How do you safely deal with range.front?

[Dukc]

On Saturday, 30 December 2017 at 19:00:07 UTC, aliak wrote:
> Instead of this:
>   auto result = range.op!f;
>   if (!result.empty) {
>     result.front.method();
>   }
>
> This:
>   range.op!f.ifFront.method();

Ah, so you don't have a problem with checking emptiness but you 
want to do it inside the expression, without having to type the 
range twice? I personally find it useful to define an "use" 
template for cases like this:

import std.typecons : Nullable;

auto ref use(alias how, T)(T what){return how(what);}

void main()
{
     import std.algorithm, std.range, std.stdio;
     auto arr = [2, 3, 4];
     foreach(unused; 0 .. 5)
     {   arr
             .map!(x => (x + 2) * x)
             .use!(x => x.empty? 0: x.front)
             .writeln;
         if (arr.empty){} else arr.popFront;
     }
     readln(); //program termination delayed until pressing enter
}

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Of course, if you want you can also define a more specialized 
template which does not require typing the lambda function.