In the previous post, we talked about creating a custom ANTLR4 grammar. Our grammar can parse formulas with basic mathematical operators and it defines some custom expressions that allow us to implement domain logic which, combined with the formula parsing, allow us to calculate domain-specific concepts like prices, amounts and conversion factors between units of measure.<\/p>\n
In this post, we\u2019ll get a decimal result out of some simple expressions and write some tests to make sure our initial calculator is working.<\/p>\n
In our grammar, we have two operators that have widespread use in probably every trading and banking system in the world. They are:<\/p>\n
- \n
- UomConvert(FromUnitOfMeasure, ToUnitOfMeasure)<\/li>\n
- FXRate(\u2018CurrencyPair\u2019)<\/li>\n<\/ul>\n
The first one converts between units of measure. The second one is a simplified FX rate. We can interpret it as an FX Spot Rate between two currencies. While FX Spot Rates <\/a>are meaningless without a date component, for the purposes of this post, we will keep it simple and always assume \u2018latest known FX Rate\u2019<\/a>.<\/p>\n
Our first step is to get a basic calculator working. To do that we\u2019ll need to be able to handle the different inputs in the following table.<\/p>\n
\n\n
\n \n Input<\/strong><\/h3>\n<\/td>\n
\n Result<\/strong><\/h3>\n<\/td>\n
\n Comments<\/strong><\/h3>\n<\/td>\n<\/tr>\n
\n 2<\/td>\n 2<\/td>\n Number identity should work.<\/td>\n<\/tr>\n \n 2 * 2<\/td>\n 4<\/td>\n Standard math operators (addition, multiplication, subtraction and division)<\/td>\n<\/tr>\n \n (2 + 2) * 4<\/td>\n 16<\/td>\n Parenthesis priority should be respected.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n Extending the base visitor<\/h2>\n
In our previous post we did all the plumbing necessary to generate some code that we can work with. This generated code is added automatically into our project thanks to the new C# project file (*.csproj) format.<\/p>\n
As a result of the plumbing work, we\u2019re telling the ANTLR runtime to generate a visitor. This visitor gives us some classes we can inherit from, and add some logic to, to get the desired result. The generated code will give us an expression tree, as well as the necessary tools to perform custom actions each time we visit a node in the tree. Nodes can be a recognized character, an operator, or an expression.<\/p>\n
Our main goal is to calculate the result of our expression. To do this, we first need to inherit from the MyGrammarBaseVisitor class, which has a generic type parameter that defines our expression\u2019s result type.<\/p>\n
To start, the visitor that will inherit from our base class will\u2014for now\u2014override the basic mathematical operations that we defined in our grammar file<\/a>. These are \u2018Number\u2019, \u2018Parens\u2019 (parentheses), and the operators \u2018ADD\u2019, \u2018SUB\u2019, \u2018MUL\u2019, and \u2018DIV\u2019, using decimal as a return type.<\/p>\n
public class MyGrammarVisitor : MyGrammarBaseVisitor<decimal>\r\n{\r\n public override decimal VisitParens(MyGrammarParser.ParensContext context)\r\n {\r\n return base.VisitParens(context);\r\n }\r\n\r\n public override decimal VisitNum(MyGrammarParser.NumContext context)\r\n {\r\n return base.VisitNum(context);\r\n }\r\n\r\n public override decimal VisitAddSub(MyGrammarParser.AddSubContext context)\r\n {\r\n return base.VisitAddSub(context);\r\n }\r\n\r\n public override decimal VisitMulDiv(MyGrammarParser.MulDivContext context)\r\n {\r\n return base.VisitMulDiv(context);\r\n }\r\n}\r\n<\/pre>\nWe override the operations defined in our grammar labeled with the # symbol (these are for convenience as they let us know which method names we need to override). The overridden methods cover all the operations we need to implement the calculator.<\/p>\n
Context<\/h2>\n
Each overridden method is handed a context variable whose type is specific to the method we\u2019re overriding. This is extremely helpful because within the method we define what steps need taking via code, and which data to perform the steps on is provided by the parameter.<\/p>\n
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