Clarify blocks being statements in doco

[zenspider]

As decided here: #9758 (comment)

example of expression/statement disparity:

const lol: u8 = if (args.len == 2)
    3
  else v: {
    std.debug.print("logging info", .{...});
    break :v 5;
};

[leecannon]

It is not true that unlabelled blocks are always statements as they can be used on the right side on an assignment and result in a type of void

It is also not true that labelled blocks are always expressions, the label can be used only for break and continue control flow without any return value.

pub fn main() void {
    const a = {
        // this block is unlabelled but it is an expression and it evaluates to `void`
    };
    @compileLog(@TypeOf(a));

    blk: {
        // this block is labelled but is not an expression
        break :blk;
    }
}

[tenderlove]

Hi, I'm very new to Zig so this may be a silly question, but is the block evaluating to void construct useful? Writing something like that seems like a mistake on the programmers side (and the compiler shouldn't allow it).

Also please take my comments with a grain of salt, I'm very new at Zig. Actually @zenspider was helping me try to understand why my if expression wasn't working, and it turned out to be due to the fact that blocks aren't expressions (IOW const x = if () { ... } is very different than const x = if () ..., and I didn't understand that)

[mitchellh]

Hi, I'm very new to Zig so this may be a silly question, but is the block evaluating to void construct useful? Writing something like that seems like a mistake on the programmers side (and the compiler shouldn't allow it).

Its the only way to initialize a void field. This is common practice in particular if you're using comptime to turn a field "on" or "off". i.e.:

const Widget = struct {
  const KnobType = if (builtin.target.isDarwin()) u64 else void;
  const knob_default = if (builtin.target.isDarwin()) 0 else {};

  knobs: KnobType = knob_default,
}

A void field takes up no space.

[InKryption]

Actually, it is one of two ways to initialize a void value. The second way is the void literal 'void{}'.

[tenderlove]

Hi, I'm very new to Zig so this may be a silly question, but is the block evaluating to void construct useful? Writing something like that seems like a mistake on the programmers side (and the compiler shouldn't allow it).

Its the only way to initialize a void field. This is common practice in particular if you're using comptime to turn a field "on" or "off". i.e.:

const Widget = struct {
  const KnobType = if (builtin.target.isDarwin()) u64 else void;
  const knob_default = if (builtin.target.isDarwin()) 0 else {};

  knobs: KnobType = knob_default,
}

A void field takes up no space.

Awesome, this makes a lot of sense. Thank you for taking the time to explain, I really appreciate it!!

[kevin-neel-oracle-com]

I'm admittedly a little new to github. I wonder if this is committed into master, why is this change not reflected in https://ziglang.org/documentation/master/#Blocks (nor 0.13.0)?

[nektro]

it was not committed

[kevin-neel-oracle-com]

What does this mean?

Clarify blocks being statements in doco
zenspider
zenspider
committed
on Feb 13, 2023

[kevin-neel-oracle-com]

Does it mean committed into his fork?

[kevin-neel-oracle-com]

Because I do find the current wording quite confusing. Maybe something like

Blocks are expressions. They return void unless you use break :labl...