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Re: LET / BIND / Assignment test cases (ISSUE-57)

From: Lee Feigenbaum <lee@thefigtrees.net>
Date: Thu, 30 Sep 2010 10:12:29 -0400
Message-ID: <4CA49ACD.80605@thefigtrees.net>
To: Andy Seaborne <andy.seaborne@epimorphics.com>, SPARQL Working Group <public-rdf-dawg@w3.org>
On 9/30/2010 5:15 AM, Andy Seaborne wrote:
>
>
> On 29/09/10 03:45, Lee Feigenbaum wrote:
>> When last we talked, we decided we wanted to look at the semantics of a
>> proposed LET/BIND/Assignment clause before choosing the keyword and
>> proceeding with a decision.
>>
>> Here are a few test cases; I use KW (keyword) below where LET or BIND or
>> whatever might go.
>>
>> Data:
>>
>> :s :one 1 .
>> :s :two 2 .
>>
>>
>> 1. Simple
>>
>> SELECT ?sum ?one ?two {
>> ?s :one ?one ; :two ?two .
>> KW (?sum := ?one + ?two)
>> }
>>
>> Result:
>>
>> ?sum ?one ?two
>> -----------------------
>> 3 1 2
>>
>>
>> 2. Calculated value matches existing binding
>>
>> SELECT ?one {
>> ?s :one ?one .
>> KW (?one := 1)
>> }
>>
>> I believe there are two options here from what I've heard
>>
>> Option 1 ("join semantics") result:
>
> Maybe this is more "FILTER semantics" because of the similarity between:
>
> SELECT ?one {
> ?s :one ?one .
> KW (?one := 1)
> }
>
> SELECT ?one {
> ?s :one ?one .
> FILTER (?one = 1)
> }
>
> and the discussion of placement and eliminating rows as below. The
> handling of errors in expressions is more FILTER like as is the
> expressions form
>
> KW (?two := ?one+1)
>
> because ?one is in scope.

...I see what you mean. Under both types of semantics LET/BIND can add 
new columns, but only with join/filter semantics can it eliminate 
bindings in the way a filter would.

>>
>> ?one
>> -----
>> 1
>>
>> Option 2 ("simple semantics") result:
>>
>> Error, can't assign to / bind a variable that's elsewhere bindable in
>> the same scope
>>
>>
>> 3. Calculated value does not match existing binding
>>
>> SELECT ?one {
>> ?s :one ?one .
>> KW (?one := 5)
>> }
>>
>> I believe there are three options here from what I've heard
>>
>> Option 1 ("join semantics") result:
>>
>> ?one
>> -----
>>
>> Option 2 ("simple semantics") result:
>>
>> Error, can't assign to / bind a variable that's elsewhere bindable in
>> the same scope
>>
>> Option 3 ("assignment semantics") result:
>>
>> ?one
>> -----
>> 5The only way to tell is
>
>>
>> NOTE: I don't believe _anyone_ is advocating or implements assignment
>> semantics like this, but I wanted to include it for completeness.
>>
>>
>> 4. Logical order of assignment/binding
>>
>> SELECT ?sum {
>> KW (?sum := ?one + 1) .
>> :s :one ?one .
>> }
>>
>> Option 1 ("in place") result:
>>
>> ?sum
>> ----
>>
>> (no result as ?one has no binding and so the expression is an error)
>>
>> Option 2 ("end of group") result:
>>
>> ?sum
>> ----
>> 2
>>
>> (assignments/binds are treated as with FILTERs, they occur logically
>> after a group's solution set has been evaluated)
>>
>>
>> I believe from the thread at
>> http://lists.w3.org/Archives/Public/public-rdf-dawg/2010JulSep/0116.html
>> that Mulgara implements what I'm calling "simple semantics". Anzo
>> (Glitter) implements "join semantics". ARQ implements "simple semantics"
>> as well.
>
> ARQ implements filter/join semantics, but "in place" not "end of group"
>
> Because of
>
> SELECT ?one {
> KW (?one := 1 )
> :s :one ?one .
> }
>
> SELECT ?one {
> :s :one ?one .
> KW (?one := 1 )
> }
>
> have the same results because of the assign-same-value rule.
>
> So the only way to tell the difference is:
>
> SELECT ?sum {
> KW (?two := ?one+1)
> :s :one ?one .
> }
>
> which causes an error evaluating (?two := ?one+1) for
> unbound-at-the-time ?one.

OK, right, I see.

> I'm happy with changing to end-of-group semantics. It's much the same as
> processing filter (algebra) and placement is very similar to filter
> placement (optimization).

Right, I handle assignments and filters basically together in my 
implementation.

Lee

>
> http://lists.w3.org/Archives/Public/public-rdf-dawg/2010JulSep/0019.html
>
>> Here was the last time we discussed this at a TC:
>>
>> http://www.w3.org/2009/sparql/meeting/2010-07-06#LET__2f_Assignment
>>
>> Please discuss if you'd like, and next week I'd like to decide on
>> inclusion or not (strong lean towards include last time) and, if we
>> choose to include the feature, the semantics based on these options or
>> any other options people suggest and the keyword.
>>
>> Lee
>>
>
>
Received on Thursday, 30 September 2010 14:20:02 GMT

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