mixins

[Paul Backus]

On Wednesday, 17 November 2021 at 14:51:58 UTC, Abby wrote:
> Hello I would like to create validation mixin or mixin template 
> which would return on error.
>
> Something like this:
>
>
> ```
> mixin template Validate(a, b)
> {
>     if(a > b)
>     {
>        writeln("invalid input");
>        return false;
>     }
> }
>
> bool test(a,b)
> {
>     mixin Validate!(a,b);
>
>     ----
>     on valid code
>     ----
>
>     return true;
> }
> ```

Sadly this can't be done with a template mixin, because the body 
of a template mixin is only allowed to contain *declarations*, 
not *statements*.

However, it can be done with a string mixin:

```d
enum string Validate(string a, string b) = q{
     if (} ~ a ~ q{ < } b ~ q{)
     {
         writeln("invalid input");
         return false;
     }
};

bool test(int a, int b)
{
     mixin(Validate!("a", "b"));

     // on valid code

     return true;
}
```

The `q{...}` syntax is a [token string][1], a special kind of 
string literal designed specifically for putting D code in a 
string.

[1]: https://dlang.org/spec/lex.html#token_strings