Rust: Tracking issue: Provide Entry-like API for Option

[shahn]

Often I have code where Option is used to indicate whether a value has been initialized or not. In these instances I often have code that either wants to use the value directly or initialize it and then use the value. So what I'm looking for is similar to as_ref()/as_mut() except they should return &T/&mut T respectively, and take either a value or a closure.

I think I would call the functions get{,_mut}_or_insert{,_with}() marrying the get_ functions (for example in hashmaps) for getting references with the or_insert Entry like functions.

[Thinkofname]

Isn't this just as_ref/mut().unwrap_or(val) and as_ref/mut().unwrap_or_else(|| val) ?

Edit: These don't modify the original Option which I guess it what you wanted

[shahn]

indeed

[tbu-]

See also #25149 and #29203. Also, this seems fixed by #39289.

[shahn]

AIUI, this is not fixed by #39289, because this issue is referenced as the tracking issue for the implementation and should stay open until it's decided whether this is to be stabilized or backed out.

[tbu-]

Ah, right. Maybe you could put "Tracking issue" in the title.

[oberien]

As this issue is quite old already (over 3 months), would it be possible to stabilize these two functions soon?

[brson]

@rfcbot merge

[brson]

@rfcbot fcp merge

[rfcbot]

Team member @brson has proposed to merge this. The next step is review by the rest of the tagged teams:

• [x] @BurntSushi
• [x] @Kimundi
• [x] @alexcrichton
• [x] @aturon
• [x] @brson
• [x] @dtolnay
• [x] @sfackler

No concerns currently listed.

Once these reviewers reach consensus, this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[alexcrichton]

To be clear, the APIs proposed here for stabilization are:

impl<T> Option<T> {
    fn get_or_insert(&mut self, v: T) -> &mut T;
    fn get_or_insert_with<F: FnOnce() -> T>(&mut self, f: F) -> &mut T;
}

[rfcbot]

:bell: This is now entering its final comment period, as per the review above. :bell:

[rfcbot]

The final comment period is now complete.

[dhardy]

This isn't Entry-like. If it was, it would be possible to do the following:

match opt_value.entry() {
    VacantEntry(entry) => entry.insert(x),
    OccupiedEntry(entry) => { assert_eq!(entry.get(), x); }
}

[SimonSapin]

@dhardy That particular example can be written as:

```rust
match opt_value {
ref mut entry @ None => *entry = Some(x),
Some(ref entry) => assert_eq!(entry, x),
}

[raphlinus]

To save time for people researching this, the feature landed in stable at Rust 1.20 (source: 64c1b23).

[shepmaster]

@raphlinus I think that commit is wrong :-) The function in question doesn't show up in the 1.20.0 docs but does in the 1.21.0 docs. Nope, never mind; I just forgot how to search, sigh.

GitHub does say that commit is only in the 1.21.0 tag though...

[shepmaster]

Ah, the 1.20.0 commit was a backport, thus why it's a different tag.