Return explicitly "empty" MapResult (fail-fast)

[Paul Backus]

On Thursday, 1 January 2026 at 17:01:25 UTC, zhade wrote:
> Example:
>
> ```d
> import std.algorithm : filter, map;
> import std.typecons : tuple;
>
> auto example_func(bool fail)
> {
>     if (fail)
>     {
>         // [...]
>     }
>
>     auto list = someExpensiveOperation();
>
>     return list
>         .filter!(a => a != "expensive")
>         .map!(a => tuple!("value", "numLetters")(a, a.length));
> }
>
> string[] someExpensiveOperation()
> {
>     return ["some", "expensive", "operation"];
> }
> ```

You could try something like this:

```d
auto example_func(bool fail)
{
     string[] list;

     if (fail)
         list = [];
     else
         list = someExpensiveOperation();

     return list
         .filter!(...)
         .map!(...);
}
```

Because `map` and `filter` are lazy, calling them on an empty 
`list` won't do anything, but they will still return the correct 
type.