01 unused connection

// Database module demonstrating phantom obligation semantics
//
// The key insight: acquiring a resource creates an OBLIGATION to USE it meaningfully.
// Automatic cleanup (RAII, IDisposable, use statements) doesn't satisfy this -
// you must actually DO something with the resource.
//
// State machine:
//   connect()  → Connection[open!]           -- creates obligation to use connection
//   begin()    → Transaction[active!]        -- consumes connection, creates tx obligation  
//   execute()  → Transaction[active]         -- uses tx without consuming (can execute many)
//   commit()   → void                        -- consumes tx obligation (meaningful end)
//   rollback() → void                        -- consumes tx obligation (meaningful end)

const std = @import("std");

const Connection = struct { handle: i32 };
const Transaction = struct { conn_handle: i32 };
const QueryResult = struct { rows: i32 };

// connect: creates connection with obligation to use it
~pub event connect { host: []const u8 }
| ok *Connection<open!>
| err []const u8

~proc connect|zig {
    const c = std.heap.page_allocator.create(Connection) catch unreachable;
    c.* = Connection{ .handle = 42 };
    return .{ .ok = c };
}

// begin: consumes connection obligation, creates transaction obligation
~pub event begin { conn: *Connection<!open> }
| ok *Transaction<active!>
| err []const u8

~proc begin|zig {
    const tx = std.heap.page_allocator.create(Transaction) catch unreachable;
    tx.* = Transaction{ .conn_handle = conn.handle };
    std.heap.page_allocator.destroy(conn);
    return .{ .ok = tx };
}

// execute: uses transaction WITHOUT consuming it (can call multiple times)
~pub event execute { tx: *Transaction<active>, sql: []const u8 }
| ok { tx: *Transaction<active>, result: QueryResult }
| err { tx: *Transaction<active>, msg: []const u8 }

~proc execute|zig {
    std.debug.print("Executing: {s}\n", .{sql});
    return .{ .ok = .{ .tx = tx, .result = .{ .rows = 1 } } };
}

// commit: consumes transaction obligation - this is a MEANINGFUL END
// [!] marks this as the default discharge for Transaction[active!]
// Void event (no branches) so [!] works correctly
~[!]pub event commit { tx: *Transaction<!active> }

~proc commit|zig {
    std.debug.print("COMMIT\n", .{});
    std.heap.page_allocator.destroy(tx);
}

// rollback: consumes transaction obligation - this is also a MEANINGFUL END
// Also marked [!] as alternative discharge
~[!]pub event rollback { tx: *Transaction<!active> }

~proc rollback|zig {
    std.debug.print("ROLLBACK\n", .{});
    std.heap.page_allocator.destroy(tx);
}