Alternative to std.range.choose

[Steven Schveighoffer]

On 7/22/20 12:33 AM, James Gray wrote:
> Is there a better way to achieve behaviour similar to rangeFuncIf
> below? f gives a contrived example of when one might want this. g is
> 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
>        .chain(only(true).repeat(c?0:1).map!(x=>F2()).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(f(2));
>   writeln(f(0));
>   writeln(g(2));
>   //writeln(g(0)); <---- runtime error
> }

I know this is a contrived example, but choose is not good for such a 
thing. It should only be used if the range types are different.

g could be:

auto g(ulong n) {
   return only(n != 0L ? 100/n : 0)
}

But in any case, changing choose to use lazy is probably a good answer.

-Steve