Return explicitly "empty" MapResult (fail-fast)

[kdevel]

On Tuesday, 27 January 2026 at 02:05:45 UTC, Ali Çehreli wrote:
> On 1/1/26 9:01 AM, zhade wrote:
>> Hi, I am trying to fail-fast and need to return "something" 
>> that works together with MapResult.
>
> A method I used in the past is to put the entire 
> range-generating code inside a function that has a default 
> argument (makeRange below):

Why put that code in a /nested/ function (template)?

> auto example_func(bool fail)
> {
>     alias filterFunc = a => a != "expensive";
>     alias mapFunc = a => tuple!("value", "numLetters")(a, 
> a.length);
>
>     auto makeRange(R)(R r = 
> typeof(someExpensiveOperation()).init) {
>         return r
>             .filter!filterFunc
>             .map!mapFunc;
>     }
>
>     if (fail)
>     {
>         return makeRange();
>     }
>
>     auto list = someExpensiveOperation();
>
>     return makeRange(list);
> }

> makeRange need not be a template but the code is a little 
> cleaner when it is.

Would it not be even cleaner with less indentation, less ifs and 
less
functions?:

    auto example_func (bool fail)
    {
       alias filterFunc = a => a != "expensive";
       alias mapFunc = a => tuple!("value", "numLetters")(a, 
a.length);

       auto list = fail
          ? null // []
          : someExpensiveOperation();

       return list
          .filter!filterFunc
          .map!mapFunc;
    }

BTW: The OP mentioned the notion of "fail fast". example_func is 
not
fail fast. It fails fast if it immediately throws:

    auto example_func (bool fail)
    {
       enforce (! fail, "oh no, failing...");

       alias filterFunc = a => a != "expensive";
       alias mapFunc = a => tuple!("value", "numLetters")(a, 
a.length);

       return someExpensiveOperation()
          .filter!filterFunc
          .map!mapFunc;
    }