What is Fail-stop?

A fail-stop subset of a computer language is one that has the same semantics as the original, except in the case where an exceptional condition arises. The fail-stop subset must report an exceptional condition whenever the superset language reports one, but may additionally report an exceptional condition in other cases.

Fail-stop languages are often used in computer systems where correctness is very important, since it is easier to make such systems fail-fast. For example, the "+" operator in many programming languages is not associative because of the possibility of floating-point overflow. Repairing these languages to fail fast when commonly assumed properties do not hold makes it much easier to write and verify correct code.