Make the helper a bit simpler to read and close handles properly

Author: Leonidas-from-XIV

test/bin/mdx-dune-gen/misc/non-deterministic/different.ml

@@ -1,38 +1,42 @@
 (* small helper to determine whether two files differ *)
 
+type comparison = Same | Different
+
+let rec compare first second =
+  match input_line first with
+  | first_line -> (
+      match input_line second with
+      | second_line ->
+          match String.equal first_line second_line with
+          | true -> compare first second
+          | false ->
+              (* we found a difference between the lines *)
+              Different
+      | exception End_of_file ->
+          (* the second file ended before the first *)
+          Different)
+  | exception End_of_file ->
+      (* the first file ended first *)
+      match input_line second with
+      | _ ->
+          (* the second file continues: a difference *)
+          Different
+      | exception End_of_file ->
+          (* the second file ended too *)
+          Same
+
 let main () =
   let first = Sys.argv.(1) |> open_in in
   let second = Sys.argv.(2) |> open_in in
-  let rec loop () =
-    match input_line first with
-    | first_line -> (
-        match input_line second with
-        | second_line ->
-            match String.equal first_line second_line with
-            | true -> loop ()
-            | false ->
-                (* we found a difference between the lines *)
-                exit 0
-        | exception End_of_file ->
-            (* the second file ended before the first *)
-            exit 0
-    )
-    | exception End_of_file ->
-        (* the first file ended first *)
-        match input_line second with
-        | _ ->
-            (* the second file continues: a difference *)
-            exit 1
-        | exception End_of_file ->
-            (* the second file ended too *)
-            ()
-  in
-  loop ();
+  let comparison = compare first second in
   close_in first;
   close_in second;
-  (* we didn't find a difference, exit with a failure code *)
-  prerr_endline "The files appear to be identical";
-  exit 1
+  match comparison with
+  | Same ->
+    prerr_endline "The files appear to be identical";
+    (* we didn't find a difference, exit with a failure code *)
+    exit 1
+  | Different -> ()
 
 let () =
   main ()