diff --git a/source/coral/utf8.zig b/source/coral/utf8.zig
index 78697d8..28e3ad7 100644
--- a/source/coral/utf8.zig
+++ b/source/coral/utf8.zig
@@ -1,3 +1,5 @@
+const debug = @import("./debug.zig");
+
 const io = @import("./io.zig");
 
 const math = @import("./math.zig");
@@ -6,6 +8,11 @@ pub const DecimalFormat = struct {
 	delimiter: []const io.Byte,
 	positive_prefix: enum {none, plus, space},
 
+	const default = DecimalFormat{
+		.delimiter = "",
+		.positive_prefix = .none,
+	};
+
 	pub fn parse(self: DecimalFormat, utf8: []const io.Byte, comptime Decimal: type) ?Decimal {
 		if (utf8.len == 0) {
 			return null;
@@ -104,5 +111,170 @@ pub const DecimalFormat = struct {
 			else => @compileError("`" ++ @typeName(Decimal) ++ "` cannot be formatted as a decimal string"),
 		}
 	}
+
+	pub fn print(self: DecimalFormat, writer: io.Writer, value: anytype) PrintError!void {
+		if (value == 0) {
+			return print_string(writer, switch (self.positive_prefix) {
+				.none => "0",
+				.plus => "+0",
+				.space => " 0",
+			});
+		}
+
+		const ValueType = @TypeOf(value);
+
+		switch (@typeInfo(ValueType)) {
+			.Int => |int| {
+				const radix = 10;
+				var buffer = [_]u8{0} ** (1 + math.max(int.bits, 1));
+				var buffer_start = buffer.len - 1;
+
+				{
+					var decomposable_value = value;
+
+					while (decomposable_value != 0) : (buffer_start -= 1) {
+						buffer[buffer_start] = @intCast((decomposable_value % radix) + '0');
+						decomposable_value = (decomposable_value / radix);
+					}
+				}
+
+				if (int.signedness == .unsigned and value < 0) {
+					buffer[buffer_start] = '-';
+				} else {
+					switch (self.positive_prefix) {
+						.none => buffer_start += 1,
+						.plus => buffer[buffer_start] = '+',
+						.space => buffer[buffer_start] = ' ',
+					}
+				}
+
+				try print_string(writer, buffer[buffer_start ..]);
+			},
+
+			else => unformattableMessage(ValueType),
+		}
+	}
 };
 
+pub const HexadecimalFormat = struct {
+	delimiter: []const u8 = "",
+	positive_prefix: enum {none, plus, space} = .none,
+	casing: enum {lower, upper} = .lower,
+
+	const default = HexadecimalFormat{
+		.delimiter = "",
+		.positive_prefix = .none,
+		.casing = .lower,
+	};
+
+	pub fn print(self: HexadecimalFormat, writer: io.Writer, value: anytype) PrintError!void {
+		// TODO: Implement.
+		_ = self;
+		_ = writer;
+		_ = value;
+
+		unreachable;
+	}
+};
+
+pub const PrintError = error {
+	PrintFailed,
+	PrintIncomplete,
+};
+
+pub fn print_string(writer: io.Writer, utf8: []const io.Byte) PrintError!void {
+	if ((writer.invoke(utf8) orelse return error.PrintFailed) != utf8.len) {
+		return error.PrintIncomplete;
+	}
+}
+
+pub fn print_formatted(writer: io.Writer, comptime format: []const u8, arguments: anytype) PrintError!void {
+	switch (@typeInfo(@TypeOf(arguments))) {
+		.Struct => |arguments_struct| {
+			comptime var arg_index = 0;
+			comptime var head = 0;
+			comptime var tail = 0;
+
+			inline while (tail < format.len) : (tail += 1) {
+				if (format[tail] == '{') {
+					if (tail > format.len) {
+						@compileError("expected an idenifier after opening `{`");
+					}
+
+					tail += 1;
+
+					switch (format[tail]) {
+						'{' => {
+							try print_string(writer, format[head .. (tail - 1)]);
+
+							tail += 1;
+							head = tail;
+						},
+
+						'}' => {
+							if (!arguments_struct.is_tuple) {
+								@compileError("all format specifiers must be named when using a named struct");
+							}
+
+							try print_string(writer, arguments[arg_index]);
+
+							arg_index += 1;
+							tail += 1;
+							head = tail;
+						},
+
+						else => {
+							if (arguments_struct.is_tuple) {
+								@compileError("format specifiers cannot be named when using a tuple struct");
+							}
+
+							try print_string(writer, format[head .. (tail - 1)]);
+
+							head = tail;
+							tail += 1;
+
+							if (tail >= format.len) {
+								@compileError("expected closing `}` or another `{` after opening `{`");
+							}
+
+							debug.assert(tail < format.len);
+
+							inline while (format[tail] != '}') {
+								tail += 1;
+
+								debug.assert(tail < format.len);
+							}
+
+							try print_value(writer, @field(arguments, format[head .. tail]));
+
+							tail += 1;
+							head = tail;
+						}
+					}
+				}
+			}
+		},
+
+		else => @compileError("`arguments` must be a struct type"),
+	}
+}
+
+noinline fn print_value(writer: io.Writer, value: anytype) PrintError!void {
+	const Value = @TypeOf(value);
+
+	return switch (@typeInfo(Value)) {
+		.Int => DecimalFormat.default.print(writer, value),
+		.Float => DecimalFormat.default.print(writer, value),
+
+		.Pointer => |pointer| switch (pointer.size) {
+			.One, .Many, .C => HexadecimalFormat.default.print(writer, @intFromPtr(value)),
+			.Slice => if (pointer.child == u8) print_string(writer, value) else @compileError(unformattableMessage(Value)),
+		},
+
+		else => @compileError(unformattableMessage(Value)),
+	};
+}
+
+fn unformattableMessage(comptime Value: type) []const u8 {
+	return "type `" ++ @typeName(Value) ++ "` is not formattable with this formatter";
+}
diff --git a/source/ona/kym.zig b/source/ona/kym.zig
index 353fb6a..997a895 100644
--- a/source/ona/kym.zig
+++ b/source/ona/kym.zig
@@ -456,9 +456,12 @@ pub const RuntimeEnv = struct {
 	}
 
 	pub fn raise(self: *RuntimeEnv, runtime_error: RuntimeError, error_message: []const u8) RuntimeError {
-		if (self.err_writer.invoke(error_message) == null) {
-			return error.SystemFailure;
-		}
+		// TODO: Print stack trace from state.
+		coral.utf8.print_formatted(self.err_writer, "{name}@{line}: {message}", .{
+			.name = "???",
+			.line = @as(u64, 0),
+			.message = error_message,
+		}) catch return error.SystemFailure;
 
 		return runtime_error;
 	}