An inference engine for RDF - Agfa

An inference engine for RDF
Definition: an inference step is the act of passing from one state to another (with
the exception of states 1 and 2).
An inference step uses a data structure InfData:
data InfData = Inf {graphs::Array Int RDFGraph, goals::Goals, stack::Stack,
lev::Level, pdata::PathData, subst::Subst,
sols::[Solution], ml::MatchList, mes::String }
This data structure is specified in an Abstract Data Structure InfADT.hs
(fig.5.10). It illustrates how complex data structures can easily be handled in
Haskell using field labels. The use of field labels provides for evolvability of the
datastructure because single fields can be easily removed or added without
provoking major changes to the program. For instance, to get access to the stack,
the instruction:
stackI = stack inf where inf is an instance of InfData, can be used.
This is a complex data structure. Its acess is further simplified by access routines
in the ADT like e.g. sstack: show the contents of the stack in string format.
The elements of the data structure are :
Array Int RDFGraph: The array containing the RDF graphs.
Goals: The list of goals that have to be proved.
Stack: The stack needed for backtracking.
Level: The inferencing level. This is needed for renumbering the variables and
backtracking.
PathData: A list of the statements that have been unified. This is used to
establish the proof of the solutions.
Subst: The current substitution. This substitution must be applied to the current
goal.
[Solution]: The list of the solutions.
MatchList: This is a list of Match constructors:
data Match = Match {msub::Subst, mgls::Goals, bs::BackStep}
Matching two statements gives a substitution, a list of goals (that may be empty)
and a backstep. A backstep is nothing else than a tuple formed by the two
matched statements. This is needed for producing the proof of a solution.
A substitution with a list of goals forms an alternative.
String: This string is used for maintaining the state of the process.
An inference step has as only input an instantiation of InfData and as only
output an instantiation of InfData. The data structure contains all the necessary
information for the next inference step.
Another mini-language for inferencing is defined in module InfML.hs.
It is shown in fig. 5.11.
Fig. 5.12 gives the source code of the module that executes the inferencing
process RDFEngine.hs.
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