BoolExParser

BoolExParser

BoolExParser is a boolean expression configuration system. It allows clients to create complex boolean operations in a declarative way. Check below some examples of how it can be used:

• Expression: "true"

var exp = new Constant(true);

• Expression: "not true"

var exp = new NotExp(new Constant(true));

• Expression: "not x"

var exp = new NotExp(new VariableExp('x'));

It also allows for composing expressions to create OR (x or y) and AND (y and x) expressions:

var yOrX = new OrExp(new VariableExp('x'), new VariableExp('y'));
var yAndX = new AndExp(new VariableExp('x'), new VariableExp('y'));

Providing an open-ended range of possibilities.

Language grammar

A simple grammar was defined for the language:

• BooleanExp $::=$ VariableExp | Constant | OrExp | AndExp | NotExp | '(' BooleanExp ')'
• AndExp $::=$ BooleanExp 'and' BooleanExp
• OrExp $::=$ BooleanExp 'or' BooleanExp
• NotExp $::=$ 'not' BooleanExp
• Constant $::=$ 'true' | 'false'
• VariableExp $::=$ 'a' | 'b' | ... | 'x' | 'y' | 'z'

The Interpreter Pattern applied

To implement such a flexible structure, the Interpreter Pattern was used. As Interpreter is just a particular form of Composite, but applied to interpret a language, the same aspects of a composite applies. The top-level Composite structure in our case is the base interface BooleanExp, which defines the methods each composite-compliant class should implement:

public interface BooleanExp
{
  public bool Evaluate(Context context);
  public BooleanExp Replace(char name, BooleanExp exp);
  public BooleanExp Copy();
  public string GetName();
}

Then, multiple classes implement BooleanExp in different ways depending on their particular needs. The class diagram below shows the full class hierarchy:

classDiagram

class BooleanExp {
    bool Evaluate()
    BooleanExp Copy()
    BooleanExp Replace()
}

class VariableExp {
    string name

    bool Evaluate()
    BooleanExp Copy()
    BooleanExp Replace()
}

class AndExp {
    BooleanExp operand1
    BooleanExp operand2

    bool Evaluate()
    BooleanExp Copy()
    BooleanExp Replace()
}

class OrExp {
    BooleanExp operand1
    BooleanExp operand2

    bool Evaluate()
    BooleanExp Copy()
    BooleanExp Replace()
}

class NotExp {
    BooleanExp exp

    bool Evaluate()
    BooleanExp Copy()
    BooleanExp Replace()
}

class Constant {
    bool value

    bool Evaluate()
    BooleanExp Copy()
    BooleanExp Replace()
}

BooleanExp <|-- VariableExp : implements
BooleanExp <|-- AndExp : implements
BooleanExp <|-- OrExp : implements
BooleanExp <|-- NotExp : implements
BooleanExp <|-- Constant : implements

Loading

the classes VariableExp, AndExp, OrExp, NotExp are also intermediate composites and therefore delegates most of its behavior to calling the methods on their children, while Constant is a leaf, representing a raw boolean value.