Re: [Caml-list] using a restricted subset of a polymorphic variant

[Romain <postmaster@doomeer.com>]

>    method get_heading = self#get_direction heading

This unifies heading (which has type [`Up | `Down]) with type dir, but 
types [`Up | `Down] and dir cannot be *unified*. However, [`Up | `Down] 
is a subtype of dir, so [`Up | `Down] can be *coerced* to type dir:

method get_heading = self#get_direction (heading :> dir)

Unification cannot guess coercions, so you have to annotate them by hand.

This may sound confusing, but I came up with an analogy some time ago 
which may help. (I'm probably not the only one who came up with it.)

A type can be seen as a set of values. For instance type [`Up | `Down] 
denotes the set of values `Up and `Down. This set is included in the set 
denoted by dir. [`Up | `Down] is a "subset" of dir, except we don't say 
"subset" but "subtype".

A polymorphic type can be seen as a set of types, i.e. a set of sets of 
values. Type [> `Up | `Down] denotes the set of all types which contain 
at least `Up and `Down. In particular, it contains [`Up | `Down]. So [> 
`Up | `Down] can be instantiated with [`Up | `Down].

Now you see that "is an instance of" is not the same as "is a subtype 
of": the first is inclusion between sets of sets of values, while the 
second is inclusion between sets of values.

Hope this helps,

-- 
Romain Bardou