alt Hacker NewsI think, more generally, "push effects to the edges" which includes validation effects like reporting errors or crashing the program. If you, hypothetically, kept all of your runtime data in a big blob, but validated its structure right when you created it, then you could pass around that blob as an opaque representation. You could then later deserialize that blob and use it and everything would still be fine -- you'd just be carrying around the validation as a precondition rather than explicitly creating another representation for it. You could even use phantom types to carry around some of the semantics of your preconditions.
Point being: I think the rule is slightly more general, although this explanation is probably more intuitive.
Systems tend to change over time (and distributed nodes of a system don’t cut over all at once). So what was valid when you serialized it may not be valid when you deserialize it later.