[FIXED] What does "|_|" mean in Rust?

Issue

I am learning how to connect database to Rust program. There something I don't quite understand:

PgConnection::establish(&database_url).unwrap_or_else(|_| panic!("Error connecting to {}", database_url))

What does this expression mean? What does |_| mean?

Solution

Ignoring the underscore _ for now, Rust uses pipes | to delimit the parameter-list of a closure _{(aka lambda-function, anonymous function, etc)}. For comparison, JavaScript and C# use normal parentheses and a fat arrow ( ) => to denote their closures (a closure is just an anonymous function with environment capture semantics), like so:

// JavaScript:
const smallerArray = bigArray.filter( ( x, idx ) => idx < 5 );

// C#:
T[] smallerArray = bigArray.Where( ( T item, Int32 idx ) => idx < 5 ).ToArray();

Instead of being a conformist and using ( ) => like everyone else, Rust opts for a different syntax for the parameter-list: it uses |a, b, c| for single-line functions and |a, b, c| { stuff } for multi-line functions, and without any arrow symbology.

...so the above JavaScript and C# would be translated into Rust like so:

let smaller_array = big_array.into_iter().filter( |item| item < 5 )

As for the underscore: Rust shares C#'s convention for using an underscore to denote a discarded or otherwise ignored parameter or local. Now you might wonder what the point of a discarded parameter is: why not just omit it entirely like in JavaScript? (e.g. bigArray.filter( x => x < 5 ) and bigArray.filter( ( x, idx ) => x < 5 ) and bigArray.filter( ( x, idx, arr ) => x < 5 ) are all equivalent.

...well, JavaScript doesn't support function overloading: each function name resolves to a single function implementation, so omitting unused parameters isn't a problem. Now while Rust doesn't support function overloading either, it's still a very large and complex language that has many situations where you will need to explicitly declare a parameter that you don't use (strictly speaking, _ represents an unbound identifier and you cannot use _ as a variable).

The main use-case for naming a function parameter to _ in Rust is because it's a value annotated with the "must_use" attribute - but if you really know you don't need to use that (bound) parameter and you don't want to be bombarded with low-value compiler warnings about must_use values, then _ is a handy solution.

Another use-cases of _ is to declare that a call-site's return-value is being willfully discarded (so this syntax signifies intent), for example:

let _ = someFunctionThatReturnsAValue();

...whereas if you simply put someFunctionThatReturnsAValue(); on its own line then anyone else reading the code, or a static-analysis tool, will think you absent-mindedly forgot to check someFunctionThatReturnsAValue's return-value - but using let _ = ... makes it clear that you really don't care about the return value such that you don't want static-analysis tools dinging you.

---

So given unwrap_or_else(|_| panic!("Error connecting to {}", database_url)), what does |_| mean?

• unwrap_or_else is a method of std::result.

• unwrap_or_else's parameter is a callback function op: FnOnce. If the result is-not-Ok then op will be invoked and op's return-value becomes the end-result of the unwrap_or_else expression (or it panics... your call).

  • Crucially, the op function in unwrap_or_else accepts one (1) parameter which is the E error-value contained within the result.
  • In this case, the panic!("Error connecting to {}", database_url)) expression doesn't use the inner error value at all (which I think is a bad idea) so the callback closure/function discards the value by using the _ syntax.

• So in conclusion, |_| in unwrap_or_else(|_| panic!("Error") means "The argument of unwrap_or_else is an anonymous function accepting an E (error value)-typed parameter - buuuut we just don't care about it, so pretend it doesn't exist".

Answered By - Dai

Answer Checked By - Marilyn (PHPFixing Volunteer)