Re: [Caml-list] Re: records with polymorphic variants?

[Martin Jambon <martin.jambon@ens-lyon.org>]

On Sun, 4 Mar 2007, Lukasz Stafiniak wrote:

> On 3/4/07, Zheng Li <li@pps.jussieu.fr> wrote:
>> 
>> Hi,
>> 
>> Eliot Handelman <eliot@generation.net> writes:
>> > type f = [ `A of int ]
>> >
>> > type r = { x : f }
>> >
>> > but then I can't do this:
>> >
>> > { x = `B "test" }
>> Here you want type f polymorphic, however as the type of a record field, 
>> its
>> polymorphism has to be reflected (bound) as type parameter in the 
>> declaration
>> of r.
>
> Yes, but the type f itself is not polymorphic, it is not [> f].

You can do this directly if you prefer:

type 'a f = 'a
   constraint 'a = [> `A of int ]

>> I guess you want the follows
>> 
>> # type 'a r = { x : 'a } constraint 'a = [> f]
>> type 'a r = { x : 'a; } constraint 'a = [> f ]
>> # {x = "test"}
>> Characters 6-12:
>>   { x = "test"};;
>>         ^^^^^^
>> This expression has type string but is here used with type [> f ]
>> # {x = `B "test"}
>> - : [> `A of int | `B of string ] r = {x = `B "test"}
>> 
> Nice! I didn't realize this is possible. Thanks.
>
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--
Martin Jambon
http://martin.jambon.free.fr