(For others, labels are optional in three senses: not all parameters need to be labelled, the compiler can emit a warning if a label is omitted but you may want

(For others, labels are optional in three senses: not all parameters need to be labelled, the compiler can emit a warning if a label is omitted but you may want this to be an error instead, and there are optional labelled arguments which may be omitted entirely in some cases. Labels are not optional in that the type system is unwilling to convert between labelled/unlabelled or optional/omitted/required parameters and the compiler mostly won’t add hidden coercions between these types) A common case where the trivial-value syntactic sugar fails looks like: foo ~some_descriptive_label:t.some_descriptive_label bar ~something:!something Or baz ~the_option:config.the_option (Why not just pass the config? Two reasons: it is nice to have the interface being explicit about what data is needed, and the function may come from a module that cannot depend on the definition of the config type.)