Reader mail bag (Rational)

[Brian Goetz]

Yes, I would frame this one as a "lucky near miss", where there's a 
removable discontinuity in Rational that permits it to work with a small 
amount of extra effort.  The author will likely do the appropriate input 
checking (and possibly GCD reduction) in the explicit ctor, but the key 
is that because of the implicit ctor, the author has to do some 
additional checking elsewhere too.

Because it can all be factored through a `denom` accessor *in this 
case*, it is not very intrusive, but I am not positioning this as a 
general property, as much as "we got lucky with Rational and might get 
lucky with other similar cases that are at the boundary."

I think much of the value of the Rational example is not "building a 
better Rational", as much as a cookbook example of something that 
_almost_ fits the implicitly-constructible mold, and a how-to guide for 
how to deal with this flavor of "almost."  If we had a few others, we'd 
have a cookbook cooking, and that would be a good thing.


On 6/1/2023 1:46 PM, Kevin Bourrillion wrote:
> On Thu, Jun 1, 2023 at 10:35 AM Brian Goetz <brian.goetz at oracle.com> 
> wrote:
>
>     Rational is unfortunate because the default representation (when
>     used improperly) can lead to DBZE, but has a sensible default of
>     zero -- except for that pesky denominator. However, I think this
>     is a removable discontinuity, where the author can make up for
>     this with some careful coding:
>
>
> It helps a bit that you want to canonicalize all 0/n to 
> /something/ anyway, and rational operations are already busy taking 
> the gcd and ensuring positive denominator as it is. The need to 
> internally represent zero as 0/0 probably adds little incremental pain 
> in *this* case, but there will be others where it does. Still, overall 
> it seems like a very fine trade-off.
>
>
> -- 
> Kevin Bourrillion | Java/Kotlin Ecosystem Team | Google, 
> Inc. |kevinb at google.com
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