(12th-July-2020)
• We require a representation of the base-level expressions that can be manipulated by the interpreter to produce answers. Initially, the base language will also be the language of definite clauses. Recall that the definite clause language is made up of terms, atoms, bodies, and clauses.
• The metalanguage refers to these syntactic elements of the base language. Thus, meta-level symbols will denote base-level terms, atoms, bodies, and clauses. Base-level terms will denote objects in the domain being modeled, and base-level predicates will denote relations in the domain.
• One of the advantages of a meta-interpreter over writing an interpreter for a whole new language is that the object level can use the meta-level constructs. When writing a logic programming meta-interpreter, there is a choice of how to represent variables. In the non-ground representation, base-level terms are represented as the same term in the metalanguage, so in particular, base-level variables are represented as meta-level variables. This is in contrast to the ground representation, where base language variables are represented as constants in the metalanguage. The non-ground representation means that meta-level unification is available to be used for unifying base-level terms. The ground representation allows the implementation of more sophisticated models of unification.
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