use decimal

Maciej Stachowiak mjs at apple.com
Wed Sep 17 18:50:26 PDT 2008


On Sep 17, 2008, at 6:22 PM, Mark S. Miller wrote:

> On Wed, Sep 17, 2008 at 2:18 PM, Sam Ruby <rubys at intertwingly.net>  
> wrote:
> On Wed, Sep 17, 2008 at 12:49 PM, Brendan Eich wrote:
> > On Sep 17, 2008, at 6:54 PM, Douglas Crockford wrote:
> >
> > > It requires that  a === b  produce the same result as
> > > typeof a == typeof b && a == b.
>
> Agreed.
>
> > Without "use decimal", typeof 1.1m must not be "number" to preserve
> > this same invariant. Otherwise (without "use decimal") 1.5m == 1.5
> > but 1.1m != 1.1, so without making typeof 1.5m != typeof 1.1m, we
> > cannot have typeof 1.5m == "number".
> >
> > I do not believe anyone wants typeof 1.5m != typeof 1.1m.
>
> I agree that typeof 1.5m == typeof 1.1m. However, I do not agree  
> that 1.1m == 1.1. These should be !=, since they represent distinct  
> real numbers.

It seems to me that Brendan said 1.1m != 1.1 and did not question this.

> I do agree that 1.5m == 1.5, but only because they represent the  
> same real number, not because they look the same. This extends the  
> notion of cohort across the decimal/binary divide.
>
> -0, 0, -0m, 0m, 0.000m
>
> may all be computationally distinct values. (Is 0.00m distinct from  
> 0m?) If they are, then they should therefore be distinguished by any  
> Object.identical() and Object.indentityHash() operations. But they  
> all represent the same real number, so they should be ==. If we  
> agree that they should also all be typeof 'number', then they should  
> also all be === and be considered generalized cohorts.

Brendan's premise is that 1.5m has different operational behavior from  
1.5 and therefore it would be wrong to make them '==='. I tend to  
agree. Making '===' treat distinct values the same and then  
introducing a new Object.identical to tell the difference seems  
inelegant.

>
>
> If typeof all these were 'number', then binary and decimal could  
> share a single NaN, Infinity, and -Infinity. Only the remaining  
> finite numeric values would be distinctly either binary or decimal.
>
> I agree with Sam and apparently Crock and everyone else that mixed  
> mode should coerce to decimal. Or, at least, I have no objection to  
> that position.

If that is the case then 1.5m / 10.0 != 1.5 / 10.0, and thus it seems  
wrong for 1.5m and 1.5 to be '==='.

> Were we to adopt this, then I think "use decimal" should condition  
> only whether an unqualified numeric literal be interpreted as binary  
> or decimal floating point. We should then have a suffix which means  
> binary floating point, so you can say it explicitly. Nothing else  
> about the numerics should be conditioned by the pragma.


Perhaps that suffix could be 'f' as in C/C++.

Regards,
Maciej

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