Using Prolog as metalanguage for teaching programming language concepts

syntax trees are represented as Prolog terms but at the same time with an acceptable concrete appearance. A defining interpreter consists of the following predicates. program(program, final-storage) statement(statement, storage-before, storage-after) expression(expression, storage, integer) condition(condition, storage, ftrue, falseg) Some of the rules of this interpreter are shown in the following; the most important ones are for the if and while statements. Notice that we reuse the storage structure and auxiliaries from the previous example. program(P, Storage) :statement(P, [], Storage). statement((Var := Expression), L1, L2):expression(Expression, L1, Value), store(Var,Value,L1,L2). statement( (S1 ; S2), L1, L3):statement(S1, L1, L2), statement(S2, L2, L3). statement( if(Cond, Smt1, Stm2), L1, L2):condition(Cond, L1, Value), (Value = true -> statement(Stm1, L1, L2) ; statement(Stm2, L1, L2)). statement( while(Cond, Stm), L1, L2):condition(Cond, L1, Value), (Value = true -> statement( (Stm ; while(Cond, Stm)),L1,L2) ; L1=L2). expression(Variable, L, V):atom(Variable), fetch(Variable,V,L).