Return explicitly "empty" MapResult (fail-fast)

[Ali Çehreli]

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):

import std;

auto someExpensiveOperation() {
     return [ "hello" ];
}

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);
}

void main() {
     example_func(true);
}

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

That solution was a part of the following presentation (step 98/121 there):

   https://youtu.be/dRORNQIB2wA?t=2628

Ali