[Caml-list] They have different arities

[ygrek <ygrek@autistici.org>]

Hello,

Consider the following : 
```
# type t' = int option;;
type t' = int option
# type t = int option = None | Some of int;;
Characters 0-40:
  type t = int option = None | Some of int;;
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Error: This variant or record definition does not match that of type
         int option
       They have different arities.
```

What is the reasoning behind this error?
I was expecting "specialisation" (in C++ terms), i.e. after `'a` in `'a option` was substituded for `int` -
type is now 0-arity and equality should hold - there is no "free" arity left..
     
I gather that in case of `t'` it is type abbreviation and for `t` it is a re-export. The manual says :
 
> Re-exported variant type or record type: an equation, a representation.
>    In this case, the type constructor is defined as an abbreviation for the type expression given in the equation, but in addition the constructors or fields given in the representation remain attached to the defined type constructor. The type expression in the equation part must agree with the representation: it must be of the same kind (record or variant) and have exactly the same constructors or fields, in the same order, with the same arguments. 

So it seems as long as representation matches - `t` should behave the same as abbreviation `t'`.

PS The real-world usage for this involves a bigger type and a need to define serialization function for a "specialized" type outside of module with generic definition, hence the requirement to explicitly list constructors by re-exporting, but type has to be monomorphic.

--