Define function based on match - rust

Question

I have the following function, which runs fine:
use chrono_tz::Tz;
use chrono::{TimeZone, NaiveDate, NaiveDateTime};
fn my_func(time_zone: &str, fmt: &str, ndt: NaiveDateTime) {
let datefmt_func = |ndt: NaiveDateTime| time_zone.parse::<Tz>().unwrap().from_utc_datetime(&ndt).format(fmt);
let res = datefmt_func(ndt).to_string();
println!("res: {res}");
}
fn main() {
let time_zone = "UTC";
let fmt = "%Y-%m-%d %H:%M:%S%z";
let ndt = NaiveDate::from_ymd_opt(2018, 9, 28).unwrap().and_hms_opt(2, 30, 0).unwrap();
my_func(time_zone, fmt, ndt);
}
I'd like to let it accept None as time_zone. Within it, I need to repeatedly call datefmt_func, so I'd like to assign to datefmt_func once and reuse it. Here's what I've tried:
use chrono_tz::Tz;
use chrono::{TimeZone, NaiveDate, NaiveDateTime};
fn my_func(time_zone: Option<&str>, fmt: &str, ndt: NaiveDateTime) {
let datefmt_func = match time_zone {
Some(time_zone) => |ndt: NaiveDateTime| time_zone.parse::<Tz>().unwrap().from_utc_datetime(&ndt).format(fmt),
None => |ndt: NaiveDateTime| ndt.format(fmt),
};
let res = datefmt_func(ndt).to_string();
println!("res: {res}");
}
fn main() {
let time_zone = Some("UTC");
let fmt = "%Y-%m-%d %H:%M:%S%z";
let ndt = NaiveDate::from_ymd_opt(2018, 9, 28).unwrap().and_hms_opt(2, 30, 0).unwrap();
my_func(time_zone, fmt, ndt);
}
However, this gives me
error[E0308]: `match` arms have incompatible types
--> src/main.rs:7:17
|
5 | let datefmt_func = match time_zone {
| ________________________-
6 | | Some(time_zone) => |ndt: NaiveDateTime| time_zone.parse::<Tz>().unwrap().from_utc_datetime(&ndt).format(fmt),
| | -----------------------------------------------------------------------------------------
| | |
| | the expected closure
| | this is found to be of type `[closure#src/main.rs:6:28: 6:48]`
7 | | None => |ndt: NaiveDateTime| ndt.format(fmt),
| | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected closure, found a different closure
8 | | };
| |_____- `match` arms have incompatible types
|
= note: expected closure `[closure#src/main.rs:6:28: 6:48]`
found closure `[closure#src/main.rs:7:17: 7:37]`
= note: no two closures, even if identical, have the same type
= help: consider boxing your closure and/or using it as a trait object
For more information about this error, try `rustc --explain E0308`.
error: could not compile `tmp` due to previous error
How can I adjust my code so that it will run? If "no two closures, even if identical, have the same type", then how can I assign a function which depends on a local variable (time_zone) to datefmt_func?

Answer 1

Do as the compiler tells you, use trait objects and Box the closures:
fn my_func(time_zone: Option<&str>, fmt: &str, ndt: NaiveDateTime) {
let datefmt_func: Box<dyn Fn(_) -> _> = match time_zone {
Some(time_zone) => Box::new(|ndt: NaiveDateTime| {
time_zone
.parse::<Tz>()
.unwrap()
.from_utc_datetime(&ndt)
.format(fmt)
}),
None => Box::new(|ndt: NaiveDateTime| ndt.format(fmt)),
};
let res = datefmt_func(ndt).to_string();
println!("res: {res}");
}
Or in your simple example just run the code instead of creating a closure which you immediately call:
fn my_func(time_zone: Option<&str>, fmt: &str, ndt: NaiveDateTime) {
let res = match time_zone {
Some(time_zone) => time_zone
.parse::<Tz>()
.unwrap()
.from_utc_datetime(&ndt)
.format(fmt),
None => ndt.format(fmt),
}
.to_string();
println!("res: {res}");
}