Map, filter and pure functions

[realhet]

On Friday, 29 November 2019 at 15:30:22 UTC, realhet wrote:
> ...

Unfortunately function purity is not the answer.

I put a very long calculation into the transform function which 
is called from "map!".
And the "filter!" is making the "map!" call my function 2 times:
First for the "filter!" to decide it allows the element or not.
And second when the map! is passing it to the foreach() where I 
write the elements out.

Is there a solution for this performance issue?

Using functional programming, my program looks beautiful. But I 
need it to be optimal too.

//To be easier to understand, I want to make the following:
foreach(a; input){
   auto temp = slowFunction(a);
   if(check(temp))
     writeln(temp);
}

//But the elegant functional form ->
input.map!slowFunction.filter!(a => check(a)).each!writeln;

//Produces the following unoptimal solution:
foreach(a; input){
   if(check(slowFunction(a)))
     writeln(slowFunction(a));
}

Is there a functional way to fix this?

Thank you in advance!