diff --git a/source/coral/files.cpp b/source/coral/files.cpp
index 2231794..d9ba825 100644
--- a/source/coral/files.cpp
+++ b/source/coral/files.cpp
@@ -106,18 +106,31 @@ export namespace coral {
 		char buffer[max + 1];
 	};
 
+	struct file_reader : public reader {
+		virtual expected<u64, io_error> seek(u64 offset) = 0;
+
+		virtual expected<u64, io_error> tell() = 0;
+	};
+
+	struct file_writer : public writer {};
+
 	/**
 	 * Platform-generalized file system interface.
 	 */
 	struct fs {
+		struct io_rules {
+			bool can_read;
+
+			bool can_write;
+		};
+
 		/**
 		 * Attempts to read the file in the file system located at `file_path` relative, calling
 		 * `then` if it was successfully opened for reading.
 		 *
 		 * Once `then` returns, access to the file is closed automatically.
 		 */
-		virtual void read_file(path const & file_path,
-			callable<void(readable const &)> const & then) = 0;
+		virtual void read_file(path const & file_path, callable<void(file_reader &)> const & then) = 0;
 
 		/**
 		 * Attempts to write the file in the file system located at `file_path` relative, calling
@@ -125,7 +138,6 @@ export namespace coral {
 		 *
 		 * Once `then` returns, access to the file is closed automatically.
 		 */
-		virtual void write_file(path const & file_path,
-			callable<void(writable const &)> const & then) = 0;
+		virtual void write_file(path const & file_path, callable<void(file_writer &)> const & then) = 0;
 	};
 }
diff --git a/source/coral/sequence.cpp b/source/coral/sequence.cpp
index 24258cc..b568637 100644
--- a/source/coral/sequence.cpp
+++ b/source/coral/sequence.cpp
@@ -200,14 +200,20 @@ export namespace coral {
 	/**
 	 * Writable type for appending data to a [sequence] containing [u8] values.
 	 */
-	struct sequence_writer : public writable {
-		sequence_writer(sequence<u8> * output_sequence) : writable{
-			[output_sequence](slice<u8 const> const & buffer) -> expected<usize, io_error> {
-				switch (output_sequence->append(buffer)) {
-					case append_result::ok: return buffer.length;
-					case append_result::out_of_memory: return io_error::unavailable;
-					default: unreachable();
-				}
-			}} {}
+	struct sequence_writer : public writer {
+		sequence_writer(sequence<u8> * output_sequence) {
+			this->output_sequence = output_sequence;
+		}
+
+		expected<usize, io_error> write(slice<u8 const> const & buffer) {
+			switch (output_sequence->append(buffer)) {
+				case append_result::ok: return buffer.length;
+				case append_result::out_of_memory: return io_error::unavailable;
+				default: unreachable();
+			}
+		}
+
+		private:
+		sequence<u8> * output_sequence;
 	};
 }