fix: file existence

book-src/01-04-file-exists.md

@@ -1,6 +1,7 @@
 # Check file existence
 
-In this example, we use `panic` to ensure `build.zig` should exist.
+In this example, `access` is utilized to verify file existence; however, for it to function correctly, one must specifically check for the `FileNotFound` error type.
+
 ```zig
 {{#include ../src/01-04.zig}}
 ```

src/01-04.zig

@@ -1,13 +1,15 @@
-//! Check file existence
+//! Test file/directory existence
 
 const std = @import("std");
 const fs = std.fs;
 
-pub fn main() void {
+pub fn main() !void {
     const filename = "build.zig";
-    fs.cwd().access(filename, .{}) catch {
-        std.debug.panic("{s} not exists", .{filename});
+    var found = true;
+    fs.cwd().access(filename, .{}) catch |e| switch (e) {
+        error.FileNotFound => found = false,
+        else => return e,
     };
 
-    std.debug.print("{s} exists", .{filename});
+    std.debug.assert(found);
 }