Polystate: Composable Finite State Machines

sdzx-1 · July 3, 2025, 3:22am

label switch causes the compiler to never halt

opened 03:05AM - 03 Jul 25 UTC

bug

### Zig Version 0.14.1 ~ Latest version ### Steps to Reproduce and Observed Be…havior ```zig const std = @import("std"); pub const Exit = union(enum) {}; pub const Method = enum { next, current, }; pub fn FSM( transition_method: Method, state: type, ) type { return struct { pub const TransitionMethod = transition_method; pub const State = state; }; } pub fn Runner() type { return struct { pub const StateId = std.math.IntFittingRange(0, 1); pub fn run_handler(curr_id: StateId, ctx: *void) ?StateId { sw: switch (curr_id) { // 0 -> A // 1 -> Exit inline 0, 1 => |idx| { const State = if (idx == 0) A else Exit; if (comptime State == Exit) return null; const handle_res = @call(.always_inline, State.handler, .{ctx}); // const handle_res = State.handler(ctx); switch (handle_res) { inline else => |new_fsm_state_wit, tag| { _ = tag; const new_fsm_state = comptime @TypeOf(new_fsm_state_wit); const new_id = if (new_fsm_state.State == A) 0 else 1; switch (new_fsm_state.TransitionMethod) { inline .next => return new_id, inline .current => continue :sw new_id, } }, } }, } } }; } // pub fn Example(method: Method, Current: type) type { return FSM(method, Current); } var k: i32 = 10; pub const A = union(enum) { // to_a: Example(.current, A), exit: Example(.next, Exit), pub fn handler(_: *void) @This() { // if (k == 1) return .to_a; return .exit; } }; pub fn main() void { std.debug.print("----------------------------\n", .{}); var ctx: void = {}; const R = Runner(); var curr_id: ?R.StateId = 0; while (curr_id) |id| { curr_id = R.run_handler(id, &ctx); } std.debug.print("----------------------------\n", .{}); } ``` ### Expected Behavior Compile correctly

I encountered a compiler bug and am waiting for an official fix.

sdzx-1 · July 3, 2025, 4:25am

New impl:

github.com/sdzx-1/polystate

src/polystate.zig

new

const std = @import("std");
const Adler32 = std.hash.Adler32;
const AVL = @import("avl.zig").AVL;

pub const Exit = union(enum) {};

// FSM       : fn (type) type , Example
// State     : type           , A, B
// FsmState  : type           , Example(A), Example(B)

pub const Mode = enum {
    no_suspendable,
    suspendable,
};

pub const Method = enum {
    next,
    current,
};

This file has been truncated. show original

examples:

Atm state graph

The blue arrow indicates Next, and the black arrow indicates Current.

The way to work around the zig compiler bug above is to have at least one black arrow.

milogreg · July 3, 2025, 9:03pm

That bug is unfortunate, I’ve ran into a few labeled switch bugs myself (they seem to be all too common).

I came up with a hack to fix it, though. Just update run_handler like so:

pub fn run_handler(curr_id: StateId, ctx: *Context) RetType {
    @setEvalBranchQuota(10_000_000);
    sw: switch (curr_id) {
        inline 0...state_map.avl.len - 1 => |idx| {
            // Remove when https://github.com/ziglang/zig/issues/24323 is fixed:
            {
                var runtime_false = false;
                _ = &runtime_false;
                if (runtime_false) continue :sw 0;
            }

            << rest of switch prong >>
        },
        else => unreachable,
    }
}