> How can you know that half the things I can represent are wrong? There are many situations where both returning a value and not returning a value are both valid. And where errors conditions may not dictate what the returned value should be.

In cases where it's valid, you use a type that makes it valid. Much of the point of having a type system is that the return type tells you exactly what the valid things for the function to return are.

Having an explicit declaration of which functions might return both a result and an error and which functions will definitely return one or the other but not both makes your code easier to understand and work with. As it stands, I would bet that most Go programmers ignore the possibility of returning both a result and an error even in functions that do it deliberately, because the idiomatic thing a function with that signature does is return one or the other but not both.