Re: [Caml-list] Cyclic type abbreviation

[Romain Bardou <romain@cryptosense.com>]

On 13/11/2015 14:37, David Allsopp wrote:
> Mr. Herr wrote:
>> On 13.11.2015 13:49, Christoph Höger wrote:
>>> Dear all,
>>>
>>> why is this type cyclic?
>>>
>>> type node = int * tree * tree
>>>   and tree = node option
>>>
>>> I cannot introduce a manifest for the option type, as there is no
>>> Option module (why is that, btw?) - so I would assume option to be
>>> special enough to be handled like any other algebraic data type.
>> type 'a option = None | Some 'a
>>
>> no need for a module, just a simple type. Maybe you confound it with other
>> languages.
>>
>> And cyclic - well, the types are referring to each other.
>>
>> Summary: what is supposedly wrong with it?
>
> I expect that what is wrong is that you can write:
>
> type node = int * tree * tree
>   and tree = Some of node
>            | None
>
> I don't know why you can't write [and tree = node option] instead.
>
>
> David
>

Interestingly, this definition is accepted:

type tree = (int * 'a * 'a) option as 'a

Here you are helping the type-checker because you give it a "canonical" 
representation that it can use when unifying; it no longer has to expand 
the type, potentially infinitely. I think the main point is that the 
type is isorecursive, but I'm not really an expert on the subject.

-- 
Romain Bardou