0000-option-result-map-or-default.md

• Feature Name: option_result_map_or_default
• Start Date: 2021-07-14
• RFC PR: rust-lang/rfcs#3148
• Rust Issue: rust-lang/rust#0000

Summary

Option has the methods unwrap, unwrap_or, unwrap_or_else and unwrap_or_default. It similarly has map, map_or, map_or_else, however map_or_default is missing. The exact same problem exists for Result. This RFC is a proposal to add this method to Option and Result.

Motivation

As mentioned before, a user might reasonably expect this method to exist, based on the existence of other or_default methods such as unwrap_or_default. Furthermore, this is a very common usecase. Searching for map_or_else in .rs files in the official Rust repository it is incredibly common to see instances like these:

.map_or_else(String::new, ...)
.map_or_else(SmallVec::new, ...)
.map_or_else(Vec::new, ...)

In fact, from a manual count at the time of writing, 25 out of the 57 occurrences of map_or_else in the Rust codebase could have been replaced with an equivalent call to map_or_default.

Reference-level explanation

The following implementation would get added to core::option::Option:

impl<T> Option<T> {
    pub fn map_or_default<U: Default, F: FnOnce(T) -> U>(self, f: F) -> U {
        match self {
            Some(t) => f(t),
            None => Default::default(),
        }
    }
}

The following implementation would get added to core::result::Result:

impl<T, E> Result<T, E> {
    pub fn map_or_default<U: Default, F: FnOnce(T) -> U>(self, f: F) -> U {
        match self {
            Ok(t) => f(t),
            Err(e) => Default::default(),
        }
    }
}

Drawbacks

It adds another method to Option and Result, which may be considered as cluttered by some.

Rationale and alternatives

• .map_or_else(Default::default, ...) can be written, although it is significantly longer.
• In case feature(default_free_fn) stabilizes a user can write .map_or_else(default, ...) after std::default::default, which is a bit shorter.

However, neither alternative solves the discrepancy between unwrap_or_default existing but map_or_default not existing.

Prior art

We already have unwrap_or_default.

Unresolved questions

None.