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Re: Pre-RFC: `unknown` versus `undef`

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Martijn Lievaart
December 19, 2021 10:46
Re: Pre-RFC: `unknown` versus `undef`
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Op 19-12-2021 om 11:41 schreef Darren Duncan:
> On 2021-12-19 2:01 a.m., Martijn Lievaart wrote:
>> So what does Excuse("unknown value") plus Excude("unauthorized) 
>> return? You just reintroduced the same problem that made you decide 
>> against NVL, unless I'm missing something.
> The answer is that attempting to do that fails because addition is not 
> defined for Excuse values.  For example, it could result in something 
> like:
>   Excuse("No_Matching_Routine_Found", "plus", [@_])
> That Excuse value can either be returned, or if desired, thrown as an 
> exception.
> A key part of what makes this NOT N-valued-logic is that the result of 
> trying to use some arbitrary non-existent routine with an Excuse value 
> always returns the same base result, an Excuse of subtype 
> "No_Matching_Routine_Found", with added info naming the name and 
> arguments attempted, and that there is NOT logic to test every 
> possible Excuse subtype to determine what kind of result to give.
> Basically an Excuse is an unthrown exception, and is treated similarly.
> In this context, the version where there is exactly the 1 singleton 
> Unknown etc is analagous to die() always throwing the same generic 
> exception "I died" without any further information.

Ahh, makes sense. I'm still not completely convinced it's a good idea 
(nor saying it isn't!), but I agree you solved the original problem.



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