Return explicitly "empty" MapResult (fail-fast)

[zhade]

On Thursday, 1 January 2026 at 19:39:15 UTC, monkyyy wrote:
> On Thursday, 1 January 2026 at 19:30:11 UTC, zhade wrote:
>> 
>>     auto list = someExpensiveOperation();
>>
>>     return list
>>         .filter!filterFunc
>>         .map!mapFunc;
>> }
>
> oh your looking to mutate the data
>
> Consider my mutmap here if expensive operation is element by 
> element: 
> https://forum.dlang.org/post/urwssqgafegghncksyio@forum.dlang.org

The actual operations performed by 'filter' and 'map' are not 
important. The original question was about how to return an empty 
MapResult.

I also thought I could change the 'auto' return type to something 
like `InputRange!(Tuple!(string, "value", int, "numLetters"))` 
but that didn't work because InputRange != MapResult despite 
MapResult effectively being an InputRange.

For now I'll stick with the additional function which applies the 
filter and map function.