I prefer a slightly more general rule: Make Illegal States Unrepresentable. The "Parse, don't validate" rule is a special case of MISU.

What's the difference? MISU applies even when there's no parsing-like transformation happening. For example, if you have a variable that represents the current state of a network connection, and let's say it can be Disconnected, or Connected to some IP address (this is an oversimplification).

Then one way to do it would be

  struct {
    connected: bool,
    peer_ip: int32
  }

The trouble is that this allows us to represent an illegal/meaningless state: we're disconnected but there's still some junk old peer_ip hanging in there. Even worse, we might have written

  struct {
    connected: bool,
    peer_ip: Option<int32>
  }

Now we could have connected = true but peer_ip = None.

The solution is to use a sum type:

  type connection = 
     Disconnected
   | Connected of int32

(sorry for using made-up syntax; I hope it's clear to anyone familiar with Rust.)

"Make Illegal States Unrepresentable" applies throughout your program, at every interface between modules or functions in the program, including but not limited to parsing input.