Discussion Thread: DIP 1038-- at nodiscard--Community Review Round 1

[Dennis]

On Wednesday, 9 December 2020 at 10:10:40 UTC, Mike Parker wrote:
> This is the discussion thread for the first round of Community 
> Review of DIP 1038, "@nodiscard":

This is a reply to Andrej Mitrovic's post in the Feedback Thread.
https://forum.dlang.org/post/gdjclwuwuoyqiftdercu@forum.dlang.org

> I think @nodiscard should not apply to void functions.
> (...)
> At least I can't think of use-cases for it having an effect on 
> void functions.

There is a pattern that replaces `goto fail;` and `goto success;` 
style error handling with calls to (inner) functions in a return 
statement. It looks something like this:

```
struct Parser {
     S result;
     string error;

     void setError(string msg) {
         error = msg;
     }

     void parse(string s) {
         if (s.length == 0) {
             return setError("empty input");
         }
         if (!s.starsWith("MAGIC")) {
             return setError("wrong header");
         }
         // etc.
     }
}
```

This pattern is used in dmd:

https://github.com/dlang/dmd/blob/97aa2ae5ee19ce6a2979ca1627479df713f99252/src/dmd/expressionsem.d#L2524

https://github.com/dlang/dmd/blob/97aa2ae5ee19ce6a2979ca1627479df713f99252/src/dmd/expressionsem.d#L5383

Functions setError() yes() and no() should be called in a return 
statement, continuing afterwards could be considered a bug. One 
might want to apply @nodiscard to those functions so it will be 
caught by the compiler when you forget to prepend `return`.

For that to work however, @nodiscard should not have a special 
case for `void`.