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Summary: Annotation Syntax Proposals

From: Andy Seaborne <andy@apache.org>
Date: Tue, 19 Jan 2021 18:10:38 +0000
To: public-rdf-star@w3.org
Message-ID: <9a0c1b89-b1a1-1efd-5766-a258ca1cb2e7@apache.org>
This is the action for issue 84.

Annotation syntax is additional syntax, for Turtle-star, to make it more 
convenient to write a triple and also add triples about that triple. It 
generates multiple triples.

A convenient way to write both:

:s :p :o .
<< :s :p :o >> :q .z .

It does not allow Turtle to say something that could not be said with 
the fundamental <<>> syntax.

The proposals are of the form:

:s :p :o annotationBlock? .

where the annotation block is some syntax and a predicateObjectList 
(Turtle grammar rule [7]) with <<>> as the subject.

== Proposals

There have been several proposals, as well as more possibilities noted 
without advocacy.

They fall into 2 categories: both have start-end delimiters and allow 
nesting, with different delimiters for start and for end.

Type 1: Use { } with a introduction character.

Simple { } would work in Turtle because {} aren't used anywhere else but 
using the same syntax for SPARQL and N3 would be visually unclear.

So type 1 syntax uses an introduction character which as "@" - others 
have been suggested.


:subject :predicate :object @{ :source :URL } .

:subject :predicate :object @{ :source :URL ;
                                :author "Alice"
                              } .

:subject :predicate :object @{ :source :URL ;
                                :author "Alice" @{ :saidBy "Bob" }
                              } .

"@{" is a new token in the grammar (no white space between the two 

Type 2: Use distinctive enclosing delimiting pair, each is a new token.

One suggestion that works is "{|" ...  "|}"

"|" is one of several possible choices to go with "{", "}"

:subject :predicate :object {| :source :URL |} .

:subject :predicate :object {| :source :URL ;
                                :author "Alice"
                             |} .

:subject :predicate :object {| :source :URL ;
                                :author "Alice" {| :saidBy "Bob" |}
                             |} .

== Requirements and notes

There are some requirements and constraints on the syntax design. Some 
are technically necessary and some are desirable. Visual appearance is 
also a factor.

+ SPARQL basic graph pattern matching uses a similar syntax to Turtle - 
it is "turtle with variables". Annotation syntax should fit naturally to 
being added to SPARQL so that it looks like Turtle.

SPARQL uses many extras characters not used for syntax in Turtle. These 
can be double up for annotation syntax (it appears to be technically 
possible) but it leaves the question of whether it is visually clear.

+ Is nesting allowed in the annotationBlock? that is, be able to add 
annotations to the extra triples inside the annotation block? (Technical 
consequence: if not, then the Turtle grammar has to have more changes to 
exclude this possibility by having versions of some grammar rules with 
and without the annotation syntax, and also in the grammar rules a 
production uses. Annotation is part of predicateObjectList.

Nesting requires a delimiter style - mark the start and end of the 
annotation block.

+ Other Turtle syntax

Turtle allows predicateObjectLists and objectLists.

A consequence of the annotationBlock is:

:s :p1 :o1 annotationBlock1? ;
    :p2 :o2 annotationBlock2? .

:s :p :o1 annotationBlock1? , :o2 annotationBlock2? .

+ RDF Collections (Turtle lists)

There has been no proposal for annotation of elements combined with list 
convenience syntax.

+ Alignment to other languages

TriG and N3 are languages that share syntax features with Turtle.

Fitting into TriG is quite natural.

Of note in N3 is that "{ ... }" is already a language feature - a 
"formula".  It contains a number of N3-triples. It can go in the 
subject, predicate or object position.

N3 CG work: https://w3c.github.io/N3/spec/

Formula rule in the grammar:

+ Non-requirement

Annotation syntax is for Turtle-star, not N-Triples/N-Quads, in a 
similar way that the syntax "[ :p 123 ]" is not part of N-triples.
Received on Tuesday, 19 January 2021 18:10:53 UTC

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