Most of the time you avoid having booleans in the first case, in favour of polymorphism (e.g. rather than having an "addOrMultiply" flag, you have separate "Add" and "Multiply" classes with a polymorphic method that does the addition or multiplication). You probably need some conditional logic in your "parser" (and whether that's "if" or pattern matching isn't so important IMO), but you should push booleans out of your core business logic and over to the edges of your program.