From: Yaron Minsky <yminsky@gmail.com>

> I've noticed what appear to be inconsistent labelling on some list
> functions, and I'm wondering if I'm properly understanding the reasons
> behind the way the labels work.
> 
> For example, in the various association list functions, in some cases
> the association list is passed with a ~map label, and sometimes with
> no label.  Another odd case is the mem and memq functions, both of
> which label the list being queried with the label ~set.  In this case,
> the labelling mostly seems kind of useless rather than inconsistent.

There are reasons for both :-)
The ~set label is there, so that you can easily define the membership
function.

   let in_a = List.mem ~set:a

Same thing for ~map in List.mem_assoc.
However, there is no label in List.remove_assoc, because there it
doesn't really make sense: it maps an association list to a new
association list.
There is no label either in List.assoc for a dirty reason:
as the result is a polymorphic variable, if there were a label, one
wouldn't be able to omit it in applications. List.assoc is used very
often.

> I'm asking all of this because I'm playing around with writing a
> labelled version of the extlib interface, and I'm wondering whether
> these are mistakes that should be fixed, or whether there are good
> reasons for them and they should be preserved.

So, there are good reasons, but you may make different choices. The
labelling of the standard library is intentionally light; in other
libraries you might want to put more. Or, conversely, if you choose to
have only a labelled version (avoids maintaining two versions), you
must be careful of using labels only where they will not get in the
way.

      Jacques Garrigue