Question

I have an ANTLR grammar that can parse and evaluate simple expressions like 1+2*4, etc.
What I would like to do is to evaluate expressions like 2+$a-$b/4 where the $ variables are dynamic variables, that come from an external source and are continuously updated.

Is there any design pattern on how to do this using ANTLR, best practices, etc?

• Shall I “substring” the $a with the updated value ($a -> 4.34)
• A nicer way to do this?

Thx

Answer 1

There is actually an example for this in the ANTLR book (the ANTLR Definitive Reference.) The pattern is to parse the variable values and add them to a dictionary in the target language:

@members { var dict = new Dictionary<string, int>(); }

decl: v=ID '=' v=expr { dict[$e.Text] = int.Parse($v.Value); };

ID  : '$' ('a'..'z'|'A'..'Z')+;

where ‘expr’ can be any valid expression (including an expression containing a variable.)