Extending the language

Extending the language.

There are two ways to extend the FBASIC programming language. 

1. To add a library with new statements and functions.
2. To add statements, functions and variable values before invoke the interpreter via C# code.  
3. To add new statements only in the main code of the interpreter (non recommended)

Adding a library 

Library is a set of statements and/or functions. To create a new library, create a new class that implements the IFBasicLibrary interface. The are only one method to implement like the following example:

    public class MyFBasicLibrary : IFBasicLibrary
    { 
        public void InstallAll(Interpreter interpreter)
        {
            interpreter.AddFunction("FUN", Fun);

            interpreter.AddStatement("STATE", State);
            interpreter.SetVar("PI",3.14159);
        }
        
        public static Value Fun(Interpreter interpreter, List<Value> args)
        {
        ...
        }
        
        public static void State(Interpreter interpreter)
      	{
      	...
      	}
   }

Note: The implementation of the functions and the statements are static methods. 

To add the library to the interpreter use the AddLibrary() method: 

Interpreter interp = new();
interp.AddLibrary(new FBasicStringFunctions());

Adding before program interpretation

Before a program execution, the developer has the ability to add Variables with values, Statements and Functions. 

Example 1:

Interpreter interp = new();
interp.AddFunction("FUN", MyClass.Fun);

interp.AddStatement("STATE", MyClass.State);
interp.SetVar("PI",3.14159);
...
... code to run (interprate) a program
...

Example 2:

executionResult result;
string basProgramFile = Directory.GetFiles(programsFolder, startupName).FirstOrDefault();
result = fBasicHelper.run(env, basProgramFile, (interp) =>
	{

    	interp.SetVar("table.column", new Value("myColumn1"));
		interp.AddLibrary(new FBasicStringFunctions());
    });
    if (result.hasError)
    {
    	Console.WriteLine(result.errorText);
        if (!string.IsNullOrEmpty(result.errorSourceLine)) Console.WriteLine(result.errorSourceLine);
    }
    else Console.WriteLine($"Result: {result.value}");

Adding new (in main code) statements.   

There are seven (7) steps to add a new statement to the FBASIC Core interpreter core code:

1. Use Interpreter_Statement.cs or/and the partial class Interpreter
2. Implement a method like:
   void MYSTATEMENT(){}
   to code how the statement works.
3. Edit Token.cs and add a new value to enumeration: Token
4. Edt Lexer.cs find the GetToken() and add a CASE to the long SWITCH converting the MYSTATEMENT to token.
5. In the Lexer.cs find the isStatement() and add CASE to the SWITCH if the added token is statement.
6. Edit code file  interpreter_Elements.cs and at method Statement() add a CASE to the long SWITCH to map the Token with the implementing method
7. Add entry for the new statement to FBasicManual.md