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Re: Missing LET (Assignment) in SPARQL 1.1

From: Paul Gearon <gearon@ieee.org>
Date: Mon, 26 Oct 2009 20:35:41 -0400
Message-ID: <a25ac1f0910261735g12808a37k8cd2eaa20e5bd3bf@mail.gmail.com>
To: Holger Knublauch <yahoo@knublauch.com>
Cc: public-rdf-dawg-comments@w3.org
Hi Holger,

While I agree with Lee's assessment, I also think that you're right in
that simple variable assignment is important.

I'd like to point out that almost every implementation is already
doing this, and I believe that they're using identical (or nearly
identical) syntax. The fact that so many parse with ARQ is a good
start. I also know that some non-ARQ versions (like mine) have just
duplicated the ARQ syntax.

So while I don't think it will make the standard (and am disappointed
that it won't) it may be possible to get a note published about it to
recommend best practice. I believe that the only impediment to this is
that it requires someone with the time to write it up.

Regards,
Paul Gearon

On Mon, Oct 26, 2009 at 7:56 PM, Holger Knublauch <yahoo@knublauch.com> wrote:
> Thanks, Lee. I appreciate you taking the time to assemble all this
> information.
> I have made some experiments with the proposal to use sub-selects plus
> project expressions in a random sample of some of my typical queries. You
> can see three cases below. Without understanding all implications from a
> SPARQL engine and algebra point of view, my impression is that the mapping
> appears to be straight forward, but that it leads to very verbose code. And
> I did not even try to find the really bad cases.
> I am therefore wondering whether LET can be introduced as syntactic sugar
> similar to some of the new OWL 2 extensions that do not change the semantics
> but only provide additional mappings from syntax to semantics - this is
> hopefully easier to manage for the WG?
> Thanks,
> Holger
>
>
> ----
> From the currency conversion example on my blog
> http://composing-the-semantic-web.blogspot.com/2009/09/currency-conversion-with-units-ontology.html
>
> The original current query is
> SELECT ?newLiteral
> WHERE {
>     LET (?fromCurrency := datatype(?arg1)) .
>     LET (?rate := currencies:getRateByCurrencies(?fromCurrency, ?arg2)) .
>     LET (?fromValue := xsd:double(?arg1)) .
>     LET (?newValue := (?fromValue * ?rate)) .
>     LET (?newLiteral := smf:cast(?newValue, ?arg2)) .
> }
> Using nested queries with well-meaning formatting would create something
> like
> SELECT ?newLiteral
> WHERE {
> {
> SELECT (datatype(?arg1) AS ?fromCurrency)
> (xsd:double(?arg1) AS ?fromValue) WHERE {}
> }
> {
>
> SELECT (currencies:getRateByCurrencies(?fromCurrency, ?arg2) AS ?rate) WHERE {}
> }
> {
> SELECT ((?fromValue * ?rate) AS ?newValue) WHERE {}
> }
> {
> SELECT (smf:cast(?newValue, ?arg2) AS ?newLiteral) WHERE {}
> }
> }
> Using a single expression would be
> SELECT (smf:cast((xsd:double(?arg1) *
> currencies:getRateByCurrencies(datatype(?arg1), ?arg2)), ?arg2) AS ?newLiteral)
> WHERE {
> }
> The example is a bit atypical because it exclusively uses LETs, and not even
> a triple match. It also uses externally pre-bound variables. But still it
> gives some insights.
>
> ---
> In the following function body, project expressions work actually ok, but
> keep fingers crossed that you do not have to return multiple of such
> computed values in the SELECT:
> SELECT ?value
> WHERE {
>     ?arg2 qud:conversionMultiplier ?M1 .
>     ?arg2 qud:conversionOffset ?O1 .
>     ?arg3 qud:conversionMultiplier ?M2 .
>     ?arg3 qud:conversionOffset ?O2 .
>     LET (?value := ((((?arg1 * ?M1) + ?O1) - ?O2) / ?M2)) .
> }
> would become
> SELECT (((((?arg1 * ?M1) + ?O1) - ?O2) / ?M2) AS ?value)
> WHERE {
>     ?arg2 qud:conversionMultiplier ?M1 .
>     ?arg2 qud:conversionOffset ?O1 .
>     ?arg3 qud:conversionMultiplier ?M2 .
>     ?arg3 qud:conversionOffset ?O2 .
> }
> ---
> Here is an example from the SPIN box computer game, using LET in SPARQL
> rules. This is a very typical use case actually:
> # Rule1: Collect and replace diamond if possible
> CONSTRUCT {
>     ?diamond spinbox:replaceWith spinbox:Space .
>     ?world boulders:diamondsCollected ?newDiamondsCount .
> }
> WHERE {
>     ?world spinbox:field ?this .
>     ?world spinbox:keyDirection ?direction .
>     LET (?diamond := spinbox:getNeighbor(?this, ?direction)) .
>     ?diamond a boulders:Diamond .
>     ?world spinbox:field ?this .
>     ?world boulders:diamondsCollected ?oldDiamondsCount .
>     LET (?newDiamondsCount := (?oldDiamondsCount + 1)) .
> }
> This would become
> # Rule1: Collect and replace diamond if possible
> CONSTRUCT {
>     ?diamond spinbox:replaceWith spinbox:Space .
>     ?world boulders:diamondsCollected ?newDiamondsCount .
> }
> WHERE {
>     ?world spinbox:field ?this .
>     ?world spinbox:keyDirection ?direction .
> {
> SELECT (spinbox:getNeighbor(?this, ?direction) AS ?diamond)
> WHERE {
> }
> }
