examples/fs: show how to read files while scanning

examples/fs/main.ml

@@ -1,21 +1,32 @@
-(* Walks the directory tree rooted at the current directory,
-   displaying all directory names (skipping hidden directories and `_build`). *)
-
-open Eio.Std
+(* Walk the directory tree rooted at the current directory,
+   showing a summary for any .mli files. *)
 
 let ( / ) = Eio.Path.( / )
 
-let rec scan t =
+let is_doc_comment = String.starts_with ~prefix:"(** "
+
+(* Print the first line of [t]'s doc-comment, if any *)
+let scan_mli t f =
+  Eio.Path.with_lines t (fun lines ->
+      Seq.find is_doc_comment lines
+      |> Option.iter (fun line ->
+          let stop = String.index_from_opt line 4 '*' |> Option.value ~default:(String.length line) in
+          Format.fprintf f "%s: %s@." (Eio.Path.native_exn t) (String.sub line 4 (stop - 4))
+        )
+    )
+
+(* Walk the tree rooted at [t] and scan any .mli files found. *)
+let rec scan t f =
   match Eio.Path.kind ~follow:false t with
   | `Directory ->
-    traceln "Visiting %a" Eio.Path.pp t;
     Eio.Path.read_dir t |> List.iter (function
-      | "_build" -> ()
-      | item when String.starts_with ~prefix:"." item -> ()
-      | item -> scan (t / item)
-    )
+        | "_build" | "_opam" -> ()                (* Don't examine these directories *)
+        | item when String.starts_with ~prefix:"." item -> () (* Skip hidden items *)
+        | item -> scan (t / item) f
+      )
+  | `Regular_file when Filename.check_suffix (snd t) ".mli" -> scan_mli t f
   | _ -> ()
 
 let () =
   Eio_main.run @@ fun env ->
-  scan (Eio.Stdenv.cwd env)
+  scan (Eio.Stdenv.cwd env) Format.std_formatter