postfix-predicate-interpreter

A postfix expression converter and interpreter for generic boolean predicates.

Note

Postfix representation, also known as Reverse Polish Notation (RPN), is an alternative way to describe mathematical expressions.
While the standard representation (infix) uses operators between operands, the postfix representation uses operators after operands.
Expressions in postfix form are easier and less expensive to evaluate than standard infix expressions: they can be evaluated linearly left-to-right, since this notation removes the concept of operator precedence and the need for parenthesis.

Features

• Instantiate an infix expression made of generic boolean predicates
• Instantiate a postfix expression made of generic boolean predicates
• Convert an infix expression to a postfix expression
• Convert a postfix expression to an infix expression
• Evaluate a postfix expression

Usage

Let's say you want to evaluate the infix expression A AND (B OR C).

We can instantiate the infix expression as follows:

let infix = InfixExpression::from_tokens(vec![
    InfixToken::Predicate("A"),
    InfixToken::Operator(Operator::And),
    InfixToken::Parenthesis(Parenthesis::Open),
    InfixToken::Predicate("B"),
    InfixToken::Operator(Operator::Or),
    InfixToken::Predicate("C"),
    InfixToken::Parenthesis(Parenthesis::Close),
])
.unwrap();

We can then convert the infix expression to the equivalent postfix expression A B C OR AND:

let postfix = infix.to_postfix();
assert_eq!(
    postfix,
    PostfixExpression::from_tokens(vec![
        PostfixToken::Predicate("A"),
        PostfixToken::Predicate("B"),
        PostfixToken::Predicate("C"),
        PostfixToken::Operator(Operator::Or),
        PostfixToken::Operator(Operator::And),
    ])
    .unwrap()
);

Finally, we can evaluate the postfix expression:

let result = postfix.evaluate(&predicate_evaluator);

Note that predicates are represented as letters in this example (A, B, C), but they can be any type that resolves to either true or false given a predicate evaluator.
The predicate evaluator is an object of a type implementing the PredicateEvaluator trait; such an object is passed as argument to the evaluate method, making it possible to solve the expression.

Note

The aim of the library is to evaluate expressions whose results depend on an external context, represented by the value of the predicate evaluator.