是否可以编写一个生成函数的宏,其中该函数的参数个数由宏决定?例如,我想在Cassandra驱动程序中使用预准备语句更容易编写一些内容。
let prepared = prepare!(session, "insert into blah (id, name, reading ) values (?, ?, ?)", int, string, float);
let stmt = prepared(1, "test".to_string(), 3.1);
session.execute(stmt);
prepare!
需要生成类似的内容(仅为了简洁而解压缩):
fn some_func(arg1, arg2, arg3) -> Statement {
let mut statement = Statement::new("insert into blah (id, name, reading ) values (?, ?, ?)", 3);
statement.bind_int(0, arg1).unwrap()
.bind_string(1, arg2).unwrap()
.bind_float(2, arg3).unwrap()
}
答案 0 :(得分:8)
Rust宏中的两件难事:计数和唯一标识。你有两个。然后,我是那个写答案的人,所以我想现在是我的问题。至少你没有问过解析字符串(如果没有编译器插件,这是完全不可能的。)
另一个不可能的事情是将类型映射到不同的方法。你不能。相反,我将假设存在一个执行此映射的辅助特征。
此外,Rust没有int
,string
,或 float
。我假设您的意思是i32
,String
和f32
。
最后,你编写调用和扩展的方式并没有真正凝聚。我不明白为什么session
参与其中;它不用于扩展。因此,我将冒昧地假装你不需要它;如果你这样做,你将不得不重新入侵。
所以,这就是我想出来的。
// Some dummy types so the following will type-check.
struct Statement;
impl Statement {
fn new(stmt: &str, args: usize) -> Self { Statement }
fn bind_int(self, pos: usize, value: i32) -> Result<Self, ()> { Ok(self) }
fn bind_float(self, pos: usize, value: f32) -> Result<Self, ()> { Ok(self) }
fn bind_string(self, pos: usize, value: String) -> Result<Self, ()> { Ok(self) }
}
struct Session;
impl Session {
fn execute(&self, stmt: Statement) {}
}
// The supporting `BindArgument` trait.
trait BindArgument {
fn bind(stmt: Statement, pos: usize, value: Self) -> Statement;
}
impl BindArgument for i32 {
fn bind(stmt: Statement, pos: usize, value: Self) -> Statement {
stmt.bind_int(pos, value).unwrap()
}
}
impl BindArgument for f32 {
fn bind(stmt: Statement, pos: usize, value: Self) -> Statement {
stmt.bind_float(pos, value).unwrap()
}
}
impl BindArgument for String {
fn bind(stmt: Statement, pos: usize, value: Self) -> Statement {
stmt.bind_string(pos, value).unwrap()
}
}
// The macro itself.
macro_rules! prepare {
// These three are taken straight from
// https://danielkeep.github.io/tlborm/book/
(@as_expr $e:expr) => {$e};
(@count_tts $($tts:tt)*) => {
<[()]>::len(&[$(prepare!(@replace_tt $tts ())),*])
};
(@replace_tt $_tt:tt $e:expr) => {$e};
// This is how we bind *one* argument.
(@bind_arg $stmt:expr, $args:expr, $pos:tt, $t:ty) => {
prepare!(@as_expr <$t as BindArgument>::bind($stmt, $pos, $args.$pos))
};
// This is how we bind *N* arguments. Note that because you can't do
// arithmetic in macros, we have to spell out every supported integer.
// This could *maybe* be factored down with some more work, but that
// can be homework. ;)
(@bind_args $stmt:expr, $args:expr, 0, $next:ty, $($tys:ty,)*) => {
prepare!(@bind_args prepare!(@bind_arg $stmt, $args, 0, $next), $args, 1, $($tys,)*)
};
(@bind_args $stmt:expr, $args:expr, 1, $next:ty, $($tys:ty,)*) => {
prepare!(@bind_args prepare!(@bind_arg $stmt, $args, 1, $next), $args, 2, $($tys,)*)
};
(@bind_args $stmt:expr, $args:expr, 2, $next:ty, $($tys:ty,)*) => {
prepare!(@bind_args prepare!(@bind_arg $stmt, $args, 2, $next), $args, 3, $($tys,)*)
};
(@bind_args $stmt:expr, $_args:expr, $_pos:tt,) => {
$stmt
};
// Finally, the entry point of the macro.
($stmt:expr, $($tys:ty),* $(,)*) => {
{
// I cheated: rather than face the horror of trying to *also* do
// unique identifiers, I just shoved the arguments into a tuple, so
// that I could just re-use the position.
fn prepared_statement(args: ($($tys,)*)) -> Statement {
let statement = Statement::new(
$stmt,
prepare!(@count_tts $(($tys))*));
prepare!(@bind_args statement, args, 0, $($tys,)*)
}
prepared_statement
}
};
}
fn main() {
let session = Session;
let prepared = prepare!(
r#"insert into blah (id, name, reading ) values (?, ?, ?)"#,
i32, String, f32);
// Don't use .to_string() for &str -> String; it's horribly inefficient.
let stmt = prepared((1, "test".to_owned(), 3.1));
session.execute(stmt);
}
以下是main
函数扩展的内容,为您提供参考框架:
fn main() {
let session = Session;
let prepared = {
fn prepared_statement(args: (i32, String, f32)) -> Statement {
let statement = Statement::new(
r#"insert into blah (id, name, reading ) values (?, ?, ?)"#,
<[()]>::len(&[(), (), ()]));
<f32 as BindArgument>::bind(
<String as BindArgument>::bind(
<i32 as BindArgument>::bind(
statement, 0, args.0),
1, args.1),
2, args.2)
}
prepared_statement
};
// Don't use .to_string() for &str -> String; it's horribly inefficient.
let stmt = prepared((1, "test".to_owned(), 3.1));
session.execute(stmt);
}