Fril
Fril is a programming language for first-order predicate calculus. It includes the semantics of Prolog as a subset, but takes its syntax from the of Logic Programming Associates and adds support for fuzzy sets, support logic, and metaprogramming. Fril was originally developed by Trevor Martin and Jim Baldwin at the University of Bristol around 1980. In 1986, it was picked up and further developed by Equipu A.I. Research, which later became Fril Systems Ltd. The name Fril was originally an acronym for Fuzzy Relational Inference Language. Prolog and Fril comparison Aside from the uncertainty-management features of Fril, there are some minor differences in Fril's implementation of standard Prolog features. Types The basic types in Fril are similar to those in Prolog, with one important exception: Prolog's compound data type is the term, with lists defined as nested terms using the . functor; in Fril, the compound type is the list itself, which forms the basis for most constructs. Variables are distinguished by identifiers containing only uppercase letters and underscores (whereas Prolog only requires the first character to be uppercase). As in Prolog, the name _ is reserved to mean "any value", with multiple occurrences of _ replaced by distinct variables.