I want to validate the existing rdfs:domain and rdfs:range statements of an existing ontology and knowledge base with SHACL.
However it seems like it is extremely verbose to do that with SHACL.
Existing Definition
:prop1 a owl:ObjectProperty;
rdfs:domain ?A;
rdfs:range ?B.
In SHACL
:AShape a sh:NodeShape;
sh:targetClass :A;
sh:property [sh:path :prop1];
sh:closed true.
:ADomainShape a sh:NodeShape;
sh:targetSubjectsOf :prop1;
sh:class :A.
:prop1RangeShape a sh:NodeShape;
sh:targetObjectsOf :prop1;
sh:ClassB.
When you have dozens of properties, this adds a large amount of ceremony over something that is already declared in the original domain and range statements.
While it is possible to speed up the process using a script or a multiline editor, it still seems unnecessary to me.
Is there a way to tell a SHACL validator like PySHACL to just validate the existing domain and range statements without requiring all those extra triples?
Let me start by saying that rdfs:domain and rdfs:range are not constraints and don't mean what you imply. They are merely producing inferences. Having said this, many people have in the past used them to "mean" constraints simply because there was no other modeling language. For background, see
https://www.topquadrant.com/owl-blog/
If you don't want to duplicate the RDFS triples as individual SHACL constraints, you can write a single generic SHACL shape that has sh:targetSubjectsOf rdfs:domain (and range) and then uses a SPARQL constraint to check that there is no instance of a class other than the domain class that has values for the given property. The end result would be that all rdfs:domain statements would be checked at once.
But arguably RDFS should be left alone. If you want to use closed-world semantics you should use a language that has been designed for that purpose, i.e. SHACL.
(BTW there is a Discord group in case you want a higher bandwidth to discuss such things https://twitter.com/HolgerKnublauch/status/1461590465304662019)
Related
After re-reading the off/on topic lists, I'm still not certain if this question is best posted to this site, so apologies in advance, if it is not.
Overview:
I am working on a project that mixes several programming languages and we are trying to determine important considerations for the command used to call one in particular.
For definiteness, I will list the specific languages; however, I think the principles ought to be general, so familiarity with these specific languages is not really essential.
Specific Context
Specifically, we are using: Maxima, KaTeX, Markdown and HTML). While building the prototype, we have used the following (I believe, standard) conventions:
KaTeX delimited by $ $ or $$ $$;
HTML delimited by < > </ > pairs;
Markdown works anywhere in the body, except within KaTeX or Maxima environments;
The only non-standard convention we used during this design phase was to call on Maxima using \comp{<Maxima commands>}. This command works within all the other environments (which is desired).
Now that we are ready to start using the platform, it has become apparent that this temporary command for calling Maxima is cumbersome for our users. The vast majority of use cases involve simply calling a single variable or function, e.g.
As such, we have $\eval{function-name()}(\eval{variable-name})$
as opposed to actually using Maxima for computation, e.g.
Here, it is clear that $\eval{a} + \eval{b} = \eval{a+b}$
(where \eval{a+b} would return the actual sum, as calculated by Maxima).
As such, our users would prefer a delimiter-less command option for invoking a single variable or function, e.g. \#<variable-name-in-Maxima> and \#<function-name>(<argument>) (where # is some reserved character not used in the other languages), while also having a delimited alternative for the (much less frequent) cases where they actually want to use Maxima for computation; perhaps something like \#{a+b}.
However, we have a general sense that this is not a best practice, even though we can't foresee any specific issue.
"Research" / Comparisons:
Indeed, there is precedence for delimit-less expressions for single arguments like x^2 (on any calculator) or Knuth's a \over b in TeX (which persists in LaTeX with \frac12 being parsed as \frac{1}{2}.
IIRC Knuth's point was that this delimit-less notation was more semantic (and so, in his view, preferable), and because delimiters can be added, ambiguity can be avoided, whenever the need arises: e.g. x^{22}, {a+b}\over{c+d} and \frac{12}{3}.
The Question, Proper:
Can anyone point to or explain actual shortcomings / risks associated with a dual solution like:
\#<var>, \#<function>(<arg>) and,
\#[<extended expression>],
(where # is a reserved (& escapable) character), for calling one language amongst others, as opposed to only using a delimited command?
