##Non-observable program requirements

The validity of a program should depend on things that can be observed when the program is treated as a black box. Examples are data written to standard output or error streams, drawing on the screen, file operations, memory usage, and runtime. Non-observable behaviors include "string operations" and "implementing the Fu-Barr algorithm". Additionally, "using Y" would fall into the non-observable category in most instances of "Do X without Y"

Why they are bad

Non-observable requirements tend to be vague, subjective, or based on false assumptions about the properties of programming languages. For example, requiring that answers use a specific algorithm is highly open to interpretation. What modifications are allowed? Can some steps be replaced with different operations that produce the same result?

###Exceptions A commonly used non-observable requirement is to avoid using a library function that solves the entire challenge. I believe these rules are OK. Like other non-observable requirements, one can find borderline cases of whether a built-in function solves all of the task or not, but the benefits of higher-quality answers outweigh the costs.

Non-observable program requirements

The validity of a program should depend on things that can be observed when the program is treated as a black box. Examples are data written to standard output or error streams, drawing on the screen, file operations, memory usage, and runtime. Non-observable behaviors include "string operations" and "implementing the Fu-Barr algorithm". Additionally, "using Y" would fall into the non-observable category in most instances of "Do X without Y"

Why they are bad

Non-observable requirements tend to be vague, subjective, or based on false assumptions about the properties of programming languages. For example, requiring that answers use a specific algorithm is highly open to interpretation. What modifications are allowed? Can some steps be replaced with different operations that produce the same result?

Exceptions

A commonly used non-observable requirement is to avoid using a library function that solves the entire challenge. I believe these rules are OK. Like other non-observable requirements, one can find borderline cases of whether a built-in function solves all of the task or not, but the benefits of higher-quality answers outweigh the costs.