Making Errors errors

[Paul Backus]

On Thursday, 28 January 2021 at 23:41:44 UTC, sighoya wrote:
> Given range indexing as an example, we would certainly think of 
> index violations as bugs. That may be true. However, the code 
> following after the random access wouldn't be executed anyway 
> and the caller may or may not depend hard on the result of the 
> code fragment.

If out-of-bounds array access is defined by the language spec as 
an unrecoverable error, an optimizing compiler is allowed to 
assume that no program ever recovers from it, and potentially 
re-order code based on that assumption. So you cannot actually be 
sure that "the code following after the random access wouldn't be 
executed."

Of course, the key phrase here is "defined by the language spec." 
As a language designer, you are free to define out-of-bounds 
indexing as either recoverable or unrecoverable. But as a 
programmer, once the decision has been made and the spec has been 
written, you do not get a choice--either you play by the rules, 
or your code is wrong.

I suspect a lot of the confusion on this issue comes from people 
mixing up the programmer's perspective and the language 
designer's perspective.