145 lines
4.2 KiB
Zig
145 lines
4.2 KiB
Zig
//
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// Being able to pass types to functions at compile time lets us
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// generate code that works with multiple types. But it doesn't
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// help us pass VALUES of different types to a function.
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//
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// For that, we have the 'anytype' placeholder, which tells Zig
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// to infer the actual type of a parameter at compile time.
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//
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// fn foo(thing: anytype) void { ... }
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//
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// Then we can use builtins such as @TypeOf(), @typeInfo(),
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// @typeName(), @hasDecl(), and @hasField() to determine more
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// about the type that has been passed in. All of this logic will
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// be performed entirely at compile time.
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//
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const print = @import("std").debug.print;
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// Let's define three structs: Duck, RubberDuck, and Duct. Notice
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// that Duck and RubberDuck both contain waddle() and quack()
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// methods declared in their namespace (also known as "decls").
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const Duck = struct {
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eggs: u8,
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loudness: u8,
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location_x: i32 = 0,
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location_y: i32 = 0,
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fn waddle(self: *Duck, x: i16, y: i16) void {
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self.location_x += x;
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self.location_y += y;
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}
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fn quack(self: Duck) void {
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if (self.loudness < 4) {
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print("\"Quack.\" ", .{});
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} else {
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print("\"QUACK!\" ", .{});
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}
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}
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};
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const RubberDuck = struct {
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in_bath: bool = false,
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location_x: i32 = 0,
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location_y: i32 = 0,
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fn waddle(self: *RubberDuck, x: i16, y: i16) void {
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self.location_x += x;
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self.location_y += y;
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}
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fn quack(self: RubberDuck) void {
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// Assigning an expression to '_' allows us to safely
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// "use" the value while also ignoring it.
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_ = self;
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print("\"Squeek!\" ", .{});
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}
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fn listen(self: RubberDuck, dev_talk: []const u8) void {
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// Listen to developer talk about programming problem.
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// Silently contemplate problem. Emit helpful sound.
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_ = dev_talk;
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self.quack();
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}
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};
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const Duct = struct {
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diameter: u32,
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length: u32,
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galvanized: bool,
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connection: ?*Duct = null,
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fn connect(self: *Duct, other: *Duct) !void {
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if (self.diameter == other.diameter) {
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self.connection = other;
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} else {
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return DuctError.UnmatchedDiameters;
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}
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}
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};
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const DuctError = error{UnmatchedDiameters};
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pub fn main() void {
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// This is a real duck!
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var ducky1 = Duck{
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.eggs = 0,
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.loudness = 3,
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};
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// This is not a real duck, but it has quack() and waddle()
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// abilities, so it's still a "duck".
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var ducky2 = RubberDuck{
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.in_bath = false,
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};
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// This is not even remotely a duck.
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var ducky3 = Duct{
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.diameter = 17,
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.length = 165,
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.galvanized = true,
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};
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print("ducky1: {}, ", .{isADuck(ducky1)});
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print("ducky2: {}, ", .{isADuck(ducky2)});
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print("ducky3: {}\n", .{isADuck(ducky3)});
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}
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// This function has a single parameter which is inferred at
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// compile time. It uses builtins @TypeOf() and @hasDecl() to
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// perform duck typing ("if it walks like a duck and it quacks
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// like a duck, then it must be a duck") to determine if the type
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// is a "duck".
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fn isADuck(possible_duck: anytype) bool {
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// We'll use @hasDecl() to determine if the type has
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// everything needed to be a "duck".
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//
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// In this example, 'has_increment' will be true if type Foo
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// has an increment() method:
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//
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// const has_increment = @hasDecl(Foo, "increment");
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//
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// Please make sure MyType has both waddle() and quack()
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// methods:
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const MyType = @TypeOf(possible_duck);
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const walks_like_duck = ???;
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const quacks_like_duck = ???;
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const is_duck = walks_like_duck and quacks_like_duck;
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if (is_duck) {
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// We also call the quack() method here to prove that Zig
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// allows us to perform duck actions on anything
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// sufficiently duck-like.
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//
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// Because all of the checking and inference is performed
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// at compile time, we still have complete type safety:
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// attempting to call the quack() method on a struct that
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// doesn't have it (like Duct) would result in a compile
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// error, not a runtime panic or crash!
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possible_duck.quack();
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}
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return is_duck;
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}
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