Re: [Caml-list] does class polymorphism need to be so complicated?

[Benjamin Geer <ben@socialtools.net>]

Benjamin Geer wrote:
> I'm still curious to know why the example I gave (returning a 
> mysql_connection from a method that was typed to return a #connection) 
> didn't compile, though ("This method has type string -> mysql_connection 
> which is less general than 'a. string -> (#connection as 'a)").

Curiously, it works if the class is parameterised instead of the method:

class type ['a] driver =
object
   constraint 'a = #connection
   method get_connection : db_name:string -> 'a
end

class mysql_driver =
object (self : 's)
   constraint 's = #connection #driver
   method get_connection ~(db_name:string) =
     new mysql_connection db_name
end

Maybe something about these different approaches should go in a FAQ 
somewhere.

Ben

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