From: Marnen Laibow-Koser <marnen@...>
Date: 2009-11-17T08:55:43+09:00
Subject: Re: Newbie question:  Defining a numeric type

Seebs wrote:
> On 2009-11-16, Marnen Laibow-Koser <marnen@marnen.org> wrote:
>> a = Stat.new(5)
>> b = Stat.new(5)
>>
>> Should a == b return true or false?
> 
>> If false, then this is probably not a value object in the conventional 
>> sense.
> 
>> If true, then it probably is.
> 
> Hmm.
> 
> I would think it should return true.

So would I.

> 
>> Poor analogy.  I think your paycheck is like your character's entire 
>> stat block, whereas the individual Stat objects seem to be analogous to 
>> the dollar amounts on each like of the paycheck -- and those are 
>> certainly value objects.
> 
> An individual stat might have:
>   base value 10
>   modifier (racial, no duration) +3
>   modifier (training, no duration) +2
>   modifier (drunk, duration 15 turns) -1
> 
> That's "a 14".  It is *equal to* another stat which has a base value of 
> 14
> and no modifiers.  But it's not the same *thing* as that other stat.

Of course it isn't.  Here's how I see this working (for simplicity, I'll 
just assume one modifier):

a = Stat.new 10, 0
b = Stat.new 10, 2
c = Stat.new 12, -2
d = Stat.new 12, -2
c == d # true
a == b # false
a == c # false
a.to_i == c.to_i # true

Does that all seem right?

> 
>> And I think you're probably wrong.
>>
>> john = Character.new
>> john.strength = Stat.new(15)
>> mary = Character.new
>> mary.strength = john.strength
>> mary.strength += 1
>>
>> Now, what should john.strength be? 15 or 16?
> 
> 15.  But you don't copy stats -- you use their values.  

Right!  In other words, they're identified by their value, not by the 
individual object reference.  Ergo, *they are value objects*, by 
definition.

> So what really
> happens is that mary defines strength= such that it applies modifiers
> to mary.strength.

Or simply creates a new immutable value object, which will save some 
headache.

> 
> In fact, in "john.strength = Stat.new(15)", what really happens is that
> john does the same thing -- adjust the already-existing stat so that it 
> will
> have an effective value of 15.

No.  If you call a constructor, you might as well use the object. :)

> 
> Hmm.
> 
> So, thinking about it, it seems like what I'm getting to is that really,
> the "Stat" class is largely internal to characters.  It's a way to 
> express
> something that, in general, no one else should use.  (Not quite true;
> you can query the modifiers that currently apply to a stat, for 
> instance.)

So you *do* want to do something like john.strength.modifiers[:racial], 
right?  If so, then the Stat class is not internal.  Personally, I think 
that's fine -- a stat is not jet a simple number, and so you might as 
well represent it with an appropriate value object.

> For the purposes of nearly any possible interaction, john.strength is
> just the value 15.

The easiest way to achieve that is with conversion and/or coercion.

> 
> You're asking some really good questions here, thanks.
> 
> -s

You're welcome!  These are questions I struggle with as well -- I like 
to learn by teaching.
Best,
--
Marnen Laibow-Koser
http://www.marnen.org
marnen@marnen.org
-- 
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