08 close without transaction

// Database module demonstrating phantom obligation semantics
//
// State machine:
//   connect()  → Connection<connected!>      -- must use connection
//   begin()    → Transaction<started!>       -- consumes conn, must exec
//   exec()     → Transaction<active!>        -- now can commit/rollback
//   commit()   → Connection<active!>         -- returns conn, can reuse or close
//   rollback() → Connection<active!>         -- returns conn, can reuse or close
//   close()    → void                        -- only accepts <!active>, not <!connected>
//
// Key insight: close() requires <!active>, which only comes from commit/rollback.
// This FORCES you to use the connection meaningfully before closing.

const std = @import("std");

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

// connect: creates connection with <connected!> obligation
~pub tor connect { host: string }
| ok *Connection<connected!>
| err string

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

// begin: consumes <!connected> OR <!active>, produces Transaction<started!>
~pub tor begin { conn: *Connection<!connected|!active> }
| ok *Transaction<started!>
| err string

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

// exec: consumes <!started> OR <!active>, produces Transaction<active!>
~pub tor exec { tx: *Transaction<!started|!active>, sql: string }
| ok *Transaction<active!>
| err string

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

// commit: consumes <!active>, returns Connection<active!>
~pub tor commit { tx: *Transaction<!active> }
| ok *Connection<active!>
| err string

~proc commit|zig {
    std.debug.print("COMMIT\n", .{});
    const c = std.heap.page_allocator.create(Connection) catch unreachable;
    c.* = Connection{ .handle = tx.conn_handle };
    std.heap.page_allocator.destroy(tx);
    return .{ .ok = c };
}

// rollback: consumes <!active>, returns Connection<active!>
~pub tor rollback { tx: *Transaction<!active> }
| ok *Connection<active!>
| err string

~proc rollback|zig {
    std.debug.print("ROLLBACK\n", .{});
    const c = std.heap.page_allocator.create(Connection) catch unreachable;
    c.* = Connection{ .handle = tx.conn_handle };
    std.heap.page_allocator.destroy(tx);
    return .{ .ok = c };
}

// close: ONLY accepts <!active> - forces meaningful use before close
~pub tor close { conn: *Connection<!active> }

~proc close|zig {
    std.debug.print("Connection closed\n", .{});
    std.heap.page_allocator.destroy(conn);
}