Insert space for additional async exercises

exercises/109_files.zig

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+//
+// Until now, we've only been printing our output in the console,
+// which is good enough for fighting aliens and hermit bookkeeping.
+//
+// However, many other tasks require some interaction with the file system,
+// which is the underlying structure for organizing files on your computer.
+//
+// The file system provides a hierarchical structure for storing files
+// by organizing them into directories, which hold files and other directories,
+// thus creating a tree structure that can be navigated.
+//
+// Fortunately, the Zig Standard Library provides a simple API for interacting
+// with the file system, see the detail documentation here:
+//
+// https://ziglang.org/documentation/master/std/#std.Io
+//
+// In this exercise, we'll try to:
+//   - create a new directory,
+//   - open a file in the directory,
+//   - write to the file.
+//
+// Note: For simplicity, we write byte-by-byte without buffering.
+// In real applications, you'd typically use a buffer for better
+// performance. We'll learn about buffered I/O in a later exercise.
+//
+const std = @import("std");
+
+pub fn main(init: std.process.Init) !void {
+    // default I/O implementation
+    const io = init.io;
+
+    // first we get the current working directory
+    const cwd: std.Io.Dir = std.Io.Dir.cwd();
+
+    // then we'll try to make a new directory /output/
+    // to store our output files.
+    cwd.createDir(io, "output", .default_dir) catch |e| switch (e) {
+        // there is a chance you might want to run this
+        // program more than once and the path might already
+        // have been created, so we'll have to handle this error
+        // by doing nothing
+        //
+        // we want to catch error.PathAlreadyExists and do nothing
+        ??? => {},
+        // if there's any other unexpected error we just propagate it through
+        else => return e,
+    };
+
+    // then we'll try to open our freshly created directory
+    // wait a minute...
+    // opening a directory might fail!
+    // what should we do here?
+    var output_dir: std.Io.Dir = try cwd.openDir(io, "output", .{});
+    defer output_dir.close(io);
+
+    // we try to open the file `zigling.txt`,
+    // and propagate any error up
+    const file: std.Io.File = try output_dir.createFile(io, "zigling.txt", .{});
+    // it is a good habit to close a file after you are done with it
+    // so that other programs can read it and prevent data corruption
+    // but here we are not yet done writing to the file
+    // if only there were a keyword in Zig that
+    // allowed you to "defer" code execution to the end of the scope...
+    file.close(io);
+
+    // you are not allowed to move these lines above the file closing line!
+    var file_writer = file.writer(io, &.{});
+    const writer = &file_writer.interface;
+
+    const byte_written = try writer.write("It's zigling time!");
+    std.debug.print("Successfully wrote {d} bytes.\n", .{byte_written});
+}
+// to check if you actually write to the file, you can either,
+// 1. open the file in your text editor, or
+// 2. print the content of the file in the console with one of these commands
+//    Linux/macOS:   >> cat ./output/zigling.txt
+//    Windows (CMD): >> type .\output\zigling.txt
+//
+//
+// More on Creating files
+//
+// notice in:
+// ... try output_dir.createFile(io, "zigling.txt", .{});
+//                                                  ^^^
+//                 we passed this anonymous struct to the function call
+//
+// this is the struct `CreateFlag` with default fields
+// {
+//      read: bool = false,
+//      truncate: bool = true,
+//      exclusive: bool = false,
+//      lock: Lock = .none,
+//      lock_nonblocking: bool = false,
+//      mode: Mode = default_mode
+// }
+//
+// Question:
+//   - what should you do if you want to also read the file after opening it?
+//   - go to the documentation of the struct `std.Io.Dir` here:
+//     https://ziglang.org/documentation/master/std/#std.Io.Dir
+//       - can you find a function for opening a file? how about deleting a file?
+//       - what kind of options can you use with those functions?