Command–query separation principle [re: @mustuse as a function attribute]

[mw]

On Wednesday, 19 October 2022 at 21:13:00 UTC, mw wrote:
> On Wednesday, 19 October 2022 at 13:17:00 UTC, Paul Backus 
> wrote:
>>
>> In other words, you cannot introduce `@mustuse` in a derived 
>> class; it must be present in the base class.
>>

I spent some more time thinking about this, and I do not agree 
with this rule even though I know what your design goals (and 
constraints) are.

I will make an improved proposal based on my initial transitive 
closure design, and I will write a longer post about it, which 
may take some time, please bear with me.

As a summary, this is what I am going to propose: introduce two 
variants of @mustuse

1) @mustUse_remedyLegacy: this annotation is to allow programmer 
flag existing legacy library code that s/he has no right to 
change, but want to set a flag and let the compiler to help to 
find violations, but the compiler only output warnings instead of 
errors.

2) @mustuse: proper, the default. For programmer to use in new 
code, or library code s/he can change (from the root of the 
inheritance tree). Violations of this annotation will cause 
compiler error.

And:

a) in both cases, this function property will be transitive 
closure, I.e. be propagated upwards and downwards, in all 
directions.

b) in both cases, removing the annotation is *not* allowed in the 
derived class if the supper class method carry such annotation.


I will write more about my rationale and considerations, and give 
examples when I get more time.