Revisiting Decimal (generic algorithms)

Brendan Eich brendan at mozilla.com
Fri Jan 16 17:30:40 PST 2009


On Jan 16, 2009, at 4:30 PM, Sam Ruby wrote:

> Indeed.  This is the first time I understood (at a high level) the
> request.  I'm not saying it wasn't explained before, or even that it
> wasn't explained well, but this is the first time I understood it
> (again at a high level, questions on details below).

It's good to get this understood widely -- it probably did not come  
through in the notes from Kona (useful as they were). Sorry on my part  
for that, kudos again to Allen.


> Like Allen says later, most small integers (i.e., the ones that fit
> exactly in a double precision binary value) can simply be retained as
> binary64.

Or machine ints -- ALU >> FPU still.


> I suspect that covers the majority of constants in deployed
> javascript.  Now let's consider the rest.
>
> First, Allen's example:
>
> function fuzz(a) { return a + 0.1}
>
> Where fuzz(0.1)===0.2 and fuzz(0.1m)===0.2m
>
> The only way I can see that working is if the constant is initially in
> a form that either is readily convertible to source, or stores both
> values.  I don't understand how multimethods (on "+"?) affect this.
> If I'm missing something, please let me know (or simply provide a
> pointer to where I can educate myself).

I did, see followup links to reading-lists, from which I'll pick a  
specific link:

http://www.artima.com/weblogs/viewpost.jsp?thread=101605


> Continuing on, let's tweak this a bit.
>
> function fuzz(a) {var b=0.1; return a+b}
>
> I would suggest that if the expectation would be that this function
> behaves the same as the previous one.

It had better!


> My interpretation is that this means that internally there are three
> data types, one that is double, one that is decimal, and one that
> somehow manages to be both.  Internally in that this implementation
> detail ideally should not be visible to the application programmer.
> Again, I could be wrong (in the need for three data types, not on the
> opinion that this should not be visible), but pressing on...

No, Allen allowed for that, but of course this generic type has to  
propagate at runtime through variable and function abstraction.


> function is_point_one(a) {var b=0.1; return b===a}
>
> Is the expectation that this would return true for *both* 0.1 and
> 0.1m?

I don't see how this could work.


>  This leads to a rather odd place where it would be possible for
> triple equals to not be transitive, i.e. a===b and b===c but not
> a!===c.

Er, a!==c ;-).


>  That alone is enough to give me pause and question this
> approach.

Me too.


> Continuing trip down this looking glass, what should typeof(0.1)
> return?  You might come to a different conclusion, and again I might
> be missing something obvious, but if these Schrödinger's catstants
> (sorry for the bad pun) can be assigned to variable, then I would
> assert that typeof(0.1) and typeof(0.1m) should both be 'number'.

It should be clear that I won't go this far. My reply to Allen was  
gently suggesting that his suggestion would not fly on implementation  
efficiency grounds, but I think you've poked bigger holes. I'm still  
interested in multimethods, including for operators.


> Finally, this has bearing on the previous json discussion.  If it is
> possible to defer the binding of a literal value to a particular
> variant of floating point (i.e., binary vs decimal), then there no
> longer is no need for a JSON parse to prematurely make this
> determination.
>
> I suspect that these last two paragraphs will make Kris happy.

The previous paragraphs should induce unhappiness that trumps that  
illusory joy, though.


> But I'll stop here.  I may very well be out in the weeds at this
> point.  But my initial take is that this approach produces a different
> (and somehow more fundamental) set of surprises that the approach than
> we had previously agreed on, and furthermore it isn't clear to me that
> this approach can be implemented in a way that has negligible
> performance impact for applications that never make use of decimal.
>
> But I hope that one or both of you (or anybody else) can point out
> something that I'm missing.

Not me, and I see David-Sarah has observed that dual representation  
cannot be confined to literals.

But I'd still like to encourage thinking outside of the narrow ES3-ish  
box in which Decimal has been cast. If not multimethods, some other  
novel (to ES, not to well-researched language design) is needed.

/be


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