[jandrese]

Imagine a case where you have 4 options. W, X, Y, Z.

Y and Z are mutually exclusive.

X can only be set if W is set.

If Y is set then X must be set.

Going down this road you end up encoding your business logic into your datatypes. Which is good to a degree, but makes things messy when new options are added or requirements change. Imagine a new option U is introduced that is only valid when W is unset and Z is set but allows X to be set. Your interface becomes very hard to manage with even a small amount of complexity added to the options.

[kccqzy]

This is an instance of inherent complexity. Your domain is complex. You either place them into a series of nested if statements (which is what majority of programmers do), or you place it into the type system. You cannot avoid complexity either way. We are merely arguing where this complexity belongs. Such complexity is hard to manage in either case.

[aiono]

What is the alternative? And is it really better than encoding into the data type?

Only option I can think of is writing unit tests for each case, which doesn't seem like too much different.

And without type encoding you never sure that invariant always hold. You can always manipulate any options in any part of the program. Then after any modification you have to perform a "sanity check" if options are in a well defined state or not. I don't see how this is better than encoding the invariant into the types.