Cookbook recipe to parse command-line arguments with stdlib only

Author: boechat107

data/cookbook/parse-command-line-arguments/00-stdlib.ml

@@ -0,0 +1,65 @@
+---
+packages: []
+---
+(* `Sys.argv` is the command-line arguments as an array of strings
+   ([standard library documentation](https://ocaml.org/manual/5.3/api/Sys.html#VALargv)).
+   Here we print each argument with its corresponding index in the array. *)
+let simplest_parser () =
+  Sys.argv
+  |> Array.to_list
+  |> List.iteri (Printf.printf "argument %d: %s\n")
+
+
+(* `Arg` is a module for parsing command-line arguments, and it is part of
+   OCaml's standard library
+   ([documentation](https://ocaml.org/manual/5.3/api/Arg.html)).
+   In this function we define the structure of the command-line arguments we
+   expect, the argument types types and their documentation. This is basically
+   the same function defined in the module's documentation.
+
+ *)
+let arg_module_parser () =
+  let usage_msg =
+    "mycmd [--verbose] <file1> [<file2>] ... -o <output>"
+  and verbose = ref false
+  and input_files = ref []
+  and output_file = ref "" in
+  (* This function is called once for each anonymous argument. *)
+  let anonymous_args_f filename =
+    input_files := filename :: !input_files
+  (* The spec list defines argument keywords, "setter" functions to handle the
+     values, and their corresponding documentation. *)
+  and spec_list =
+    [("--verbose", Arg.Set verbose, "Output debug information");
+     ("-o", Arg.Set_string output_file, "Set output file name")]
+  in
+  Arg.parse spec_list anonymous_args_f usage_msg;
+  Printf.printf "verbose: %b\n" !verbose;
+  Printf.printf "input files: %s\n"
+    (!input_files |> String.concat ", ");
+  Printf.printf "output file: %s\n" !output_file
+
+(* Given a command-line like `mycmd --verbose file1 -o /tmp/out`, we should
+   expect the following output:
+
+   ```
+   === Simplest parser ===
+   argument 0: mycmd
+   argument 1: --verbose
+   argument 2: file1
+   argument 3: -o
+   argument 4: /tmp/out
+
+   === Arg.parse ===
+   verbose: true
+   input files: file1
+   output file: /tmp/out
+   ``` *)
+let () =
+  print_endline "=== Simplest parser ===";
+  simplest_parser ();
+
+  Printf.printf "\n%!";
+
+  print_endline "=== Arg.parse ===";
+  arg_module_parser ()