For the purpose of understanding tools like Bison and Flex, I have been building a parser for RapCAD despite the fact that it will probably one day use a library version of OpenSCAD as the back end. My reasons for doing this are two fold, firstly I have never writtern a parser in Bison or a Lexer in Flex before, I have only writtern a trivial Recursive Decent Parser using .NET and Its something I have wanted to try for a while. Secondly I am interested to see how to implement such things as “variables that will”, and functions with function bodies, so that I might sometime provide a patch for OpenSCAD so that it can also do these things.
Things are moving steadily along, and I am now able to parse a simple test file containing the following:
The parser then reads the text and turns it into an abstract syntax tree representation, which is stored in various objects. The objects all have a
toString() method which when called produces following text dump:
Parameters: Param: a Param: b Expression: (Literal: 1 Operator: + Literal: 2 ) Param: c Expression: (Literal: 1 Operator: - Literal: 2 ) Param: d Expression: (Literal: 1 Operator: / Literal: 2 ) Param: e Expression: (Literal: 1 Operator: % Literal: 2 ) Param: f Expression: (Literal: 1 Operator: Literal: 2 ) Param: h Expression: (Literal: 1 Operator: * Literal: 2 )
So, I am thinking about the parsing of functions now, and its got me wondering about function bodies. There is a problem which I am not sure the best way to resolve. I would like to allow a function to have a function body consisting of statements. It would be meaningless however to have a module instance inside a function body, but for the current grammar to work a module instance is a type of statement. I have two options, either re-write my grammar to make a module instance inside a function body as a syntax error, or the other option which would simply be to make the syntax acceptable, but when it comes to evaluate it it will be a compile error.