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- | Computables in the knowledge representation serve for computing relations instead of inferring them. This can be done by querying external sources of information or by combining the information in the knowledge base in order to create new statements. | + | #REDIRECT doc:defining_computables |
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- | Currently, there are two kinds of computables: | + | |
- | * computable classes, which create instances of their target class, and | + | |
- | * computable properties, which compute relations between instances. | + | |
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- | For each of them, two techniques are supported: | + | |
- | * SQL computables, | + | |
- | * Prolog computables, | + | |
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- | Some general remarks: | + | |
- | * Computables should be defined in their own module and also compute local relations. For example, the computables for temporal relations (before, after, during etc) are defined in the module comp_temporal and compute comp_temporal: | + | |
- | * The &bla; identifiers are XML entities that are replaced with the string specified at the beginning of the file when the file is being loaded. They are useful for making the code a bit shorter and more readable, and also for ensuring consistency. | + | |
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- | We will explain the two techniques (SQL and Prolog) for computable properties, while computable classes work in a rather similar way and will only briefly be described. | + | |
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- | == General concept == | + | |
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- | For computable properties to work, the following things need to be defined: | + | |
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- | * the property to be computed, including its domain and range (datatype, object prop and one-of supported) | + | |
- | * an instance of ComputableProperty that has this property as its target | + | |
- | * either a set of SQL queries or a Prolog predicate that is accessible when the query is executed | + | |
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- | For including computable relations into the reasoning process, use rdf_triple(P, | + | |
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- | ; rdfs_computable_compute_property_concatenation(SubProperty, | + | |
- | ; catch( rdfs_computable_triple(SubProperty, | + | |
- | ; user: | + | |
- | ). | + | |
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- | rdf_triple first searches for sub-properties of the current property, and for each of them, calls first rdf_has for reading information asserted in the knowledge base directly, then tries to find property concatenations (currently unused), calls all computable triples and finally creates a hook where other modules can attach (currently e.g. GrAM action models). | + | |
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- | rdfs_computable_triple handles caching issues and calls rdfs_computable_triple_1, | + | |
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- | ; catch(rdfs_computable_prolog_triple(Property, | + | |
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- | The whole code for managing the computables is contained in rdfs_computable.pl in the module semweb. | + | |
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- | === Caveats === | + | |
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- | When something does not work, check the following. Most likely, there is a minor inconsistency in the OWL names that makes the whole process break. | + | |
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- | * is the right property being computed? knowrob: | + | |
- | * is there a typo anywhere in the namespaces or base URLs? | + | |
- | * check if there is a hash sign # between the URL and the property name (if using XML entities, check if the hash is either part of the entity or written where the entity is being used in the code) | + | |
- | * debug with the following calls and step through to check that the right computable is being found, that the right query is sent, or that your predicate performs the right computations. | + | |
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- | == SQL properties == | + | |
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- | As mentioned before, the first step is to define the target to be computed. In this example, which can be found in ias_gram_human.owl, | + | |
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- | We use this property as the target for the computable that is arbitrarily named ' | + | |
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- | Apart from the target and some auth information to access the database, the most important parts are the three SQL queries. Depending on which variables in the query are bound (subject, object, or none of them), the system automatically chooses the right one. If the subject is bound and the object unbound, it uses valueSelect, | + | |
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- | The results are further processed: In the case of datatype properties, they are wrapped in the literal(...) predicate, specifying their XSD type. For object properties, the result is extended with the namespace of the property' | + | |
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- | The above property can then be called with | + | |
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- | == Prolog properties == | + | |
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- | For Prolog properties, we will have a look at the knowrob: | + | |
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- | Then, the computable that is attached to this property needs to be specified: | + | |
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- | Note that the target is described as in SQL properties, but instead of covering the different cases of bound/free variables with different SQL queries, we only specify one Prolog predicate (comp_after) that is supposed to handle them correctly. comp_after needs to be specified somewhere where it is available once knowrob.owl is loaded, which, in our case, is the gram_base.pl. | + | |
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- | % after(TimePoint, | + | |
- | comp_after(Pre, | + | |
- | rdf_has(Pre, | + | |
- | rdf_has(After, | + | |
- | term_to_atom(P, | + | |
- | term_to_atom(A, | + | |
- | P<A. | + | |
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- | This predicate first checks if the subject and property have the correct types, then transforms the time points into numerical values using term_to_atom, | + | |
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- | * Prolog computables can do much more than just compare values: Using the Java interface JPL, we included the ehow.com plan import as a Prolog computable datatype property which links Strings like "set a table" to plan specifications in OWL. The same technique was used to interface Prolog to ROS and read object poses from the jlo service. Rule knowledge (for unary or binary predicates) can be encoded in Prolog computables, | + | |
- | * When trying to use rdf_assert inside a Prolog computable, you are likely to get an error " | + | |
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- | == SQL classes == | + | |
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- | The definition of computable SQL classes is rather similar to properties and will only be shortly explained. Obviously, the target class has to exist for the method to work... | + | |
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- | computeReaching reads human motion frames labeled as ' | + | |
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- | Here, we do not have the frame/value scheme, but rather one command for creating instances (command) and one for checking if an instance belongs to the respective class (testCommand). | + | |
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- | == Prolog classes == | + | |
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- | The definition of computable prolog classes is similar to the preceding ones, you simply need to describe the class (here: DrinkingMug) and a binary predicate (here: comp_objectsOnTable(Inst, | + | |
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