Re: [Caml-list] Heterogeneous dictionary

[Yaron Minsky <yminsky@janestreet.com>]

On Thu, Apr 4, 2013 at 2:48 PM, Anthony Tavener
<anthony.tavener@gmail.com> wrote:
> The problem here...
>
>   module Keys : sig
>     val recovery : int Univ_map.Key.t
>     val resist_pain : float  Univ_map.Key.t
>     ...
>
> Would be that Keys would now have dependence on types spanning the codebase.

Sure, but there's nothing that requires these to be stored together.
They can be scattered to the four winds, and yet all used to keep
things in the same Univ_map.t.

It's honestly very much like using a dynamic language, where you get
to declare new items at will that can be cast in and out from a
universal type.

It's probably worth reading over the Univ module in Core as well,
since Univ_map is built on it, and if Univ_map doesn't quite do what
you want, you can probably built what you need precisely on top of
Univ.

> Say these modifiers use Wounds.t, Fatigue.t, Ability.Score.t, ... There are
> a
> lot of types, which mostly have a limited scope (their own module and a few
> others). Wouldn't it be a problem to have all types brought into this one
> module, which every other module also becomes dependent on? Maybe it just
> feels like a problem but it's just an aesthetic -- Would you do this? A few
> hundred keys involving types from half the modules of the codebase?
>
>
> I'm trying to use these "modifiers" for code organisation -- declaring
> snippets of functionality (all of signature 'a -> 'a... to return modified
> input) in a lightweight manner, which can be applied elsewhere.
>
> For example, the Virtue module can add a lot of modifers to an entity (UID).
> Say one entity has virtues of Agility, Inspirational, and Sun-cyclic Magic,
> which each adding a few modifier functions keyed to various contexts
> (sun-cyclic magic: "Casting" is +3 while the sun is up). The associated
> modifiers would be picked up in code spread throughout the application.
> Rules
> for combat, spellcasting, even character dialog...
>
> Rather than having character dialog checking for "does he have this virtue,
> or
> that one? How about this ability? Spell-effects? Reputations? ..." I want to
> apply all appropriate/active modifiers for entity and situation which come
> from other rules. So, dialog code might have a current_value, then...
> eg.
>   let modified_value = apply_modifiers_for_context entity `CharmRoll
> current_value
>
> There is a lot of work on my project which I've been avoiding because the
> direct approach is building hairy nests of checks which call out to code
> everywhere... effectively splitting logic between the point of origin of a
> rule, and the point of application of it. I'd rather declare the individual
> rules, each in one piece, and "magically" apply the aggregate of rules.
>
>
> In a way, what I'm looking for is having a grand-central lookup, which
> *trusts* that I'm using the right keys at the right time (where the keys are
> purely symbolic with no type info)... but it would be nice if once the whole
> program is compiled it could identify whether that trust was broken
> afterall.
> A bit of a pipe-dream, and maybe even flawed logic. :)
>
> It might sound like I'm being too picky or even whiney... that declaring
> keys
> throughout the code is unacceptable, or having a file dependent on all types
> is problematic... maybe I am? The first seems disorganised and adds a
> mental burden to deciding and knowing where keys live; the second is a
> problem, isn't it?
>
> Dynamic languages can do what I want at the cost of typesafety. So I might
> just prefer to make that same tradeoff for one mechanism in my code...
> "famous
> last words"? I hope not. I hope it just works and I don't have nightmares
> about lurking segfaults. :)
>
>
> On Thu, Apr 4, 2013 at 3:04 AM, David House <dhouse@janestreet.com> wrote:
>>
>> I don't quite understand the problem. Here's an example of how one
>> might use univ_map:
>>
>> open Core.Std
>>
>> module Keys : sig
>>   val recovery : int Univ_map.Key.t
>>   val resist_pain : float  Univ_map.Key.t
>> end = struct
>>   let recovery = Univ_key.Key.create "recovery" Int.sexp_of_t
>>   let resist_pain = Univ_key.Key.create "resist_pain" Float.sexp_of_t
>> end
>>
>> (In practice this might be two files: keys.ml with the implementation
>> and keys.mli with the signature.) You can then add things as follows:
>>
>> let add map ~key ~data = Univ_map.add_exn map key data in
>> let map =
>>   Univ_map.empty
>>   |> add ~key:Keys.recovery ~data:4
>>   |> add ~key:Keys.resist_pain ~data:10.
>> in
>> ...
>>
>> On 4 April 2013 09:37, Anthony Tavener <anthony.tavener@gmail.com> wrote:
>> > Thank-you for the advice and pointers, folks...
>> >
>> > Well, the common problem is still the same one I've been struggling
>> > with:
>> > "creating keys", and having to access them.
>> >
>> > I can't create keys "type-free" in a common module. As I figured...
>> > having
>> > "modifier.ml" with a bunch of Key.create will have monomorphic types
>> > which
>> > can't be resolved since with no usage in that module to make the type
>> > concrete. I had a nagging feeling I'd need a "whole-program" compiler...
>> >
>> > Instead I'd have to create keys in modules where they are used... but
>> > then I
>> > might have a mess of keys like Wounds.recovery, Combat.resist_pain, ...
>> > the
>> > problem being that only a fraction of these keys actually make sense
>> > being
>> > associated to a particular module, and it gets confusing to know which
>> > (of
>> > several candidates) I decided to stash them into. This was the
>> > attraction to
>> > polymorphic variants (which I rarely use) -- they give a pre-ordained
>> > unique
>> > ID based on a simple name... no declaration, and no module prefixing,
>> > which
>> > seems important to me for this case.
>> >
>> > Note that I have a "database" of tables with different types
>> > (implemented by
>> > first-class modules!), and it works great for the bulk of my game-state,
>> > but
>> > each table is well-populated and heavily used in consistent manner.
>> > These
>> > modifiers though... they're a bit like ad-hoc message passing, where I
>> > can
>> > submit any message and anywhere else add a snippet of code to interpret
>> > it
>> > (not that I have any of that going on, otherwise it might hold the
>> > solution!).
>> >
>> >
>> >
>> > On Thu, Apr 4, 2013 at 1:38 AM, Raphaël Proust <raphlalou@gmail.com>
>> > wrote:
>> >>
>> >> On Thu, Apr 4, 2013 at 1:45 AM, Anthony Tavener
>> >> <anthony.tavener@gmail.com> wrote:
>> >> > […]
>> >>
>> >> And yet-another-solution, Ocsigen's Polytable:
>> >> http://ocsigen.org/ocsigenserver/api/Polytables
>> >>
>> >>
>> >> Cheers,
>> >> --
>> >> ______________
>> >> Raphaël Proust
>> >
>> >
>
>