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64 lines
2.2 KiB
Zig
64 lines
2.2 KiB
Zig
//
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// We've seen that Zig implicitly performs some evaluations at
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// compile time. But sometimes you'll want to explicitly request
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// compile time evaluation. For that, we have a new keyword:
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//
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// . . . o . . * . . .
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// . * | . . . . . . * . .
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// --o-- comptime * | .. .
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// * | * . . . . --*-- . * .
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// . . . . . . . . . | . . .
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//
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// When placed before a variable declaration, 'comptime'
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// guarantees that every usage of that variable will be performed
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// at compile time.
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//
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// As a simple example, compare these two statements:
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//
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// var bar1 = 5; // ERROR!
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// comptime var bar2 = 5; // OKAY!
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//
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// The first one gives us an error because Zig assumes mutable
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// identifiers (declared with 'var') will be used at runtime and
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// we have not assigned a runtime type (like u8 or f32). Trying
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// to use a comptime_int of undetermined size at runtime is
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// a MEMORY CRIME and you are UNDER ARREST.
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//
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// The second one is okay because we've told Zig that 'bar2' is
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// a compile time variable. Zig will help us ensure this is true
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// and let us know if we make a mistake.
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//
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const print = @import("std").debug.print;
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pub fn main() void {
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//
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// In this contrived example, we've decided to allocate some
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// arrays using a variable count! But something's missing...
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//
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comptime var count = 0;
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count += 1;
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const a1: [count]u8 = .{'A'} ** count;
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count += 1;
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const a2: [count]u8 = .{'B'} ** count;
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count += 1;
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const a3: [count]u8 = .{'C'} ** count;
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count += 1;
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const a4: [count]u8 = .{'D'} ** count;
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print("{s} {s} {s} {s}\n", .{ a1, a2, a3, a4 });
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// Builtin BONUS!
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//
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// The @compileLog() builtin is like a print statement that
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// ONLY operates at compile time. The Zig compiler treats
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// @compileLog() calls as errors, so you'll want to use them
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// temporarily to debug compile time logic.
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//
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// Try uncommenting this line and playing around with it
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// (copy it, move it) to see what it does:
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//@compileLog("Count at compile time: ", count);
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}
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