Any alternative suggestions for how to achieve the ease-of-use and more semantic code enabled by the above solution, while keeping the code unambiguous would be very much welcome and appreciated.
I basically don't look for an answer on how to do something but I found how to do it, yet want more information. Hope this kind of question is OK here.
Since I just discovered this the code of a game I'm modding I don't have any idea what should I google for.
In Lua, I can have for example:
Account = {balance = 0}
function Account.withdraw (v)
self.balance = self.balance - v
end
I can have (in another lua file)
function Account.withdrawBetter (v)
if self.balance > v then
self.balance = self.balance - v
end
end
....
--somewhere in some function, with an Account instance:
a1.withdraw = a1.withdrawBetter
`
What's the name for this "technique" so I can find some more information about it (possible pitfalls, performance considerations vs. override/overwrite, etc)? note I'm only changing withdraw for the particular instance (a1), not for every Account instance.
Bonus question: Any other oo programming languages with such facility?
Thanks
OO in Lua
First of all, it should be pointed out that Lua does not implement Object Oriented Programming; it has no concept of objects, classes, inheritance, etc.
If you want OOP in Lua, you have to implement it yourself. Usually this is done by creating a table that acts as a "class", storing the "instance methods", which are really just functions that accept the instance as its first argument.
Inheritance is then achieved by having the "constructor" (also just a function) create a new table and set its metatable to one with an __index field pointing to the class table. When indexing the "instance" with a key it doesn't have, it will then search for that key in the class instead.
In other words, an "instance" table may have no functions at all, but indexing it with, for example, "withdraw" will just try indexing the class instead.
Now, if we take a single "instance" table and add a withdraw field to it, Lua will see that it has that field and not bother looking it up in the class. You could say that this value shadows the one in the class table.
What's the name for this "technique"
It doesn't really have one, but you should definitely look into metatables.
In languages that do support this sort of thing, like in Ruby (see below) this is often done with singleton classes, meaning that they only have a single instance.
Performance considerations
Indexing tables, including metatables takes some time. If Lua finds a method in the instance table, then that's a single table lookup; if it doesn't, it then needs to first get the metatable and index that instead, and if that doesn't have it either and has its own metatable, the chain goes on like that.
So, in other words, this is actually faster. It does use up some more space, but not really that much (technically it could be quite a lot, but you really shouldn't worry about that. Nonetheless, here's where you can read up on that, if you want to).
Any other oo programming languages with such facility?
Yes, lots of 'em. Ruby is a good example, where you can do something like
array1 = [1, 2, 3]
array2 = [4, 5, 6]
def array1.foo
puts 'bar'
end
array1.foo # prints 'bar'
array2.foo # raises `NoMethodError`
I am creating an ontology based on OWL/RDF/RDFS. My first ontology schema has namespaces as :
#prefix abc: https://example.com/a#
I want to change the namespace the next version of the ontology as
#prefix def: https://example-new.com/b#
But I dont want the previous users of the ontology to be affected at all. I was thinking if there is a way to define equivalent namespaces, and classify that the first name space will be deprecated. I am not sure if there is any provision in the OWL/RDF or even Dublin-core to so do.
Any help is appreciated. Thanks.
You can't do this at the namespace level but you can declare all old classes and properties as equivalent to the new ones; you'd keep the old IRIs in the equivalent axioms and declaration axioms only. Then any third part that uses a reasoner would be able to run queries just as before; parties not using a reasoner would have to rewrite their queries following equivalent axioms (something that they might already be doing for other similar use cases).
I wonder why we have to load an ontology, also provide its namespace while querying it? Why loading the ontology is not enough?
To understand my question better, here is a sample code:
g = rdflib.Graph()
g.parse('ppp.owl', format='turtle')
ppp = rdflib.Namespace('http://purl.org/xxx/ont/ppp/')
g.bind('ppp', ppp)
In line 2, we have opened the ontology (ppp.owl), but in line 3 we also provided its namespace. Does namespace show the program how to handle the ontology?
