ziglings/exercises/065_builtins2.zig

//
// Zig has builtins for mathematical operations such as...
//
//      @sqrt        @sin           @cos
//      @exp         @log           @floor
//
// ...and lots of type casting operations such as...
//
//      @as          @errorFromInt  @floatFromInt
//      @ptrFromInt  @intFromPtr    @intFromEnum
//
// Spending part of a rainy day skimming through the complete
// list of builtins in the official Zig documentation wouldn't be
// a bad use of your time. There are some seriously cool features
// in there. Check out @call, @compileLog, @embedFile, and @src!
//
//                            ...
//
// For now, we're going to complete our examination of builtins
// by exploring just THREE of Zig's MANY introspection abilities:
//
// 1. @This() type
//
// Returns the innermost struct, enum, or union that a function
// call is inside.
//
// 2. @typeInfo(comptime T: type) @import("std").builtin.Type
//
// Returns information about any type in a data structure which
// will contain different information depending on which type
// you're examining.
//
// 3. @TypeOf(...) type
//
// Returns the type common to all input parameters (each of which
// may be any expression). The type is resolved using the same
// "peer type resolution" process the compiler itself uses when
// inferring types.
//
// (Notice how the two functions which return types start with
// uppercase letters? This is a standard naming practice in Zig.)
//
const print = @import("std").debug.print;

const Narcissus = struct {
    me: *Narcissus = undefined,
    myself: *Narcissus = undefined,
    echo: void = undefined, // Alas, poor Echo!

    fn fetchTheMostBeautifulType() type {
        return @This();
    }
};

pub fn main() void {
    var narcissus: Narcissus = Narcissus{};

    // Oops! We cannot leave the 'me' and 'myself' fields
    // undefined. Please set them here:
    narcissus.me = &narcissus;
    narcissus.myself = &narcissus;

    // This determines a "peer type" from three separate
    // references (they just happen to all be the same object).
    const Type1 = @TypeOf(narcissus, narcissus.me.*, narcissus.myself.*);

    // Oh dear, we seem to have done something wrong when calling
    // this function. We called it as a method, which would work
    // if it had a self parameter. But it doesn't. (See above.)
    //
    // The fix for this is very subtle, but it makes a big
    // difference!
    const Type2 = Narcissus.fetchTheMostBeautifulType();

    // Now we print a pithy statement about Narcissus.
    print("A {s} loves all {s}es. ", .{
        maximumNarcissism(Type1),
        maximumNarcissism(Type2),
    });

    //   His final words as he was looking in
    //   those waters he habitually watched
    //   were these:
    //       "Alas, my beloved boy, in vain!"
    //   The place gave every word back in reply.
    //   He cried:
    //            "Farewell."
    //   And Echo called:
    //                   "Farewell!"
    //
    //     --Ovid, The Metamorphoses
    //       translated by Ian Johnston

    print("He has room in his heart for:", .{});

    // A StructFields array
    const fields = @typeInfo(Narcissus).Struct.fields;

    // 'fields' is a slice of StructFields. Here's the declaration:
    //
    //     pub const StructField = struct {
    //         name: []const u8,
    //         type: type,
    //         default_value: anytype,
    //         is_comptime: bool,
    //         alignment: comptime_int,
    //     };
    //
    // Please complete these 'if' statements so that the field
    // name will not be printed if the field is of type 'void'
    // (which is a zero-bit type that takes up no space at all!):
    if (fields[0].type != void) {
        print(" {s}", .{@typeInfo(Narcissus).Struct.fields[0].name});
    }

    if (fields[1].type != void) {
        print(" {s}", .{@typeInfo(Narcissus).Struct.fields[1].name});
    }

    if (fields[2].type != void) {
        print(" {s}", .{@typeInfo(Narcissus).Struct.fields[2].name});
    }

    // Yuck, look at all that repeated code above! I don't know
    // about you, but it makes me itchy.
    //
    // Alas, we can't use a regular 'for' loop here because
    // 'fields' can only be evaluated at compile time.  It seems
    // like we're overdue to learn about this "comptime" stuff,
    // doesn't it? Don't worry, we'll get there.

    print(".\n", .{});
}

// NOTE: This exercise did not originally include the function below.
// But a change after Zig 0.10.0 added the source file name to the
// type. "Narcissus" became "065_builtins2.Narcissus".
//
// To fix this, I've added this function to strip the filename from
// the front of the type name in the dumbest way possible. (It returns
// a slice of the type name starting at character 14 (assuming
// single-byte characters).
//
// We'll be seeing @typeName again in Exercise 070. For now, you can
// see that it takes a Type and returns a u8 "string".
fn maximumNarcissism(myType: anytype) []const u8 {
    // Turn '065_builtins2.Narcissus' into 'Narcissus'
    return @typeName(myType)[14..];
}