>     ?diamond a boulders:Diamond .
>     ?world spinbox:field ?this .
>     ?world boulders:diamondsCollected ?oldDiamondsCount .
> {
>      SELECT ((?oldDiamondsCount + 1) AS ?newDiamondsCount)
> WHERE {
> }
> }
> }
>
> On Oct 25, 2009, at 8:34 PM, Lee Feigenbaum wrote:
>
> Hi Holger,
>
> Thanks for the feedback. Unfortunately, assignment is not on the current
> Working Group's road map for standardization at this time. Here's how we got
> to this point:
>
> From roughly March through May, the WG considered around 40 potential new
> features[1] for the SPARQL landscape, including assignment[2]. At the time,
> we documented two implementations (ARQ and Open Anzo) and the support that
> you expressed for the feature back in March[3].
>
> In going through the features, the WG discussed Assignment in our March, 31
> teleconference. You can see the discussion at the time at [4], the results
> of which was a straw poll result of 7/6/3, indicating some support and
> several notes of concern.
>
> Later in the process, we took a survey of WG member's prioritized
> preferences of the proposed features. Steve Harris whipped up the Condorcet
> results of the survey which you can see at [5]. Assignment was in the middle
> of the pack.
>
> At the group's first face-to-face meeting in May, assignment was discussed
> once more[6], with significant concerns expressed from Garlik and OpenLink
> Software, strong support from Clark & Parsia, and expressions ranging from
> indifference to mild support from other WG members (as I read the minutes &
> recollect the conversation). In the end, the group resolved to accept the
> list of feature proposals at [7], and Kendall's concerns about the
> relationship between assignment and projected expressions was addressed by
> Steve H at [8].
>
> Since then, the group has been rechartered with a specific mandate to work
> on the features decided during the first phase of the group's lifetime[9].
> It's my hope & belief that if projected expressions do not end up fulfilling
> most users' needs, that implementors will extend their SPARQL
> implementations with assignment or a similar capability, and we will then
> revisit this in the next round of SPARQL standardization.
>
> hope this is helpful,
> Lee
>
>
> [1] http://www.w3.org/2009/sparql/wiki/Category:Features
> [2] http://www.w3.org/2009/sparql/wiki/Feature:Assignment
> [3]
> http://lists.w3.org/Archives/Public/public-rdf-dawg-comments/2009Mar/0009.html
> [4] http://www.w3.org/2009/sparql/meeting/2009-03-31#assignment
> [5] http://plugin.org.uk/misc/votes2.svg
> [6] http://www.w3.org/2009/sparql/meeting/2009-05-06
> [7]
> http://www.w3.org/2009/sparql/wiki/index.php?title=FeatureProposal&oldid=744
> [8] http://lists.w3.org/Archives/Public/public-rdf-dawg/2009AprJun/0231.html
> [9] http://www.w3.org/2009/05/sparql-phase-II-charter
>
> Holger Knublauch wrote:
>
> Dear WG,
>
> reading through the drafts (great to have them already!) I am confused about
> the future of Assignments (LET keyword in Jena) which has proven to be
> absolutely essential for many of our customers projects. The SPARQL 1.1
> working group seems to have converged in favor of supporting Project
> expressions and subqueries only, but these IMHO fail to address the
> requirements below.
>
> Problem 1: How to create new values for CONSTRUCT queries
>
> Project expressions solve some problems for SELECT queries, but the major
> use cases of LET have been in CONSTRUCT queries. I only see subqueries as a
> (poor) way of creating new values for use in the CONSTRUCT clause. Creating
> a subquery for every LET looks like a very user unfriendly mechanism to me.
>
> Problem 2: Verbosity
>
> We often work with complex transformations such as string operations that
> are best split into multiple steps. Project expressions do not allow using
> intermediate variables, such as below and would force users to chain
> together very long spaghetti expressions such as SELECT (?x
> ex:function3(ex:function2(ex:function1(?y))). Imagine this with some more
> complex expressions and it quickly becomes completely unreadable. Also,
> consider you would want to reuse intermediate values in multiple places, to
> avoid duplicate processing.
>
> *SELECT* ?x ?r
>
> *WHERE* {
>
> ?x ex:property ?y .
>
> *LET* (?helper1 := ex:function1(?y)) .
>
> *LET* (?helper2 := ex:function2(?helper1)) .
>
> *LET* (?r := ex:function3(?helper2)) .
>
> }
>
> The LET keyword has solved both problems nicely and in the most general way,
> and would make project expressions superfluous.
>
> I would appreciate pointers to the discussions that led to the decision to
> not support Assignments at this stage.
>
> Thanks
>
> Holger
>
> PS: For a parallel thread on jena-dev (with Andy's response), see
>
> http://tech.groups.yahoo.com/group/jena-dev/message/41903
>
>
Received on Tuesday, 27 October 2009 00:36:18 GMT

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