generate a proper --help/manpage thanks to cmdliner

Author: gasche

bin/mustache_cli.ml

@@ -43,6 +43,51 @@ let run json_filename template_filename =
 let run_command =
   let open Cmdliner in
   let doc = "renders Mustache template from JSON data files" in
+  let man = [
+    `S Manpage.s_description;
+    `P "$(tname) is a command-line tool coming with the $(i,ocaml-mustache) library,
+        an OCaml implementation of the Mustache template format.
+        $(tname) takes a data file and a template file as command-line parameters;
+        it renders the populated template on standard output.";
+
+    `P "Mustache is a simple and popular template format,
+        with library implementations in many programming languages.
+        It is named from its {{..}} delimiters.";
+
+    `I ("Mustache website",
+        "https://mustache.github.io/");
+    `I ("Mustache templates documentation",
+        "https://mustache.github.io/mustache.5.html");
+    `I ("ocaml-mustache website:",
+        "https://github.com/rgrinberg/ocaml-mustache");
+
+    `P "The $(i,ocaml-mustache) implementation is tested against
+        the Mustache specification testsuite.
+        All features are supported, except for lambdas and setting delimiter tags.";
+    `S Manpage.s_examples;
+    `Pre
+      {|
+\$ cat data.json
+{ "name": "OCaml",
+  "qualities": [{"name": "simple"}, {"name": "fun"}] }
+
+\$ cat hello.mustache
+Hello {{name}}!
+Mustache is:
+{{#qualities}}
+- {{name}}
+{{/qualities}}
+
+\$ $(tname) data.json hello.mustache
+Hello OCaml!
+Mustache is:
+- simple
+- fun
+|};
+    `S Manpage.s_bugs;
+    `P "Report bugs on https://github.com/rgrinberg/ocaml-mustache/issues";
+  ]
+  in
   let json_file =
     let doc = "data file in JSON format" in
     Arg.(required & pos 0 (some file) None & info [] ~docv:"DATA.json" ~doc)
@@ -52,7 +97,7 @@ let run_command =
     Arg.(required & pos 1 (some file) None & info [] ~docv:"TEMPLATE.mustache" ~doc)
   in
   Term.(const run $ json_file $ template_file),
-  Term.info "mustache" ~doc
+  Term.info "mustache" ~doc ~man
 
 
 let () =