Re: [Caml-list] Cyclic type abbreviation

["Mr. Herr" <misterherr@freenet.de>]

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.
>
>
>
Ah, did not see the problem at first sight...

Maybe that is why people so often define their own empty element:

# type node = Emptynode | Node of (int * node * node) ;;
type node = Emptynode | Node of (int * node * node)

# type node = Emptynode | Node of int * node * node ;; (* just had this parens
discussion... *)
type node = Emptynode | Node of int * node * node


/Str.