Struct Fold 

pub struct Fold<'a> { /* private fields */ }

Reading a whole settings struct out of one resolution.

Generated code reads each field in turn and calls Fold::finish once, which is what makes the errors a list rather than the first thing that went wrong.

Implementations

impl Fold<'_>

pub fn optional<T: FromValue>(&mut self, id: PropId) -> Option<T>

A setting that may have no value, as Option<T>.

Absence is not an error here: a setting with no default and nothing set is a field that holds None, which is what option<T> in the spec means.

pub fn required<T: FromValue>(&mut self, id: PropId) -> Option<T>

A setting that must have a value, for a field that is not an Option.

Returns None only when it has recorded an error, so code that has already called Fold::finish and found it Ok may unwrap what this returned. That is the contract generated code is written against: the fold reports, and the struct is built afterwards.

pub fn errors(&self) -> &[ReadError]

What has gone wrong so far, for a caller that wants to add to the list.

pub fn finish(self) -> Result<(), ReadErrors>

Every setting read, or every reason one could not be.

pub fn into_errors(self) -> ReadErrors

Everything that went wrong, whether or not anything did.

What a lossy fold ends with: Fold::finish answers “did this work”, and the answer there is always “not entirely” or the caller would not be folding lossily.