In this exercise you are to construct an interpreter for prefix expressions with the addition of a single memory location (like the interpreter you previously constructed, but implemented in Prolog this time). Your grammar should contain parameters that build an abstract syntax tree of the expression. Then, you should write Prolog rules that evaluate the abstract syntax tree to get the resulting expression. The grammar for prefix expressions is given in example 8.3. The grammar is LL(1) so no modifications of it are necessary to generate a parser for it in Prolog.

To complete this project you will want to use the readln predicate described in section 7.6. However, to make things easier while parsing, you should preprocess the list so that an expression like “+ S 5 R”, which readln returns as [+,s,5,r], will look like [+,s,num(5),r] after preprocessing. The num structure for numbers will help you when you write the parser.

HINT: This assignment is very closely related to the attribute grammar given in example 8.4. The main predicate for the interpreter should approximate this: