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