### Querying lists correctly In this section we show how you can correctly query ordered lists in SPARQL. Staying with our example list, let's support we want to generate HTML widgets that enumerate the steps required for a specific recipe. We want to build a function in SPARQL that has the following signature: (`rdf:List`) → `rdf:HTML`. We can define this signature in SHACL Advanced: ```turtle prefix listf: prefix list-param: listf:elements a sh:SPARQLFunction; sh:parameter list-param:list; sh:returnType rdf:HTML; sh:select '...'. list-param:list a sh:Parameter; sh:class rdf:List; sh:order 1; sh:path list-param:list. ``` Notice the following details: - We define an instance of `sh:SPARQLFuncton`. - We define a parameter list with `sh:parameter`. - We define individual parameters as instances of `sh:Parameter`. - We define the return type with `sh:returnType`. The only thing that remains to be specified is a SPARQL query string (`sh:select`) that implements this function signature. The essence of our query is the insight that the index at which an element occurs in a list is equal to the number of intermediate sublists to the start of the list (see [Diagram 1](#user-content-diagram-1)). The number of sublists can be counted using the aggregate function `count`. Ordering the elements based on this count will return the list in the correct order. ```sparql select (strdt(concat('

',group_concat(str(?element);separator='
'),'

'),rdf:HTML) as ?elements) { { select ?element { $list rdf:rest* ?sublist. ?sublist rdf:rest*/rdf:first ?element. } group by ?element order by (count(?sublist)) } } ``` Notice the following details: - We use function [`strdt`](strdt) to create a type literal with datatype `rdf:HTML`. - We use function [`concat`](concat) and aggregate function [`group_concat`](group_concat) in the projection to create a valid ordered list serialization in HTML5. - We use a [sub-select](sub-select) to order the elements internally. - We use the [`order by`](order-by) clause to perform the ordering. - We use [property path](#property-path) notation to traverse the linked list structure (using the `*`-operator and the `/`-operator). Most importantly, notice that we only used standardized features from RDF, SPARQL, and SHACL. When we insert [Query 1](#user-content-query-1) into the `sh:select` literal of [Snippet 3](#user-content-snippet-3), we finalize our implementation of the SPARQL Function. When we load this snippet into a standards-conforming SPARQL implementation, we can retrieve the ordered list of elements as follows: ```sparql prefix listf: select (listf:elements(?stepsList) as ?stepsHtml) { recipe:123 def:steps ?stepsList. } ```

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