std/core/error
std/core/src/error.trb
Error, the trait-typed failure that lets an application pass a reason up through layers that have nothing to say
about it, and its one member: cause, the chain to whatever error caused this one.
trait Error
trait Error with Show"Some error, hand it up." Result<Value, Error> is the trait-typed failure: every error type that carries this
trait fits into it, so an application can pass a failure up through layers that have nothing to say about it
without declaring a wrapper type per layer.
Precise error types stay the norm for libraries. A library that answers Result<Config, ConfigError> lets its
callers match on what went wrong; one that answers Result<Config, Error> does not. The trait is for the other
direction: fn main(): Result<Void, Error> and everything in between, where the only thing left to do with a
failure is to report it.
? needs nothing new for that: a concrete failure that carries Error becomes the trait value through the
ordinary conversion of a value to a trait value, exactly as List<Shape> takes a Circle.
cause() is the chain. An error that was caused by another one hands it out, so a report can unwind it:
error: the server did not start / caused by: app.trb: no such file. The default is None, so a type that
has nothing to add implements the trait by writing with Error and nothing else.
fn cause
fn cause(): Error?What this error was caused by, if it wraps one. The end of the chain answers None.