Comparing Exceptions and Errors

[Paul Backus]

On Saturday, 4 June 2022 at 11:57:32 UTC, kdevel wrote:
> 2. Since 2017 or so I have written some 10 KLOC of D, maybe 
> about two dozen
>    classes deriving from Exception. But I did not annotate any 
> of my methods or
>    function with "nothrow" nor did I author any class deriving 
> from `Error`.
>
>    What does that mean? Am I `Error` blind?

Generally you do not need to subclass `Error` yourself. The most 
common way of throwing an `Error` in user code is to use 
`assert`, which (with default compiler flags) throws an 
`AssertError` on failure. Function contracts and struct/class 
invariants work the same way.

> 3. Can you provide some piece of code which *must* throw 
> `Error` and cannot
>    throw an appropriate Exception?

This is entirely a question of API design. If it should be the 
caller's responsibility to check for some condition before 
calling the function, then you can throw an `Error` when that 
condition does not hold (or more likely, use an `assert` or an 
`in` contract to check for it). If it should be the callee's 
responsibility to check, you should throw an `Exception` (or use 
`enforce`).