我已经花了一整天的时间在以下代码上进行工作,(这里是playpen)
/// The rule that moves state from one to another.
///
/// `S` - the type parameter of state.
///
/// `T` - the type parameter of input symbol.
#[deriving(PartialEq, Eq, Hash)]
pub struct Rule<S, T> {
pub state: S,
pub symbol: Option<T>,
pub next_state: S
}
impl<S: PartialEq, T: PartialEq> Rule<S, T> {
/// determine whether the rule applies to the given state and symbol
pub fn apply_to(&self, state: &S, symbol: &Option<T>) -> bool {
self.state == *state && self.symbol == *symbol
}
}
/// The transition relation in NFA,
/// containing all the rules needed by the NFA.
pub struct NFATransitions<S, T> {
pub rules: HashSet<Rule<S, T>>
}
impl<S: Eq + Hash + Clone, T: Eq + Hash> NFATransitions<S, T> {
pub fn next_states(&self, states: &HashSet<S>, symbol: &Option<T>) -> HashSet<S> {
states.iter().flat_map(|state| {
// error goes here: borrowed value does not live long enough
self.next_states_for(state, symbol).iter().map(|s| s.clone())
}).collect()
// Howover, the following code which have the same behavior can compile
// let mut result = HashSet::new();
// for state in states.iter() {
// result.extend(self.next_states_for(state, symbol).iter().map(|s| s.clone()));
// }
//
// result
}
/// get the next state for the given state and symbol
fn next_states_for(&self, state: &S, symbol: &Option<T>) -> HashSet<S> {
self.rules.iter().filter_map(|rule| {
if rule.apply_to(state, symbol) { Some(rule.next_state.clone()) } else { None }
}).collect()
}
}
这段代码只是一个哈希集合的包装器,用于nfa转换规则。(这不是我关心的)
flat_map
是我得到编译错误的地方。
对我来说很奇怪,因为我认为有着与 flat_map
相同行为的被注释掉的代码行可以正常运行。
我无法想象出 error: borrowed value does not live long enough
错误是如何产生的。
任何想法?
next_states_for
的结果是临时的。 - user395760