Cheers,
RF
To specify an element over the semantic web you need its URI: Unique Resource Identifier, which is composed of the namespace and the localname. For example, consider Person an RDF class; how would you differentiate the Person DBpedia class http://dbpedia.org/ontology/Person from Person in some other ontology somewhere? you need the namespace http://dbpedia.org/ontology/ and the local name Person. Which both uniquely identify the class.
Now coming back to your specific question, when you query the ontology, you might use multiple namespaces, some namespaces may not be the one of your ontology. You need other namespaces for querying your own ontology, e.g. rdf, rdfs, and owl. As an example, you can rarely write an arbitrary query without rdf:type property, which is included under the rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns> namespace, not your ontology namespace. As a consequence, you need to specify the namespace.
Well, now as you should know why to use a namespace, then we can proceed. Why to repeat the whole string of the namespace each time it is needed? It is nothing more than a prefix string appended to the local names to use in the query, to avoid writing exhaustively the full uri. See the difference between <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> and type.
Edit
As #AKSW says, as a conclusion, there is no need to declare a namespace in order to work with the ontology but it increases the convenience when working quite often with resources whose URI has particular namespace.
I read quite a lot about the visitor pattern and its supposed advantages. To me however it seems they are not that much advantages when applied in practice:
"Convenient" and "elegant" seems to mean lots and lots of boilerplate code
Therefore, the code is hard to follow. Also 'accept'/'visit' is not very descriptive
Even uglier boilerplate code if your programming language has no method overloading (i.e. Vala)
You cannot in general add new operations to an existing type hierarchy without modification of all classes, since you need new 'accept'/'visit' methods everywhere as soon as you need an operation with different parameters and/or return value (changes to classes all over the place is one thing this design pattern was supposed to avoid!?)
Adding a new type to the type hierarchy requires changes to all visitors. Also, your visitors cannot simply ignore a type - you need to create an empty visit method (boilerplate again)
It all just seems to be an awful lot of work when all you want to do is actually:
// Pseudocode
int SomeOperation(ISomeAbstractThing obj) {
switch (type of obj) {
case Foo: // do Foo-specific stuff here
case Bar: // do Bar-specific stuff here
case Baz: // do Baz-specific stuff here
default: return 0; // do some sensible default if type unknown or if we don't care
}
}
The only real advantage I see (which btw i haven't seen mentioned anywhere): The visitor pattern is probably the fastest method to implement the above code snippet in terms of cpu time (if you don't have a language with double dispatch or efficient type comparison in the fashion of the pseudocode above).
Questions:
So, what advantages of the visitor pattern have I missed?
What alternative concepts/data structures could be used to make the above fictional code sample run equally fast?
For as far as I have seen so far there are two uses / benefits for the visitor design pattern:
Double dispatch
Separate data structures from the operations on them
Double dispatch
Let's say you have a Vehicle class and a VehicleWasher class. The VehicleWasher has a Wash(Vehicle) method:
VehicleWasher
Wash(Vehicle)
Vehicle
Additionally we also have specific vehicles like a car and in the future we'll also have other specific vehicles. For this we have a Car class but also a specific CarWasher class that has an operation specific to washing cars (pseudo code):
CarWasher : VehicleWasher
Wash(Car)
Car : Vehicle
Then consider the following client code to wash a specific vehicle (notice that x and washer are declared using their base type because the instances might be dynamically created based on user input or external configuration values; in this example they are simply created with a new operator though):
Vehicle x = new Car();
VehicleWasher washer = new CarWasher();
washer.Wash(x);
Many languages use single dispatch to call the appropriate function. Single dispatch means that during runtime only a single value is taken into account when determining which method to call. Therefore only the actual type of washer we'll be considered. The actual type of x isn't taken into account. The last line of code will therefore invoke CarWasher.Wash(Vehicle) and NOT CarWasher.Wash(Car).
If you use a language that does not support multiple dispatch and you do need it (I can honoustly say I have never encountered such a situation though) then you can use the visitor design pattern to enable this. For this two things need to be done. First of all add an Accept method to the Vehicle class (the visitee) that accepts a VehicleWasher as a visitor and then call its operation Wash:
Accept(VehicleWasher washer)
washer.Wash(this);
The second thing is to modify the calling code and replace the washer.Wash(x); line with the following:
x.Accept(washer);
Now for the call to the Accept method the actual type of x is considered (and only that of x since we are assuming to be using a single dispatch language). In the implementation of the Accept method the Wash method is called on the washer object (the visitor). For this the actual type of the washer is considered and this will invoke CarWasher.Wash(Car). By combining two single dispatches a double dispatch is implemented.
