From: William Chesters <williamc@paneris.org>

> Here's something I've wondered about for a while and just encountered again.
> How come this class ..
> 
>     class virtual ['a] c = object
>       method virtual b: bool c
>     end
> 
> ... gets a constraint 'a = bool?  I.e. for instance this doesn't compile:
> 
>     class virtual ['a] c = object
>       method virtual b: bool c
>       method virtual i: int c
>           (* "This type int should be an instance of type bool" *)
>     end

This is almost a FAQ.
The deep reason is that ocaml object types are defined by structure,
and as a result only regular recursion is allowed. If we were to allow
polymorphic recursion in class types as required by your example, the
type could not be represented as a regular tree anymore.
(to be fully exact, in this particular case it is possible to
represent it as a regular tree, but this would break the stronger
invariant used by the compiler, which requires all occurences of a
type in its own expansion to have the same parameters)

Note that this applies only to structural types.
For defined types you can use polymorphic recursion.

type 'a t = { b: bool t }

> Note that the following does work in C++:
> 
>     template <class a>
>     struct c {
>       virtual c<bool> b() = 0;
>       virtual c<int> i() = 0;
>     };
> 
> I guess there are workarounds but it seems unnecessary that ML should find
> recursive parameterised classes harder than C++?

In C++ classes are not equal by structure, so the second case applies.

Jacques Garrigue

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