Infinite range of nullable elements

[anonymous via Digitalmars-d-learn]

On Friday, 17 July 2015 at 07:42:09 UTC, Roland Hadinger wrote:
> Here's how I would implement the basic behaviour (could be 
> extended to also forward bidirectional and random access 
> functions):
>
> ---
>     auto cushion(R)(R r)
>         if (isInputRange!R)
>     {
>         static if (isInfinite!R) { return r; } else {
>
>             struct _Cushion(R)
>             {
>                 R r;
>
>                 alias E = ElementType!R;
>                 alias NE = Nullable!E;
>
>                 @property bool empty() { return false; }
>
>                 @property NE front()
>                 {
>                     return !r.empty ? NE(r.front) : NE();
>                 }
>
>                 void popFront()
>                 {
>                     if (!r.empty) r.popFront();
>                 }
>             }
>
>             return _Cushion!R(r);
>         }
>     }
>
> ---
>
> I didn't find anything like this Phobos. Did I miss something?

The building blocks are there. You're `map`ping the original 
range to `Nullable`, and then you're `chain`ing an infinite range 
(`cycle`) of nulls behind.

----
import std.range: isInputRange;

auto cushion(R)(R r)
     if (isInputRange!R)
{
     import std.algorithm: map;
     import std.range: chain, cycle, ElementType, only;
     import std.typecons: Nullable;

     alias E = ElementType!R;
     alias NE = Nullable!E;

     return chain(r.map!NE, NE().only.cycle);
}
----