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127 lines
3.9 KiB
Zig
127 lines
3.9 KiB
Zig
//
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// Remember our ant and bee simulator constructed with unions
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// back in exercises 55 and 56? There, we demonstrated that
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// unions allow us to treat different data types in a uniform
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// manner.
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//
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// One neat feature was using tagged unions to create a single
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// function to print a status for ants *or* bees by switching:
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//
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// switch (insect) {
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// .still_alive => ... // (print ant stuff)
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// .flowers_visited => ... // (print bee stuff)
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// }
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//
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// Well, that simulation was running just fine until a new insect
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// arrived in the virtual garden, a grasshopper!
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//
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// Doctor Zoraptera started to add grasshopper code to the
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// program, but then she backed away from her keyboard with an
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// angry hissing sound. She had realized that having code for
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// each insect in one place and code to print each insect in
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// another place was going to become unpleasant to maintain when
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// the simulation expanded to hundreds of different insects.
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//
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// Thankfully, Zig has another comptime feature we can use
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// to get out of this dilemma called the 'inline else'.
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//
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// We can replace this redundant code:
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//
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// switch (thing) {
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// .a => |a| special(a),
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// .b => |b| normal(b),
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// .c => |c| normal(c),
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// .d => |d| normal(d),
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// .e => |e| normal(e),
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// ...
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// }
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//
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// With:
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//
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// switch (thing) {
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// .a => |a| special(a),
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// inline else => |t| normal(t),
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// }
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//
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// We can have special handling of some cases and then Zig
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// handles the rest of the matches for us.
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//
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// With this feature, you decide to make an Insect union with a
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// single uniform 'print()' function. All of the insects can
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// then be responsible for printing themselves. And Doctor
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// Zoraptera can calm down and stop gnawing on the furniture.
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//
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const std = @import("std");
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const Ant = struct {
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still_alive: bool,
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pub fn print(self: Ant) void {
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std.debug.print("Ant is {s}.\n", .{if (self.still_alive) "alive" else "dead"});
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}
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};
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const Bee = struct {
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flowers_visited: u16,
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pub fn print(self: Bee) void {
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std.debug.print("Bee visited {} flowers.\n", .{self.flowers_visited});
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}
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};
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// Here's the new grasshopper. Notice how we've also added print
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// methods to each insect.
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const Grasshopper = struct {
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distance_hopped: u16,
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pub fn print(self: Grasshopper) void {
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std.debug.print("Grasshopper hopped {} meters.\n", .{self.distance_hopped});
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}
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};
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const Insect = union(enum) {
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ant: Ant,
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bee: Bee,
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grasshopper: Grasshopper,
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// Thanks to 'inline else', we can think of this print() as
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// being an interface method. Any member of this union with
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// a print() method can be treated uniformly by outside
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// code without needing to know any other details. Cool!
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pub fn print(self: Insect) void {
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switch (self) {
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inline else => |case| return case.print(),
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}
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}
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};
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pub fn main() !void {
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const my_insects = [_]Insect{
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Insect{ .ant = Ant{ .still_alive = true } },
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Insect{ .bee = Bee{ .flowers_visited = 17 } },
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Insect{ .grasshopper = Grasshopper{ .distance_hopped = 32 } },
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};
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std.debug.print("Daily Insect Report:\n", .{});
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for (my_insects) |insect| {
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// Almost done! We want to print() each insect with a
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// single method call here.
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???
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}
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}
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// Our print() method in the Insect union above demonstrates
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// something very similar to the object-oriented concept of an
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// abstract data type. That is, the Insect type doesn't contain
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// the underlying data, and the print() function doesn't
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// actually do the printing.
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//
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// The point of an interface is to support generic programming:
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// the ability to treat different things as if they were the
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// same to cut down on clutter and conceptual complexity.
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//
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// The Daily Insect Report doesn't need to worry about *which*
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// insects are in the report - they all print the same way via
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// the interface!
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//
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// Doctor Zoraptera loves it.
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