Function affected 

pub fn affected(
    project: &Project,
    sources: &SourceMap,
    root: &Path,
    reference: &str,
) -> Result<Vec<BeamId>, EngineError>

Every beam affected by what changed since reference, in project order: the beams whose inputs match a changed path or whose Beamfile changed, closed over their dependents. root is the project root git runs from; paths git reports are relative to it.