_action a () = action := a in
  let speclist = [
    ("-a", Arg.Unit (set_action Ast), "Print the AST"); ] 
  in  
  let usage_msg = "usage: ./mylanguage.native [-a|-s|-l|-c] [file.mylanguage]" in
  let channel = ref stdin in
  Arg.parse speclist (fun filename -> channel := open_in filename) usage_msg;
  
  let lexbuf = Lexing.from_channel !channel in
  let ast = mylanguage.program Scanner.token lexbuf in  
  match !action with
    Ast -> print_string (Ast.string_of_program ast)
  | _ -> let sast = Semant.check ast in
  Llvm_analysis.assert_valid_module m;
  print_string (Llvm.string_of_llmodule m)

Following the format in dune documentation I’ve tried variations of this

dune exec -a ./mylanguage.exe ../tests/test-hello.mylanguage
dune: unknown option `-a'.

How do I pass in my “-a” as an option? I’ve tried just a, --a, no difference

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