diff --git a/source/app.cpp b/source/app.cpp
index 865eb20..5097886 100644
--- a/source/app.cpp
+++ b/source/app.cpp
@@ -13,10 +13,23 @@ import oar;
 
 namespace app {
 	struct directory : public coral::fs {
+		struct rules {
+			bool can_read;
+
+			bool can_write;
+		};
+
+		virtual rules access_rules() const = 0;
+
 		virtual coral::slice<char const> native_path() const = 0;
 	};
 
-	struct file_reader : public coral::file_reader {
+	struct file : public coral::file_reader, public coral::file_writer {
+		enum class open_mode {
+			read_only,
+			overwrite,
+		};
+
 		enum class [[nodiscard]] close_result {
 			ok,
 			io_unavailable,
@@ -29,7 +42,7 @@ namespace app {
 			not_found,
 		};
 
-		file_reader(directory * file_directory) : rw_ops{nullptr} {
+		file(directory * file_directory) : rw_ops{nullptr} {
 			this->file_directory = file_directory;
 		}
 
@@ -45,7 +58,7 @@ namespace app {
 			return this->rw_ops != nullptr;
 		}
 
-		open_result open(coral::path const & file_path) {
+		open_result open(coral::path const & file_path, open_mode mode) {
 			if (this->is_open()) switch (this->close()) {
 				case close_result::ok: break;
 				case close_result::io_unavailable: return open_result::io_unavailable;
@@ -67,21 +80,43 @@ namespace app {
 			// No room for zero terminator.
 			if (path_buffer.is_full()) return open_result::not_found;
 
-			this->rw_ops = ::SDL_RWFromFile(reinterpret_cast<char const *>(path_buffer.begin()), "r");
+			switch (mode) {
+				case open_mode::read_only: {
+					if (!this->file_directory->access_rules().can_read)
+						return open_result::access_denied;
+
+					this->rw_ops = ::SDL_RWFromFile(
+						reinterpret_cast<char const *>(path_buffer.begin()), "rb");
+
+					break;
+				}
+
+				case open_mode::overwrite: {
+					if (!this->file_directory->access_rules().can_write)
+						return open_result::access_denied;
+
+					this->rw_ops = ::SDL_RWFromFile(
+						reinterpret_cast<char const *>(path_buffer.begin()), "wb");
+
+					break;
+				}
+
+				default: coral::unreachable();
+			}
 
 			if (this->rw_ops == nullptr) return open_result::not_found;
 
 			return open_result::ok;
 		}
 
-		coral::expected<coral::usize, coral::io_error> read(coral::slice<coral::u8> const & buffer) override {
+		coral::expected<coral::usize, coral::io_error> read(coral::slice<coral::u8> const & data) override {
 			if (!this->is_open()) return coral::io_error::unavailable;
 
-			coral::usize const bytes_read{::SDL_RWread(this->rw_ops, buffer.pointer, sizeof(uint8_t), buffer.length)};
+			coral::usize const data_read{::SDL_RWread(this->rw_ops, data.pointer, sizeof(uint8_t), data.length)};
 
-			if ((bytes_read == 0) && (::SDL_GetError() != nullptr)) return coral::io_error::unavailable;
+			if ((data_read == 0) && (::SDL_GetError() != nullptr)) return coral::io_error::unavailable;
 
-			return bytes_read;
+			return data_read;
 		}
 
 		coral::expected<coral::u64, coral::io_error> seek(coral::u64 offset) override {
@@ -106,6 +141,16 @@ namespace app {
 			return static_cast<coral::u64>(byte_position);
 		}
 
+		coral::expected<coral::usize, coral::io_error> write(coral::slice<coral::u8 const> const & data) override {
+			if (!this->is_open()) return coral::io_error::unavailable;
+
+			coral::usize const data_written{::SDL_RWwrite(this->rw_ops, data.pointer, sizeof(uint8_t), data.length)};
+
+			if ((data_written == 0) && (::SDL_GetError() != nullptr)) return coral::io_error::unavailable;
+
+			return data_written;
+		}
+
 		private:
 		directory * file_directory;
 
@@ -136,6 +181,13 @@ struct base_directory : public app::directory {
 		::SDL_free(this->directory_path.begin());
 	}
 
+	rules access_rules() const override {
+		return {
+			.can_read = true,
+			.can_write = false,
+		};
+	}
+
 	coral::slice<const char> native_path() const override {
 		return this->directory_path;
 	}
@@ -143,13 +195,14 @@ struct base_directory : public app::directory {
 	void read_file(coral::path const & file_path, coral::callable<void(coral::file_reader &)> const & then) override {
 		if (this->directory_path.length == 0) return;
 
-		app::file_reader reader{this};
+		app::file file{this};
 
-		if (reader.open(file_path) != app::file_reader::open_result::ok) return;
+		if (file.open(file_path, app::file::open_mode::read_only) != app::file::open_result::ok)
+			return;
 
-		then(reader);
+		then(file);
 
-		if (reader.close() != app::file_reader::close_result::ok) return;
+		if (file.close() != app::file::close_result::ok) return;
 	}
 
 	void write_file(coral::path const & file_path, coral::callable<void(coral::file_writer &)> const & then) override {
@@ -183,6 +236,13 @@ struct user_directory : public app::directory {
 		::SDL_free(this->directory_path.begin());
 	}
 
+	rules access_rules() const override {
+		return {
+			.can_read = true,
+			.can_write = false,
+		};
+	}
+
 	coral::slice<const char> native_path() const override {
 		return this->directory_path;
 	}
@@ -190,17 +250,27 @@ struct user_directory : public app::directory {
 	void read_file(coral::path const & file_path, coral::callable<void(coral::file_reader &)> const & then) override {
 		if (this->directory_path.length == 0) return;
 
-		app::file_reader reader{this};
+		app::file file{this};
 
-		if (reader.open(file_path) != app::file_reader::open_result::ok) return;
+		if (file.open(file_path, app::file::open_mode::read_only) != app::file::open_result::ok)
+			return;
 
-		then(reader);
+		then(file);
 
-		if (reader.close() != app::file_reader::close_result::ok) return;
+		if (file.close() != app::file::close_result::ok) return;
 	}
 
 	void write_file(coral::path const & file_path, coral::callable<void(coral::file_writer &)> const & then) override {
-		// Directory is read-only.
+		if (this->directory_path.length == 0) return;
+
+		app::file file{this};
+
+		if (file.open(file_path, app::file::open_mode::overwrite) != app::file::open_result::ok)
+			return;
+
+		then(file);
+
+		if (file.close() != app::file::close_result::ok) return;
 	}
 
 	protected: