Function learn 

fn learn(
    path: &Path,
    ids: &[usize],
    text: &dyn Fn(usize) -> String,
    messages: &[Message],
    options: &Options,
    word_ngrams: u32,
    lr: f64,
    cancel: &Cancel,
) -> Result<FastText, LearnError>

fastText’s classifier learned from the messages ids.