Now to eleborate on your remark of the terms like Accept and Visit and Visitor being very unspecific. That is absolutely true. But it is for a reason.
Consider the requirement in this example to implement a new class that is able to repair vehicles: a VehicleRepairer. This class can only be used as a visitor in this example if it would inherit from VehicleWasher and have its repair logic inside a Wash method. But that ofcourse doesn't make any sense and would be confusing. So I totally agree that design patterns tend to have very vague and unspecific naming but it does make them applicable to many situations. The more specific your naming is, the more restrictive it can be.
Your switch statement only considers one type which is actually a manual way of single dispatch. Applying the visitor design pattern in the above way will provide double dispatch.
This way you do not necessarily need additional Visit methods when adding additional types to your hierarchy. Ofcourse it does add some complexity as it makes the code less readable. But ofcourse all patterns come at a price.
Ofcourse this pattern cannot always be used. If you expect lots of complex operations with multiple parameters then this will not be a good option.
An alternative is to use a language that does support multiple dispatch. For instance .NET did not support it until version 4.0 which introduced the dynamic keyword. Then in C# you can do the following:
washer.Wash((dynamic)x);
Because x is then converted to a dynamic type its actual type will be considered for the dispatch and so both x and washer will be used to select the correct method so that CarWasher.Wash(Car) will be called (making the code work correctly and staying intuitive).
Separate data structures and operations
The other benefit and requirement is that it can separate the data structures from the operations. This can be an advantage because it allows new visitors to be added that have there own operations while it also allows data structures to be added that 'inherit' these operations. It can however be only applied if this seperation can be done / makes sense. The classes that perform the operations (the visitors) do not know the structure of the data structures nor do they have to know that which makes code more maintainable and reusable. When applied for this reason the visitors have operations for the different elements in the data structures.
Say you have different data structures and they all consist of elements of class Item. The structures can be lists, stacks, trees, queues etc.
You can then implement visitors that in this case will have the following method:
Visit(Item)
The data structures need to accept visitors and then call the Visit method for each Item.
This way you can implement all kinds of visitors and you can still add new data structures as long as they consist of elements of type Item.
For more specific data structures with additional elements (e.g. a Node) you might consider a specific visitor (NodeVisitor) that inherits from your conventional Visitor and have your new data structures accept that visitor (Accept(NodeVisitor)). The new visitors can be used for the new data structures but also for the old data structures due to inheritence and so you do not need to modify your existing 'interface' (the super class in this case).
In my personal opinion, the visitor pattern is only useful if the interface you want implemented is rather static and doesn't change a lot, while you want to give anyone a chance to implement their own functionality.
Note that you can avoid changing everything every time you add a new method by creating a new interface instead of modifying the old one - then you just have to have some logic handling the case when the visitor doesn't implement all the interfaces.
Basically, the benefit is that it allows you to choose the correct method to call at runtime, rather than at compile time - and the available methods are actually extensible.
For more info, have a look at this article - http://rgomes-info.blogspot.co.uk/2013/01/a-better-implementation-of-visitor.html
By experience, I would say that "Adding a new type to the type hierarchy requires changes to all visitors" is an advantage. Because it definitely forces you to consider the new type added in ALL places where you did some type-specific stuff. It prevents you from forgetting one....
This is an old question but i would like to answer.
The visitor pattern is useful mostly when you have a composite pattern in place in which you build a tree of objects and such tree arrangement is unpredictable.
Type checking may be one thing that a visitor can do, but say you want to build an expression based on a tree that can vary its form according to a user input or something like that, a visitor would be an effective way for you to validate the tree, or build a complex object according to the items found on the tree.
The visitor may also carry an object that does something on each node it may find on that tree. this visitor may be a composite itself chaining lots of operations on each node, or it can carry a mediator object to mediate operations or dispatch events on each node.
You imagination is the limit of all this. you can filter a collection, build an abstract syntax tree out of an complete tree, parse a string, validate a collection of things, etc.