Help understanding the `ref t` syntax

help
1

Hello, I need some help understanding the ref t syntax.

From: std::option - Rust

fn check_optional(optional: Option<Box<i32>>) {
    match optional {
        Some(ref p) => println!("has value {}", p),

Here the statement is that try to pull Box<i32> as a ref p. In equation terms- ref p = Box(9). Deref both sides => p = Box(9).deref(). p = 9.

But from: std::option - Rust

msg = Some( "howdy" )
if let Some(ref m) = msg {

doesn't seem to be following that equation, Some( "howdy" ) = Some(ref m) => m = "howdy".deref() ?

How do I understand the ref syntax conceptually?

Is this deliberately inconsistent for language ergonomics?

Only to mention- Scala would have done it slightly differently which would have been clearer too-

1-

match optional{
  Some( Ref( p ) ) => ...

2-


if let Some(Deref(m)) = msg

This is both mathematically consistent and IMHO easier to glance.

2

ref is not related to Deref. The deref coercion is automatically inserted to some places but doesn't affect the matched variables directly.

If type T is matched to ref x then type of x is &T. But then why not just match for x and use &x? Consider this code:

#[derive(Debug)]
struct S;
fn use_s(_: &S) {}

let x: Option<S> = Some(S);
if let Some(ref y) = x {
    /* y : &S */
    use_s(y);
}
dbg!(x);

If I remove ref,

if let Some(y) = x {
    /* y : S */
    use_s(&y);
}
dbg!(x);

It doesn't compile. The error tells use x is moved by the pattern matching but later used.

It is similar to that "&some_struct.field" doesn't move some_struct out.
Technically, pattern matching matches against "place" instead of "value" (if you are familiar with C++, you can read "place" as "lvalue").

Today, the ref pattern is almost superseded by the "match ergonomics" which automatically insert & pattern and ref pattern to appropriate places.
The Option example can be written as:

let msg: Option<&str> = Some("howdy");
if let Some(m) = &msg { /* m : &&str */ }
2 Likes
3
4

This topic was automatically closed 90 days after the last reply. New replies are no longer allowed.