Merge pull request #181 from chrboesch/interfaces

Interfaces

build.zig

@@ -461,6 +461,10 @@ const exercises = [_]Exercise{
     //     .output = "ABCDEF",
     //     .@"async" = true,
     // },
+    .{
+        .main_file = "092_interfaces.zig",
+        .output = "Daily Insect Report:\nAnt is alive.\nBee visited 17 flowers.\nGrasshopper hopped 32 meters.",
+    },
     .{
         .main_file = "999_the_end.zig",
         .output = "\nThis is the end for now!\nWe hope you had fun and were able to learn a lot, so visit us again when the next exercises are available.",

exercises/092_interfaces.zig

@@ -0,0 +1,127 @@
+//
+// Remember our ant and bee simulator constructed with unions
+// back in exercises 55 and 56? There, we demonstrated that
+// unions allow us to treat different data types in a uniform
+// manner.
+//
+// One neat feature was using tagged unions to create a single
+// function to print a status for ants *or* bees by switching:
+//
+//   switch (insect) {
+//      .still_alive => ...      // (print ant stuff)
+//      .flowers_visited => ...  // (print bee stuff)
+//   }
+//
+// Well, that simulation was running just fine until a new insect
+// arrived in the virtual garden, a grasshopper!
+//
+// Doctor Zoraptera started to add grasshopper code to the
+// program, but then she backed away from her keyboard with an
+// angry hissing sound. She had realized that having code for
+// each insect in one place and code to print each insect in
+// another place was going to become unpleasant to maintain when
+// the simulation expanded to hundreds of different insects.
+//
+// Thankfully, Zig has another comptime feature we can use
+// to get out of this dilema called the 'inline else'.
+//
+// We can replace this redundant code:
+//
+//   switch (thing) {
+//       .a => |a| special(a),
+//       .b => |b| normal(b),
+//       .c => |c| normal(c),
+//       .d => |d| normal(d),
+//       .e => |e| normal(e),
+//       ...
+//   }
+//
+// With:
+//
+//   switch (thing) {
+//       .a => |a| special(a),
+//       inline else |t| => normal(t),
+//   }
+//
+// We can have special handling of some cases and then Zig
+// handles the rest of the matches for us.
+//
+// With this feature, you decide to make an Insect union with a
+// single uniform 'print()' function. All of the insects can
+// then be responsible for printing themselves. And Doctor
+// Zoraptera can calm down and stop gnawing on the furniture.
+//
+const std = @import("std");
+
+const Ant = struct {
+    still_alive: bool,
+
+    pub fn print(self: Ant) void {
+        std.debug.print("Ant is {s}.\n", .{if (self.still_alive) "alive" else "dead"});
+    }
+};
+
+const Bee = struct {
+    flowers_visited: u16,
+
+    pub fn print(self: Bee) void {
+        std.debug.print("Bee visited {} flowers.\n", .{self.flowers_visited});
+    }
+};
+
+// Here's the new grasshopper. Notice how we've also added print
+// methods to each insect.
+const Grasshopper = struct {
+    distance_hopped: u16,
+
+    pub fn print(self: Grasshopper) void {
+        std.debug.print("Grasshopper hopped {} meters.\n", .{self.distance_hopped});
+    }
+};
+
+const Insect = union(enum) {
+    ant: Ant,
+    bee: Bee,
+    grasshopper: Grasshopper,
+
+    // Thanks to 'inline else', we can think of this print() as
+    // being an interface method. Any member of this union with
+    // with a print() method can be treated uniformly by outside
+    // code without needing to know any other details. Cool!
+    pub fn print(self: Insect) void {
+        switch (self) {
+            inline else => |case| return case.print(),
+        }
+    }
+};
+
+pub fn main() !void {
+    var my_insects = [_]Insect{
+        Insect{ .ant = Ant{ .still_alive = true } },
+        Insect{ .bee = Bee{ .flowers_visited = 17 } },
+        Insect{ .grasshopper = Grasshopper{ .distance_hopped = 32 }, },
+    };
+
+    std.debug.print("Daily Insect Report:\n", .{});
+    for (my_insects) |insect| {
+        // Almost done! We want to print() each insect with a
+        // single method call here.
+        ???
+    }
+}
+
+// Our print() method in the Insect union above demonstrates
+// something very similar to the object-oriented concept of an
+// abstract data type. That is, the Insect type doesn't contain
+// the underlying data, and the print() function doesn't
+// actually do the printing.
+//
+// The point of an interface is to support generic programming:
+// the ability to treat different things as if they were the
+// same to cut down on clutter and conceptual complexity.
+//
+// The Daily Insect Report doesn't need to worry about *which*
+// insects are in the report - they all print the same way via
+// the interface!
+//
+// Doctor Zoraptera loves it.

patches/patches/092_interfaces.patch

@@ -0,0 +1,4 @@
+109c109
+<         ???
+---
+>         insect.print();