modifying a field while pattern matching on it

pattern-matching, pointers, rust

Solution

Okay, with the clarified variant of the question, here is my revised answer:

enum State { Outside, InATag(~str) }
struct Tokenizer { state: State }

impl Tokenizer {
    fn toggle(&mut self) {
        match self {
            &Tokenizer { state: InATag(*) } => { self.state = Outside }
            &Tokenizer { state: Outside }   => { self.state = InATag(~"baz") }
        }
    }
}

fn main() {
    let mut t1 = Tokenizer { state: InATag(~"foo") };
    match t1 {
        Tokenizer { state: InATag(*) } => { t1.state = Outside }
        Tokenizer { state: Outside } => { /* impossible */ }
    }
    io::println(fmt!("Hello t1: %?", t1));
    let mut t2 = Tokenizer { state: InATag(~"bar") };
    t2.toggle();
    io::println(fmt!("World t2: %?", t2));
}

I admit, I actually did not expect it to be as easy as the above, and I can easily believe that small changes to the above code could cause it to start failing to borrow-check. But without a more fleshed out example from the asker, it is hard to tell whether the above code would suit his purposes or not.

Oh, here's the output when I compile and run the code:

% rustc /tmp/m.rs
warning: no debug symbols in executable (-arch x86_64)
% /tmp/m
Hello t1: {state: Outside}
World t2: {state: Outside}
%

Problem

I tried my hands at Rust for the first time today (writing a XML tokenizer), and naturally don’t understand everything: I have a struct with field that can take an enum value: ``` enum State { Outside, InATag(~str) } struct Tokenizer { state: State } ``` In a `impl Tokenizer`, I want to match on the current state, and change it in some cases, however this always gives a `use of moved value` error. H to access and/or declare the state field so that I can match on it and change its value inside a match branch? Sorry for the confusion, I meant to change the Tokenizer’s state field, not the state’s String field! ``` match self.state { InATag(name) => self.state = Outside, Outside => () } ```

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