[salamanderman]

Moving preconditions up depends what the definition of precondition is. There's some open source code I've done a deep dive in (Open cascade) and at some point they had an algorithm that assumed the precondition that the input was sorted, and that precondition was pushed up. Later they swapped out the algorithm for one that performs significantly better on randomized input and can perform very poorly on certain sorted input. Since the precondition was pushed up, though, it seems they didn't know how the input was transformed between the initial entrance function and the final inner function. Edit - if the precondition is something that can be translated into a Type then absolutely move the precondition up and let the compiler can enforce.

[deredede]

"Moving preconditions up" means moving the code that checks the precondition up. The precondition still needs to be documented (in the type system is ideal, with an assertion otherwise, in a comment if necessary) close to where it's assumed.