docs/feat: add stdin support via - filename

bin/main.ml

@@ -14,7 +14,22 @@ let read_file filename =
   close_in ic;
   Bytes.unsafe_to_string s
 
+let read_stdin () =
+  let buf = Buffer.create 4096 in
+  (try
+     while true do
+       Buffer.add_channel buf stdin 4096
+     done
+   with End_of_file -> ());
+  Buffer.contents buf
+
 let ical2rem (args : CommandLine.cli_args) ical_files =
+  (* Validate: '-' (stdin) may appear at most once *)
+  let stdin_count = List.length (List.filter (( = ) "-") ical_files) in
+  if stdin_count > 1 then begin
+    Printf.eprintf "Error: '-' (stdin) may appear at most once.\n";
+    exit 1
+  end;
   (* Validate --source with multiple files *)
   (match args.source with
   | Some _ when List.length ical_files > 1 ->
@@ -31,11 +46,17 @@ let ical2rem (args : CommandLine.cli_args) ical_files =
   let good_rems =
     ListLabels.fold_left ~init:[] ical_files ~f:(fun good_rems_acc filename ->
         try
-          let file_content = read_file filename in
-          let basename =
-            match args.source with
-            | Some name -> name
-            | None -> Filename.remove_extension (Filename.basename filename)
+          let file_content, basename =
+            if filename = "-" then
+              ( read_stdin (),
+                match args.source with
+                | Some name -> name
+                | None -> "stdin" )
+            else
+              ( read_file filename,
+                match args.source with
+                | Some name -> name
+                | None -> Filename.remove_extension (Filename.basename filename) )
           in
           match Icalendar.parse file_content with
           | Error e ->