diff --git a/.gitignore b/.gitignore
index e35d885..83659d7 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1 +1,2 @@
 _build
+inputs/
diff --git a/bin/dune b/bin/dune
index 6ed8299..2548b78 100644
--- a/bin/dune
+++ b/bin/dune
@@ -1,2 +1,3 @@
 (executable
- (name inspector))
+ (name main)
+ (libraries inspector))
diff --git a/bin/inspector.ml b/bin/inspector.ml
deleted file mode 100644
index f11b17e..0000000
--- a/bin/inspector.ml
+++ /dev/null
@@ -1,40 +0,0 @@
-module State = struct
-  type t = { lines : string array; cursor : int; size : int }
-
-  let create (fname : string) : t =
-    let lines =
-      In_channel.with_open_text fname In_channel.input_lines |> Array.of_list
-    in
-    { lines; cursor = 0; size = Array.length lines }
-
-  let show_line s : string = s.lines.(s.cursor)
-
-  let rec repl s =
-    print_string "inspector> ";
-    flush stdout;
-    try
-      match read_line () |> String.lowercase_ascii with
-      | "exit" | "quit" -> print_endline "bye"
-      | "next" ->
-          if s.cursor < s.size - 1 then repl { s with cursor = s.cursor + 1 }
-          else repl s
-      | "prev" ->
-          if s.cursor > 0 then repl { s with cursor = s.cursor - 1 } else repl s
-      | "show" ->
-          Printf.printf "[%d]: %s\n" s.cursor (show_line s);
-          repl s
-      | str ->
-          print_endline str;
-          repl s
-    with End_of_file -> print_endline "bye"
-end
-
-let () =
-  if Array.length Sys.argv < 2 then (
-    Printf.eprintf "Usage: %s <logfile>\n" (Filename.basename Sys.argv.(0));
-    exit 1);
-
-  let s = State.create Sys.argv.(1) in
-  Printf.printf "Loaded %d lines\n" s.size;
-
-  State.repl s
diff --git a/bin/main.ml b/bin/main.ml
new file mode 100644
index 0000000..a6c3791
--- /dev/null
+++ b/bin/main.ml
@@ -0,0 +1,10 @@
+let () =
+  let open Inspector in
+  if Array.length Sys.argv < 2 then (
+    Printf.eprintf "Usage: %s <logfile>\n" (Filename.basename Sys.argv.(0));
+    exit 1);
+
+  let s = State.create Sys.argv.(1) in
+  Printf.printf "Loaded %d lines\n" (State.size s);
+
+  Repl.loop s
diff --git a/lib/dune b/lib/dune
new file mode 100644
index 0000000..7c8a4d0
--- /dev/null
+++ b/lib/dune
@@ -0,0 +1,2 @@
+(library
+ (name inspector))
diff --git a/lib/repl.ml b/lib/repl.ml
new file mode 100644
index 0000000..597ee68
--- /dev/null
+++ b/lib/repl.ml
@@ -0,0 +1,15 @@
+let rec loop state =
+  print_string "inspector> ";
+  flush stdout;
+  try
+    match read_line () |> String.lowercase_ascii with
+    | "exit" | "quit" -> print_endline "bye"
+    | "next" -> loop (State.next state)
+    | "prev" -> loop (State.prev state)
+    | "show" ->
+        Printf.printf "[%d]: %s\n" (State.cursor state) (State.show_line state);
+        loop state
+    | str ->
+        print_endline str;
+        loop state
+  with End_of_file -> print_endline "bye"
diff --git a/lib/repl.mli b/lib/repl.mli
new file mode 100644
index 0000000..cc32f76
--- /dev/null
+++ b/lib/repl.mli
@@ -0,0 +1,21 @@
+(** Interactive command loop for the inspector.
+
+    The REPL (Read–Eval–Print Loop) repeatedly reads commands from standard
+    input and applies them to the current navigation state.
+
+    Each command may update the state (for example moving the cursor) or display
+    information about the current log line.
+
+    The loop continues until the user enters ["quit"] or ["exit"], or until an
+    end-of-file condition is encountered on standard input (for example when
+    pressing Ctrl-D). *)
+
+val loop : State.t -> unit
+(** [loop state] starts the interactive command loop.
+
+    The given [state] represents the current navigation state of the loaded log
+    file. Commands executed by the user may derive new states (for example via
+    {!State.next} or {!State.prev}) which are then used in the next iteration of
+    the loop.
+
+    The function runs until the user exits the program. *)
diff --git a/lib/state.ml b/lib/state.ml
new file mode 100644
index 0000000..649cde3
--- /dev/null
+++ b/lib/state.ml
@@ -0,0 +1,18 @@
+type t = { lines : string array; cursor : int }
+(** Invariant: cursor >= 0 cursor < Array.length lines *)
+
+let size s = Array.length s.lines
+let cursor s = s.cursor
+
+let create (fname : string) : t =
+  let lines =
+    In_channel.with_open_text fname In_channel.input_lines |> Array.of_list
+  in
+  { lines; cursor = 0 }
+
+let show_line s : string = s.lines.(s.cursor)
+
+let next s =
+  if s.cursor < size s - 1 then { s with cursor = s.cursor + 1 } else s
+
+let prev s = if s.cursor > 0 then { s with cursor = s.cursor - 1 } else s
diff --git a/lib/state.mli b/lib/state.mli
new file mode 100644
index 0000000..5e9f96a
--- /dev/null
+++ b/lib/state.mli
@@ -0,0 +1,24 @@
+type t
+(** Log navigation state.
+
+    A [t] represents a loaded log file and a cursor pointing to the currently
+    active line. *)
+
+val create : string -> t
+(** [create file] loads the log file into memory and initializes the cursor at
+    the first line. *)
+
+val show_line : t -> string
+(** [show_line s] returns the line currently pointed to by the cursor. *)
+
+val next : t -> t
+(** [next s] moves the cursor to the next line if possible. *)
+
+val prev : t -> t
+(** [prev s] moves the cursor to the previous line if possible. *)
+
+val cursor : t -> int
+(** Current cursor position. *)
+
+val size : t -> int
+(** Number of lines in the log. *)