Add ex 50 'no values' (help further address #25)

build.zig

@@ -260,7 +260,10 @@ const exercises = [_]Exercise{
         .output = "A  B  C  Cv Bv Av",
         .hint = "Now you're writting Zig!",
     },
-    // 50 null vs undefined
+    .{
+        .main_file = "50_no_value.zig",
+        .output = "That is not dead which can eternal lie / And with strange aeons even death may die.",
+    },
     // 51 pass-by-value and const fn params
     // 52 slices!
 };

exercises/50_no_value.zig

@@ -0,0 +1,84 @@
+//
+//    "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 = ???;
+    first_line1 = "That is not dead";
+
+    var first_line2: Err!*const [21]u8 = ???;
+    first_line2 = "which can eternal lie";
+
+    std.debug.print("{s} {s} / ", .{ first_line1, first_line2 });
+
+    printSecondLine();
+}
+
+fn printSecondLine() ??? {
+    var second_line2: ?*const [18]u8 = ???;
+    second_line2 = "even death may die";
+
+    std.debug.print("And with strange aeons {s}.\n", .{second_line2.?});
+}

patches/patches/50_no_value.patch

@@ -0,0 +1,14 @@
+68c68
+<     var first_line1: *const [16]u8 = ???;
+---
+>     var first_line1: *const [16]u8 = undefined;
+71c71
+<     var first_line2: Err!*const [21]u8 = ???;
+---
+>     var first_line2: Err!*const [21]u8 = Err.Cthulhu;
+79,80c79,80
+< fn printSecondLine() ??? {
+<     var second_line2: ?*const [18]u8 = ???;
+---
+> fn printSecondLine() void {
+>     var second_line2: ?*const [18]u8 = null;