@omochimetaru と Error Handling Rational and Proposal の英語は難しかった話をしたけど、今見てもやっぱり難しい。↓ Universal error についての一節。

This combination of requirements means that all operations must be implicitly "unwindable" starting from almost any call site it makes. For the stability of the system, this unwinding process must restore any invariants that might have been temporarily violated; but the compiler cannot assist the programmer in this. The programmer must consciously recognize that an error is possible while an invariant is broken, and they must do this proactively --- that, or track it down when they inevitably forget. This requires thinking quite rigorously about one's code, both to foresee all the error sites and to recognize that an important invariant is in flux. How much of a problem this poses depends quite a lot on the code being written. There are some styles of programming that make it pretty innocuous. For example, a highly functional program which conscientiously kept mutation and side-effects to its outermost loops would naturally have very few points where any invariants were in flux; propagating an error out of an arbitrary place within an operation would simply abandon all the work done up to that point. However, this happy state falls apart quite quickly the more that mutation and other side-effects come into play. Complex mutations cannot be trivially reversed. Packets cannot be unsent. And it would be quite amazing for us to assert that code shouldn't be written that way, understanding nothing else about it. As long as programmers do face these issues, the language has some responsibility to help them.