What's the best way to mutate a nested enum? - rust

Question

I have an enum which roughly looks like this simplified example:
use std::collections::BTreeMap;
enum Value {
Null,
Object(BTreeMap<String, Value>)
}
Now I would like to, based on a list of field names, drill down into a Value::Object and initialize it with the given fields. The leaf field should always receive the Value::Null variant.
What I would like to, and am used to, do is the following:
fn set_null_to_how_i_want_it(fields: &[&str], mut v: &mut Value) {
debug_assert!(fields.len() > 0);
for (fid, field) in fields.iter().enumerate() {
v =
match *v {
Value::Object(ref mut map) => {
let val_to_insert =
if fid == fields.len() - 1 {
Value::Null
} else {
Value::Object(Default::default())
};
map.entry(field.to_string()).or_insert(val_to_insert)
}
_ => unreachable!(),
}
}
}
However, the borrow checker will not allow it.
Interestingly, what worked is the following, more complex and probably less performant implementation:
fn set_null_how_borrow_chk_allows_it(fields: &[&str], v: &mut Value) {
let next =
match *v {
Value::Object(ref mut map) => {
let val_to_insert =
if fields.len() == 1 {
Value::Null
} else {
Value::Object(Default::default())
};
map.entry(fields[0].to_string()).or_insert(val_to_insert)
},
_ => unreachable!()
};
if fields.len() > 1 {
set_null_how_borrow_chk_allows_it(&fields[1..], next)
}
}
With the version above, all the following code runs fine:
let mut v = Value::Object(Default::default());
let fields = ["foo", "bar", "baz"];
set_null_how_borrow_chk_allows_it(&fields, &mut v);
let mut map_count = 0;
for (fid, field) in fields.iter().enumerate() {
let next =
match v {
Value::Object(mut map) => {
map_count += 1;
map.remove(&field.to_string()).unwrap()
},
_ => unreachable!()
};
v = next;
}
assert_eq!(map_count, fields.len());
match v {
Value::Null => (),
Value::Object(_) => unreachable!(),
}
Is there a better way to initialize a nested enumeration or to get set_null_to_how_i_want_it(...) to work ?
How to reproduce
You can have a look at the complete example on github, and play around with it as follows:
git clone https://github.com/Byron/depot
cd depot/src/rust/hello
cargo test --test lang
# edit src/rust/hello/tests/lang.rs for testing
Meta
➜ hello git:(master) ✗ rustc --version --verbose
rustc 1.1.0-nightly (4b88e8f63 2015-05-11) (built 2015-05-12)
binary: rustc
commit-hash: 4b88e8f63eeaf557c916a0a1e73150b028c44c52
commit-date: 2015-05-11
build-date: 2015-05-12
host: x86_64-apple-darwin
release: 1.1.0-nightly

Answer 1

The borrow checker works on names, and so some examples like your first approach don’t work; this can be worked around by moving the value v to a different name first and then assigning the new value to v:
fn set_null_to_how_i_want_it(fields: &[&str], mut v: &mut Value) {
debug_assert!(fields.len() > 0);
for (fid, field) in fields.iter().enumerate() {
let tmp = v;
v = match *tmp {
Value::Object(ref mut map) => {
let val_to_insert =
if fid == fields.len() - 1 {
Value::Null
} else {
Value::Object(Default::default())
};
map.entry(field.to_string()).or_insert(val_to_insert)
}
_ => unreachable!(),
};
}
}