Re: ocaml-dns- quick ocaml question

[Richard Mortier <Richard.Mortier@xxxxxxxxxxxxxxxx>]

On 18 Jun 2012, at 18:38, Raphael Proust wrote:

> On Mon, Jun 18, 2012 at 5:26 PM, Richard Mortier
> <Richard.Mortier@xxxxxxxxxxxxxxxx> wrote:
>> i have two types, rr_type and q_type. rr_type is a subset of q_type, 
>> extended it by about 5 elements. both are currently implemented in 
>> Dns.Packet as cenums which are translated to variant types.
>> 
>> i've just realised that (of course?) this means that the last definition 
>> (rr_type) overrides the first (q_type) as they have overlapping elements.
>> 
>> is there a nice/idiomatic way around this without either going to 
>> polymorphic variant types (that's what the `T types are called, right?) or 
>> just defining the cenum q_type with rr_type aliased to it (for slightly 
>> better readability), and using that everywhere?
>> 
>> eg., should i do something like create submodules Qtype and RRtype of 
>> Packet, and define cenum t in each?
> 
> 
> I see several possibilities:
> 
> - use polymorphic variants (yes that's what `Ts are called)

unfortunately cstruct targets variants only at the moment.

> - use one variant type and add assertion checks or other dynamic failure 
> (bad!)

ack- i thought that didn't "feel" right :)

> - use several submodules
> 
> - prefix the variants (very common) (feels similar to submodules but prevents
>  "open"-induced ambiguities)
> type q =
>  | Q_Foo
>  | Q_Bar of int
> type rr =
>  | RR_Foo
>  | RR_Bar of int
>  | RR_Baz of float

ok- i'll probably go with one of these options then.  (likely the latter.)

was just wondering if there was a "better" way.

> - use GADTs (requires OCaml 4.00 (still in beta)) (I am unsure of the proper
>  syntax, check the manual if need be)
> type q
> type rr
> type 'a t =
>  | Foo : 'a t
>  | Bar : int -> 'a t
>  | Baz : float -> q t (* This branch forces the ['a] to be instantiated with
>                          [q], thus preventing anyuse with functions requiring
>                          [rr t]
>                        *)

interesting!  but possibly a little advanced for me for now :)

thanks!

-- 
Cheers,

R.




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