Scala + HermiT API and REPL

• The SHAR API
• The SHAR REPL
  • Command Line Interface
  • Language
• Replacing HermiT
• References and Examples

SHAR features a collection of abstract data types for description logic expressions and axioms in a robust API for working with the HermiT reasoner (as well as other OWL-API based reasoners), that includes additional tools such as parsers from two DL notations (formal Unicode-based and an ASCII-only variant). Secondly, SHAR is also a REPL and interpreter for working with HermiT (or other DL (OWL-API) reasoners) directly via an external DSL for defining axioms (and/or loading OWL ontologies) and checking axioms for entailment and satisfiability.

The SHAR API

SHAR features a collection of abstract data types for representing DL concept expressions and axioms, and interacting with reasoners. Consider the following minimal example, where first the SHAR framework is initialized (which can keep track of prefixes and similar state-related things) as well as an empty instance of Hermit:

@main def example: Unit =

  // Instantiate the SHAR framework...
  val shar = Shar()
  import shar._

  // ...and a HermiT intance.
  val kb = mkHermit()

Next, we create a few IRI from Strings, and then add them to the knowledge base; finally, we prove an entailed axiom and print the result:

  val r = for 
    // Parse (and validate) IRIs.
    c1 <- Iri.fromString("<http://example.org#ComputerScientist>")
    c2 <- Iri.fromString("<http://example.org#Scientist>")
    c3 <- Iri.fromString("<http://example.org#Person>")
  yield 
    // Add axioms to the KB.
    kb.addAxiom(Subsumption(NamedConcept(c1), NamedConcept(c2)))
    kb.addAxiom(Subsumption(NamedConcept(c2), NamedConcept(c3)))

    // Prove entailment of axiom.
    kb.prove(Subsumption(NamedConcept(c1), NamedConcept(c3)))

  println(r) // prints Right(true)

Alternatively, we can also initialize HermiT with an OWL ontology, by giving its IRI to the SHAR framework:

@main def exampleOWL: Unit =

  // Instantiate the SHAR framework...
  for 
    ontology <- Iri.fromString("<https://www.w3.org/TR/owl-guide/wine.rdf>")
  do 
    val shar = Shar(OntologyInitialization(ontology))
    import shar._
    val kb = mkHermit()
    val wine = "http://www.w3.org/TR/2003/PR-owl-guide-20031209/wine#"

    val r = for 
      c1 <- Iri.fromParts(wine, "RedWine")
      c2 <- Iri.fromParts(wine, "Wine")
    yield 
      kb.prove(Subsumption(NamedConcept(c1), NamedConcept(c2)))

    println(r) // prints Right(true)

For a concrete project utilizing the SHAR API, see also From Shapes to Shapes.

The SHAR REPL

SHAR features a REPL and interpreter that can be most easily invoked via the included script shar. Let us first look at using shar to interpret script files. Consider a file kb.shar containing the following axiom definitions:

:NiceChild ⊑ :Child ⊓ :Nice
:Child ⊑ :Person
:Person ⊑ :Agent
:Agent ⊑ ∃:knows.:Agent

Invoking ./shar resources/kb.shar parses and then loads these four axioms in HermiT, before terminating and printing nothing. Exciting. So let us extend the SHAR source file with a few more lines, introducing some addtional commands and entailment checks (example.shar):

-- This is the knowledge base (and a comment).
:NiceChild ⊑ :Child ⊓ :Nice
:Child ⊑ :Person
:Person ⊑ :Agent
:Agent ⊑ ∃:knows.:Agent
info.

-- These are the tests to be performed.
⊢ :NiceChild ⊑ ∃:knows.:Agent
result.

Both "info." and "result." are commands. These commands (recognizable from the "." terminating them) perform certain actions, in this case, "info." prints all preceeding, not-yet-printed axioms added to the KB, and "result." prints the results of all preceeding entailment tests that were not yet printed. Another useful command -- when in a REPL session -- is "help." for help and "quit." to quit. Most commands can be shortened to a single letter (e.g., "i." instead of "info."), though this is again mostly useful in the interactive REPL.

The preceeding document, when passed to shar (./shar resources/example.shar), will print all axioms and the entailment result. It will also terminate with return code '0', because the final entailment is true. If we were only interested in the result of this single entailment, we could also call ./shar resources/kb.shar --entails ":Child ⊑ :Agent" (with the initial four-lined file, otherwise, we would print stuff) and the exit code would indicate the result. We could also make this again more human-readable by using ./shar resources/kb.shar --entails ":Child ⊑ :Agent" --command "result.", which will (finally) call the "result." command.

