> From: "Nick Alexander" <nalexander@amavi.com>
>
>> module type TEST =
>>   sig
>>     type outer = [ `A | `B]
>>     and inner = [ `B]
>>     val x : outer -> outer
>>     val y : inner
>>   end
>> module Test : TEST
>> # let p = Test.y;;
>> val p : Test.inner = `B
>> # let q = Test.x p;;
>>                  ^
>> This expression has type Test.inner = [ `B] but is here used with type
>>   Test.outer = [ `A | `B]
>> The first variant type does not allow tag(s) `A
>>
>> I'm confused.  Can I get an explanation for why the parameter to a
>> function with a sum variant type needs all summands to be possible
>> arguments?
>
> Inner is a subtype of outer, but they cannot be unified (unification
> requires equality, not subtyping).
>
> You can get subtyping by writing explicit coercions:
>
> # let q = Test.x (p :> Test.outer);;
> val q : Test.outer = `A
>
> You can also give polymorphic types to your values in the interface:
>  val x : [< outer] -> outer
> and
>  val y : [> inner]
> are both valid, and one of them is enough to allow (Test.x p) without
> coercion. I.e., [< outer] can be unified with inner, and [> inner] can
> be unified with outer.

Thank you Jacques.  I didn't know that you could name < and > types in
type declarations.  Perfect :)
Now, can anyone point me at docs about polymorphic records?  I can't find
a thing :(  They were mentioned in the 3.05 release notes as'- Support for polymorphic methods and record fields.'
which could be an extension of the object-based polymorphic records or a
really useful extension like the polymorphic variants.
Cheers,
Nick


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