Re: Query Review Part 2

From: Steve Harris <steve.harris@garlik.com>
Date: Fri, 16 Dec 2011 11:30:25 +0000
Cc: birte.glimm@uni-ulm.de, SPARQL Working Group <public-rdf-dawg@w3.org>
To: Andy Seaborne <andy.seaborne@epimorphics.com>
On 2011-12-15, at 11:30, Andy Seaborne wrote:

> One @@Steve.

…

>> Yes, I see. I'm not particularly happy with the notation of μ(expr) used to denote evaluating expr w.r.t. μ. Previously I used •, as in μ • expr but that didn't work for some people.
>>
>> Note, this isn't specifically an issue for implicit grouping, it applies on any grouping with only constants, though those would be odd use cases, e.g. GROUP BY 23, or GROUP BY NOW() will end up in the same place.
>>
>> I don't really have a sensible suggestion for how to fix this, the best I can think of is some explanatory text saying what μ(expr) is trying to express.
>
> @@Steve
>
> μ is a partial mapping of variables to values.
>
> μ(expr) would be the a substitution of the variables for the terms given by μ.  But not the value of the expression.
>
> So with μ being ?x=1 and expr being ?x+2 then μ(expr) is the expression "1+2" but nothing causes a call to "+" to turn that into 3 (a value).
>
> If you want the value of expr_i given μ, then you need something like:
>   expr_i(μ)
>
> which is used elsewhere (from 1.0 days).
>
> I have explicitly stated that "expr(μ)" is the value of the expr given that solution mapping μ in "18.1.8 Solution Mapping"

OK, great. I'll try and track down any uses of this and change it.

Cheers,
Steve

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