027 default discharge annotation

// Test: [!] annotation marks default discharge for ambiguous cases
//
// When multiple events can discharge an obligation, auto-discharge
// fails with "ambiguous". The [!] annotation marks which one to use.

~import "$std/io"

const std = @import("std");

const File = struct { handle: i32, dirty: bool };

~event open { path: []const u8 }
| opened { file: *File[opened!] }

~proc open {
    const f = std.heap.page_allocator.create(File) catch unreachable;
    f.* = File{ .handle = 42, .dirty = false };
    return .{ .opened = .{ .file = f } };
}

// Two discharge options - [!] marks the default
~[!]event close { file: *File[!opened] }

~proc close {
    std.heap.page_allocator.destroy(file);
}

~event flush_close { file: *File[!opened] }

~proc flush_close {
    std.debug.print("flushing...\n", .{});
    std.heap.page_allocator.destroy(file);
}

// This flow doesn't explicitly discharge - auto-discharge should use close (the [!] one)
~open(path: "test.txt")
| opened _ |> _