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86 lines
3.0 KiB
Zig
86 lines
3.0 KiB
Zig
//
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// "We live on a placid island of ignorance in the midst
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// of black seas of infinity, and it was not meant that
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// we should voyage far."
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//
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// from The Call of Cthulhu
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// by H. P. Lovecraft
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//
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// Zig has at least four ways of expressing "no value":
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//
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// * undefined
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//
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// var foo: u8 = undefined;
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//
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// "undefined" should not be thought of as a value, but as a way
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// of telling the compiler that you are not assigning a value
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// _yet_. Any type may be set to undefined, but attempting
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// to read or use that value is _always_ a mistake.
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//
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// * null
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//
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// var foo: ?u8 = null;
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//
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// The "null" primitive value _is_ a value that means "no value".
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// This is typically used with optional types as with the ?u8
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// shown above. When foo equals null, that's not a value of type
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// u8. It means there is _no value_ of type u8 in foo at all!
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//
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// * error
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//
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// var foo: MyError!u8 = BadError;
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//
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// Errors are _very_ similar to nulls. They _are_ a value, but
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// they usually indicate that the "real value" you were looking
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// for does not exist. Instead, you have an error. The example
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// error union type of MyError!u8 means that foo either holds
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// a u8 value OR an error. There is _no value_ of type u8 in foo
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// when it's set to an error!
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//
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// * void
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//
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// var foo: void = {};
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//
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// "void" is a _type_, not a value. It is the most popular of the
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// Zero Bit Types (those types which take up absolutely no space
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// and have only a semantic value. When compiled to executable
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// code, zero bit types generate no code at all. The above example
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// shows a variable foo of type void which is assigned the value
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// of an empty expression. It's much more common to see void as
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// the return type of a function that returns nothing.
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//
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// Zig has all of these ways of expressing different types of "no value"
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// because they each serve a purpose. Briefly:
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//
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// * undefined - there is no value YET, this cannot be read YET
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// * null - there is an explicit value of "no value"
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// * errors - there is no value because something went wrong
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// * void - there will NEVER be a value stored here
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//
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// Please use the correct "no value" for each ??? to make this program
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// print out a cursed quote from the Necronomicon. ...If you dare.
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//
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const std = @import("std");
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const Err = error{Cthulhu};
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pub fn main() void {
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var first_line1: *const [16]u8 = ???;
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first_line1 = "That is not dead";
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var first_line2: Err!*const [21]u8 = ???;
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first_line2 = "which can eternal lie";
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// Note we need the "{!s}" format for the error union string.
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std.debug.print("{s} {!s} / ", .{ first_line1, first_line2 });
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printSecondLine();
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}
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fn printSecondLine() ??? {
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var second_line2: ?*const [18]u8 = ???;
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second_line2 = "even death may die";
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std.debug.print("And with strange aeons {s}.\n", .{second_line2.?});
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}
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