Re: [Caml-list] Cyclic type abbreviation

[Romain Bardou <romain@cryptosense.com>]

On 13/11/2015 14:46, Romain Bardou wrote:
> 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.
>

My bad, I had actually redefined the option type as a polymorphic 
variant before:

type 'a option = [ `None | `Some of 'a ]

and I forgot about it when I tested the definition of tree above.

So yeah you can do that with polymorphic variants even though they are 
kind of type abbreviations.

Thanks to Daniel for pointing this out.

-- 
Romain