Replace exercise 074_comptime9

The exercise no longer needed any modifications to pass due to
advancements in Zig.  This new exercise attempts to teach about
@compileError, @compileLog, and some comptime debugging.  It tries to
help prepare users for the "super bonus challenge" in 075_quiz8.

1 files changed, 99 insertions, 50 deletions

diff --git a/exercises/074_comptime9.zig b/exercises/074_comptime9.zig
index 4088aad..50894fd 100644
--- a/exercises/074_comptime9.zig
+++ b/exercises/074_comptime9.zig
@@ -1,62 +1,111 @@
+const print = @import("std").debug.print;
+
+// We're going to (ab)use the power of Zig to make animal hybrid creatures!
+// What do you think a GatorMouse would look like?  Eek.
 //
-// In addition to knowing when to use the 'comptime' keyword,
-// it's also good to know when you DON'T need it.
-//
-// The following contexts are already IMPLICITLY evaluated at
-// compile time, and adding the 'comptime' keyword would be
-// superfluous, redundant, and smelly:
-//
-//    * The container-level scope (outside of any function in a source file)
-//    * Type declarations of:
-//        * Variables
-//        * Functions (types of parameters and return values)
-//        * Structs
-//        * Unions
-//        * Enums
-//    * The test expressions in inline for and while loops
-//    * An expression passed to the @cImport() builtin
-//
-// Work with Zig for a while, and you'll start to develop an
-// intuition for these contexts. Let's work on that now.
-//
-// You have been given just one 'comptime' statement to use in
-// the program below. Here it is:
-//
-//     comptime
-//
-// Just one is all it takes. Use it wisely!
+// Let's try a MouseLlama instead.
 //
-const print = @import("std").debug.print;
+// We'll make a function that runs at comptime and takes a short code describing
+// the desired creature. A Mouse is represented by "m" and a Llama is "lm".
+// A MouseLlama hybrid, then, would be represented by "mlm".
+
+const Animal = enum {
+    Mouse,
+    Llama,
+    Gator,
+};
+
+// makeCreature takes the count of animals making up the hybrid creature (so we
+// know how big a pen we'll need) and a format string, like the "mlm" for
+// MouseLlama.
+fn makeCreature(comptime count: usize, comptime fmt: []const u8) [count]Animal {
+
+    // Since not every animal is represented by a single character, we need to
+    // track the state of things as we move along. For example, if we see an
+    // "m", is that a new Mouse or the end of a Llama?
+    const State = enum {
+        start, // Ready to start a new animal.
+        l, // This means we've seen an "l", so if we see an "m", we know it's a Llama.
+    };
+    var state = State.start;
 
-// Being in the container-level scope, everything about this value is
-// implicitly required to be known compile time.
-const llama_count = 5;
+    // We return an array of animals representing the creature. (This is why we
+    // really needed the 'count' parameter. Arrays need a size.)
+    var animals: [count]Animal = .{undefined} ** count;
+    var next_animal: usize = 0;
 
-// Again, this value's type and size must be known at compile
-// time, but we're letting the compiler infer both from the
-// return type of a function.
-const llamas = makeLlamas(llama_count);
+    inline for (fmt) |char| {
 
-// And here's the function. Note that the return value type
-// depends on one of the input arguments!
-fn makeLlamas(count: usize) [count]u8 {
-    var temp: [count]u8 = undefined;
-    var i = 0;
+        // This is a good spot to add a @compileLog() call if you need to debug
+        // any variables... (Come back here after you see main().)
 
-    // Note that this does NOT need to be an inline 'while'.
-    while (i < count) : (i += 1) {
-        temp[i] = i;
+        switch (state) {
+            .start => switch (char) {
+                // We've seen the start of a Llama.
+                'l' => state = .l,
+
+                // Mice are smaller.  An "m" is a full Mouse.
+                'm' => {
+                    animals[next_animal] = .Mouse;
+                    next_animal += 1;
+                },
+
+                // @compileError lets us stop the build immediately if something
+                // is wrong. It's like @compileLog but it prints a message
+                // instead of inspecting values.
+                //
+                // What do you think happens with Gators? Do they join with
+                // other animals or is this an error?
+                'g' => ???,
+
+                else => @compileError("No animal starts with '" ++ char ++ "'!"),
+            },
+
+            .l => switch (char) {
+                // We've seen the end of a Llama.
+                'm' => {
+                    animals[next_animal] = .Llama;
+                    next_animal += 1;
+                    // Something is missing here. After we finish a Llama, we
+                    // need to be ready to _start_ over with a new animal...
+                    ???
+                },
+
+                else => @compileError("Only llamas start with 'l'!"),
+            },
+        }
     }
 
-    return temp;
+    if (state != .start) {
+        @compileError("Oh no, an incomplete llama!");
+    }
+    if (next_animal != count) {
+        @compileError("Creature is missing an animal (format string too short).");
+    }
+
+    return animals;
 }
 
 pub fn main() void {
-    print("My llama value is {}.\n", .{llamas[2]});
+    // Once you've fixed the ??? marks above, this makeCreature call will still
+    // only succeed if you move it outside of main, so it will run at comptime.
+    //
+    // With the call here, Zig will try to make the creature at runtime, and
+    // you'll get an interesting error.
+    //
+    // You may think the state got mixed up, but if you use @compileLog to check
+    // some variables in makeCreature, you'll see that Zig is trying to compare
+    // comptime values with "[runtime value]", which will never match.
+    //
+    // You can solve this by adding "comptime" to two of the variables in
+    // makeCreature...
+    const creature = makeCreature(2, "mlm");
+
+    for (creature) |animal| {
+        // @tagName gives us a string representing which variant of an enum we
+        // have. This lets us print the names of animals without repeating them
+        // here.
+        print("{s}", .{@tagName(animal)});
+    }
+    print(" joins the crew!", .{});
 }
-//
-// The lesson here is to not pepper your program with 'comptime'
-// keywords unless you need them. Between the implicit compile
-// time contexts and Zig's aggressive evaluation of any
-// expression it can figure out at compile time, it's sometimes
-// surprising how few places actually need the keyword.