public Expression ParseIfExpression()
{
var expression = new IfExpression()
{
Token = this.currentToken
};
if (!this.Expect(TokenType.LPAREN))
{
return(null);
}
this.NextToken();
expression.Condition = this.ParseExpression(Precedence.LOWEST);
if (!this.Expect(TokenType.RPAREN))
{
return(null);
}
if (!this.Expect(TokenType.LBRACE))
{
return(null);
}
expression.Cons = this.ParseBlockStatement();
if (this.IsPeekTokenTypeEqualTo(TokenType.ELSE))
{
this.NextToken();
if (!this.Expect(TokenType.LBRACE))
{
return(null);
}
expression.Els = this.ParseBlockStatement();
}
return(expression);
}
private Object EvalIfExpression(IfExpression expression, Environment env)
{
var condition = this.Eval(expression.Condition, env);
if (this.IsError(condition))
{
return(condition);
}
if (this.IsTruthy(condition))
{
return(this.Eval(expression.Cons, env));
}
else if (expression.Els != null)
{
return(this.Eval(expression.Els, env));
}
else
{
return(Null);
}
}