Alternative to std.range.choose

James Gray test at
Wed Jul 22 04:33:20 UTC 2020

Is there a better way to achieve behaviour similar to rangeFuncIf
below? f gives a contrived example of when one might want this. g 
how one might try and achieve the same with std.range.choose.

import std.stdio;
import std.range : only, chain, join, choose;
import std.algorithm : map;

auto rangeFunctIf(alias F1, alias F2)(bool c)
  if ( __traits(compiles,F1().chain(F2())))
    return only(true).repeat(c?1:0).map!(x=>F1()).join

auto f(ulong n) {
  return (n!=0uL).rangeFuncIf!(()=>only(100/n), ()=>only(0));
auto g(ulong n) {
  return choose(n!=0uL,only(100/n),only(0));

void main()
  //writeln(g(0)); <---- runtime error

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