Errors

Option, Result, the ? operator, ?? and ?., the Error trait, declaring an error type, and panic.

What belongs here#

What does not belong here: the control flow constructs themselves, which are in syntax/. Every page in this folder is a reference page: an example first, then the syntax, then numbered rules, then what the construct is not.

Pages#

• Result - A function that can fail answers Result<Value, Failure>, whose cases are Ok and Fail. The postfix question mark unwraps an Ok or returns the Fail from the surrounding function.
• Optional chaining - ?. is Option.map, or Option.flatMap when the member itself answers an Option, so chaining never nests. ?? is the trait OrElse, which gives a lazy fallback for an absent Option, a failed Result or any type that comes with it.
• The question mark operator - A postfix ? unwraps an Ok or a Some and returns the Fail or None from the surrounding function early, converting the error type through From when they differ.
• The Error trait - Error is a trait, not a base type; a failure that implements it fits into Result<Value, Error> for the layers that only need to report it, and cause() gives the chain.
• Declaring an error type - An error type is a type with cases like any other; a case that wraps one value of a type no other case wraps gets From generated, which is what makes ? convert on its own.
• panic - panic prints panic, the message and the site to standard error, exits with 101, and runs nothing else on the way out - it is for bugs, never for an expected failure.
• Errors at the top level - A ? at the top level of an entry file or a script is not a panic; it prints the error and exits with 1, walking cause() one line per link, and in the dev profile names every ? the failure went through.