Re: [Caml-list] Cyclic type abbreviation

[Ben Millwood <bmillwood@janestreet.com>]

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To be clear, options aren't being handled differently from any other
*pre-existing* user-defined type.

utop # type 'a my_type = One of 'a | Two of 'a * 'a;;
type 'a my_type = One of 'a | Two of 'a * 'a
utop # type node = int * tree * tree and tree = node my_type;;
Error: The type abbreviation node is cyclic

Using a real ADT instead of a type abbrevation works. David Allsopp already
gave one way you can do it, here is another which preserves the use of the
option type:

utop # type node = Node of int * tree * tree and tree = node option;;
type node = Node of int * tree * tree
and tree = node option

Note that here [tree] is still a type abbreviation, but [node] is not, so
the cycle is not a problem.

Note also that your original example works fine with the -rectypes flag.
But my general opinion is that if you need -rectypes to compile your code,
you should write different code :)

On 13 November 2015 at 22:24, Romain Bardou <romain@cryptosense.com> wrote:

> 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
>
>
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