feat: Support basic expression

bin/main.ml

@@ -1 +1,19 @@
-let () = print_endline "Hello, World!"
+(* bin/main.ml *)
+
+open Printf
+open Ast
+
+let () =
+  let lexbuf = Lexing.from_channel stdin in
+  let res =
+    try Parser.main Lexer.token lexbuf with
+    | Lexer.Error c ->
+        fprintf stderr "Lexical error at line %d: Unknown character '%c'\n"
+          lexbuf.lex_curr_p.pos_lnum c;
+        exit 1
+    | Parser.Error ->
+        fprintf stderr "Parse error at line %d:\n" lexbuf.lex_curr_p.pos_lnum;
+        exit 1
+  in
+  let _ = res in
+  Printf.printf "%s\n" (string_of_source_file res)