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Re: Missing LET (Assignment) in SPARQL 1.1

From: Holger Knublauch <yahoo@knublauch.com>
Date: Fri, 30 Oct 2009 10:49:25 -0700
To: SPARQL Working Group Comments <public-rdf-dawg-comments@w3.org>
Message-Id: <CEDD120F-0955-4E2E-8D52-3182A205A1DD@knublauch.com>
Thanks for pointing this out, Andy. Its good to hear there is already  
some control over the order of execution this way, and this mechanism  
would make the job easier for a LET macro.

But this also dramatically highlights how unattractive the notation  
below would be for real world usage. Not only is it way too verbose,  
but it also requires a mind set that the average user does not have -  
at least my intuition was misleading. Related working groups (in  
particular OWL) have made similar mistakes in the past, trying to  
design a language that is theoretically sound and clear, but  
unintuitive to use as it does not align with the traditional  
imperative/object-oriented way of thinking. I have a long list of  
customer stories (both for SPARQL and OWL) where people were  
disappointed because the language did not produce the results they  
expected. In particular the lack of reliable ordering of FILTER  
clauses often leads to very very slow queries and people believe the  
whole semantic web stack is not ready for prime time yet.

Holger


On Oct 30, 2009, at 3:32 AM, Seaborne, Andy wrote:

> I think you need to write it as:
>
> SELECT ?newLiteral
> WHERE {
>       { SELECT (smf:cast(?newValue, ?arg2) AS ?newLiteral)
>         WHERE
>         {
>           SELECT ((?fromValue * ?rate) AS ?newValue)
>           WHERE
>           {
>             SELECT (datatype(?arg1) AS ?fromCurrency) (xsd:double(? 
> arg1) AS ?fromValue)
>             WHERE
>             {
>                SELECT (currencies:getRateByCurrencies(? 
> fromCurrency, ?arg2) AS ?rate)
>                WHERE {}
>             }
>         }
>     }
> }
>
> To put the inner calculations into the scope of the outer ones.
>
> Whereas
>
> 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 {}
>                 }
> }
>
> evaluates each SELECT separately and joins the results together.  So  
> for example:
>
> SELECT ((?fromValue * ?rate) AS ?newValue) WHERE {}
>
> Is
>
> ------------
> | newValue |
> ============
> |          |
> ------------
>
> And does not see ?rate from the other SELECT
>
>                 Andy
>
> From: public-rdf-dawg-comments-request@w3.org [mailto:public-rdf- 
> dawg-comments-request@w3.org] On Behalf Of Holger Knublauch
> Sent: 26 October 2009 23:56
> To: public-rdf-dawg-comments@w3.org
> Subject: Re: Missing LET (Assignment) in SPARQL 1.1
>
> 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 Friday, 30 October 2009 17:50:15 GMT

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