050_no_value.zig

//
//    "We live on a placid island of ignorance in the midst
//     of black seas of infinity, and it was not meant that
//     we should voyage far."
//
//     from The Call of Cthulhu
//       by H. P. Lovecraft
//
// Zig has at least four ways of expressing "no value":
//
// * undefined
//
//       var foo: u8 = undefined;
//
//       "undefined" should not be thought of as a value, but as a way
//       of telling the compiler that you are not assigning a value
//       _yet_. Any type may be set to undefined, but attempting
//       to read or use that value is _always_ a mistake.
//
// * null
//
//       var foo: ?u8 = null;
//
//       The "null" primitive value _is_ a value that means "no value".
//       This is typically used with optional types as with the ?u8
//       shown above. When foo equals null, that's not a value of type
//       u8. It means there is _no value_ of type u8 in foo at all!
//
// * error
//
//       var foo: MyError!u8 = BadError;
//
//       Errors are _very_ similar to nulls. They _are_ a value, but
//       they usually indicate that the "real value" you were looking
//       for does not exist. Instead, you have an error. The example
//       error union type of MyError!u8 means that foo either holds
//       a u8 value OR an error. There is _no value_ of type u8 in foo
//       when it's set to an error!
//
// * void
//
//       var foo: void = {};
//
//       "void" is a _type_, not a value. It is the most popular of the
//       Zero Bit Types (those types which take up absolutely no space
//       and have only a semantic value. When compiled to executable
//       code, zero bit types generate no code at all. The above example
//       shows a variable foo of type void which is assigned the value
//       of an empty expression. It's much more common to see void as
//       the return type of a function that returns nothing.
//
// Zig has all of these ways of expressing different types of "no value"
// because they each serve a purpose. Briefly:
//
//   * undefined - there is no value YET, this cannot be read YET
//   * null      - there is an explicit value of "no value"
//   * errors    - there is no value because something went wrong
//   * void      - there will NEVER be a value stored here
//
// Please use the correct "no value" for each ??? to make this program
// print out a cursed quote from the Necronomicon. ...If you dare.
//
const std = @import("std");

const Err = error{Cthulhu};

pub fn main() void {
    var first_line1: *const [16]u8 = undefined;
    first_line1 = "That is not dead";

    var first_line2: Err!*const [21]u8 = Err.Cthulhu;
    first_line2 = "which can eternal lie";

    std.debug.print("{s} {s} / ", .{ first_line1, first_line2 });

    printSecondLine();
}

fn printSecondLine() void {
    var second_line2: ?*const [18]u8 = null;
    second_line2 = "even death may die";

    std.debug.print("And with strange aeons {s}.\n", .{second_line2.?});
}