Translator – Command Processing

When processing tokens in a statement, it is necessary to know what command is currently being processed. For example, the SPC and TAB functions are only valid in the PRINT statement. If the command is pushed onto the hold stack, there will be other tokens on top of the command token. Therefore the current command can't be determined by looking at the top of the hold stack. This means another variable would be needed to store the current command.

Commands can also be nested within a line (for example, an assignment or PRINT inside an IF-THEN-ELSE statement), so a stack is still the ideal solution. The command tokens need to be pushed onto a separate stack from the hold stack, a command stack. The precedence of a command token will still be used to empty the hold stack, but the command token will be pushed onto the command stack instead. The current command can be accessed by looking at the top of the command stack.

Before getting started with designing and implementing the commands, it's important to mention that there is no attempt being made to follow any particular standard or existing BASIC for this project. Being a heavy GW-Basic user in the 80's and early 90's, a lot of the inspiration for this project comes from that experience.

Multiple statements will be supported – as way to fit more code on the screen. In GW-Basic, the is a general lack of any rigid multiple line control structures. There are limited ones like FOR-NEXT, WHILE-WEND, DEF-ENDDEF, but these are easily abused (mainly due to the interpreted nature of GW-Basic). Factor in the GOTO, ON-GOTO GOSUB and ON-GOSUB commands (not being supported here), and GW-Basic code could look very ugly.

For this project, there will be new modern control structures including some structures that don't exist in other BASIC or other languages. It is time to get started on the commands, which will be defined before implemented. One command has already been implemented, the assignment statement, less the optional LET keyword, so it's now time to implement this optional keyword...