Let's next call ./shar --repl resources/kb.shar. This will throw us into an interactive REPL session, after executing the kb.shar script, that is, the respective axioms are already loaded. Since we supplied a script file, SHAR assumes that we want to be in entailment mode: Any axiom we enter at the REPL will be checked for entailment. If we want to add additional axioms instead, we can use the command "normal." to return to normal mode, where axioms are added, unless prefixed with "⊢", just as when using a script file. Usually, it is easiest to write axioms to a file, load it into the REPL, and then experiment in entailment mode. Of course, all commands are available at the REPL as well.

Command Line Interface

We present the command line interface of SHAR by example. We start with printing helpful information about the CLI.

./shar --help

Usually, a REPL session is started when the flag --repl is supplied. However, in some cases (e.g., when no script file, no entailment test, and no command is supplied), SHAR will automatically launch a REPL.

./shar --repl

Start a REPL session, loading axioms from resources/kb.shar.

./shar --repl resources/kb.shar

Run a script file (without starting a REPL) and possibly produce output (running commands in the script, if any). Return, as exit code, the result of the final entailment or satisfiability test, if any.

./shar resources/example.shar

Load axioms from a file but supress any output; terminate with exit code depending on the entailment of the supplied axiom.

./shar --silent resources/kb.shar --entails ":NiceChild ⊑ ∃:knows.:Agent"

Start a REPL session, loading the Wine ontology and an appropriate default prefix, defined in SPARQL syntax in the file wine.prefix.

./shar --owl "https://www.w3.org/TR/owl-guide/wine.rdf" --prefixes resources/wine.prefix

Do the same as before, but instead of launching a REPL session, prove that RedWine is Wine and terminate with the appropriate exit code. Note, that this makes SHAR a useful tool in the context of more general purpose shell scripting.

./shar --owl "https://www.w3.org/TR/owl-guide/wine.rdf" --prefixes resources/wine.prefix --entails ":RedWine ⊑ :Wine"

A bonus feature of the launcher script allows for watching a file for changes and refreshing SHAR, by relaunching it automatically. This requires the utility tool entr and is only supported on GNU/Linux and macOS systems (and possible other UNIX or UNIX-like systems).

./shar --watch resources/example.shar

Language

The SHAR DSL has a few basic features, outlined in this section. Each line is processed as one of a few categories: Comments, prefixed with either //, #, or --; commands, terminated with .; entailment tests (in normal mode) prefixed with ⊢ or :-; satisfiability tests ending with ? (optional); and axioms to be added to the knowledge base (in normal mode; in entailment mode, any axiom is interpreted as an entailment test).

Concept expressions, entailment tests, and axioms can be specified using a formal Unicode-based syntax, or an ASCII-only variant. The correspondence and meaning of each symbol is listed in the following table.

Symbol Name	ASCII	Unicode
Top	#t	⊤
Bottom	#f	⊥
Union	|	⊔
Intersection	&	⊓
Universal	#A	∀
Existential	#E	∃
Complement	!	¬
Inclusion	<<	⊑
Equivalency	==	≡
Entailment	:-	⊢
Inverse (Role)	-	-
Inclusion (Role)	<-	<-

Replacing HermiT

When using any OWL-API based reasoner (i.e., a reasoner created by a OWLReasonerFactory), such as JFact or Openllet, the generic OwlApiReasoner can be used. See the following two examples. Note, that the respective dependencies must be added as well.

import uk.ac.manchester.cs.jfact.JFactFactory

/** A reasoner that uses JFact. */
val jfact = OwlApiReasoner(JFactFactory())

import openllet.owlapi.OpenlletReasonerFactory

/** A reasoner that uses Openllet. */
val openllet = OwlApiReasoner(OpenlletReasonerFactory())

Other custom reasoner implementations can be supplied by instantiating the DLReasoner trait:

class FalseReasoner(init: ReasonerInitialization) extends DLReasoner(init):
  def addAxioms(axioms: AxiomSet): Unit = ()
  def prove(axiom: Axiom): Boolean = false
end FalseReasoner

val kb = mkReasoner(FalseReasoner(_))

References and Examples

• HermiT Reasoner
• Openllet Reasoner
• JFact Reasoner
• OWL API
• The project From Shapes to Shapes uses SHAR