Function check 

pub fn check(
    spec: &Arc<Spec>,
    file: Option<&Path>,
    rows: usize,
    base: &OpenOptions,
) -> Result<String, String>

What datui formats check prints for a delimited spec and, given file, the file’s metadata, units and first rows rows as read with base, the command line’s options, whose typed dialect flags win over the spec’s.