// Rule
// Constant | Function | Variable
public Rule ParseRule()
{
Rule r;
if (currentToken.Type == TokenType.Identifier)
{
if (lexer.Peek().Type == TokenType.LeftParen) //Function
{
r = new Rule(Type.Function);
r.Label = currentToken.Lexeme;
Advance();
Advance(); //'('
r.ArgList = ParseArgList();
Advance(); // ')'
}
else // Constant
{
r = new Rule(Type.Constant);
r.Label = currentToken.Lexeme;
Advance();
}
}
else // variable
{
r = new Rule(Type.Variable);
r.Label = currentToken.Lexeme;
Advance();
}
return r;
}
/// <summary>
/// The rules for unifying Rules in predicates.
/// </summary>
/// <param name="Rulei"></param>
/// <param name="Rulej"></param>
/// <returns> true if the two rules can unify, false if they can't. </returns>
private bool CanUnify(Rule Rulei, Rule Rulej)
{
// Variables can resolve with other variables.
if ((Rulei.Type == Type.Variable && Rulej.Type == Type.Variable) || (Rulej.Type == Type.Variable && Rulei.Type == Type.Variable))
return true;
// Constants can resolve with variables.
if ((Rulei.Type == Type.Variable && Rulej.Type == Type.Constant) || (Rulei.Type == Type.Constant && Rulej.Type == Type.Variable))
return true;
// A constant can resolve with the variable of a function.
if ((Rulei.Type == Type.Constant && Rulej.Type == Type.Function) || (Rulei.Type == Type.Function && Rulej.Type == Type.Constant))
return true;
// A variable can resolve with a function
if ((Rulei.Type == Type.Variable && Rulej.Type == Type.Function) || (Rulei.Type == Type.Function && Rulej.Type == Type.Variable))
return true;
// A function can resolve with another function of the same type.
if (((Rulei.Type == Type.Function && Rulej.Type == Type.Function) || (Rulej.Type == Type.Function && Rulei.Type == Type.Function)) && Rulei.Label == Rulej.Label)
return true;
return false;
}