[makeitdouble]

The base assumption is parsing upfront cost less than validating along. I thinks it's a common case, but not common enough to apply it as a generic principle.

For instance if validating parameter values requires multiple trips to a DB or other external system, weaving the calls in the logic can spare duplicating these round trips. Light "surface" validation can still be applied, but that's not what we're talking about here I think.

[MrJohz]

It's not about costing less, it's about program structure. The goal should be to move from interface type (in this case a series of strings passed on the command line) to internal domain type (where we can use rich data types and enforce invariants like "if server, then all server properties are specified") as quickly as possible. That way, more of the application can be written to use those rich data types, avoiding errors or unnecessary defensive programming.

Even better, that conversion from interface type to internal type should ideally happen at one explicit point in the program - a function call which rejects all invalid inputs and returns a type that enforces the invariants we're interested in. That way, we gave a clean boundary point between the outside world and the inside one.

This isn't a performance issue at all, it's closer to the "imperative shell, functional core" ideas about structuring your application and data.

[lmm]

> if validating parameter values requires multiple trips to a DB or other external system, weaving the calls in the logic can spare duplicating these round trips

Sure, but probably at the cost of leaving everything in a horribly inconsistent state when you error out partway through. Which is almost always not worth it.