Parse, don't validate (2019)

Note that this basically requires your language to have ergonomic support for sum types, immutable "data classes", pattern matching.

The point is to parse the input into a structure which always upholds the predicates you care about so you don't end up continuously defensively programming in ifs and asserts.

I think we'll eventually come to regard `if` as we do `goto`.

Using pattern matching instead or something else?

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.