making a struct an inputRange with free functions

[Jonathan M Davis]

On Monday, April 16, 2018 21:10:03 Johannes Loher via Digitalmars-d-learn 
wrote:
> Is there a way to do this? Here is a naive implementation:
> https://run.dlang.io/is/JKvL80 .
>
> It does not pass `isInputRange` (I think, because the free functions are
> not visible in the scope of `isInputRange`).
>
> Trying to iterate over it with a foreach loop results in a compile error:
> Error: invalid foreach aggregate NoRange(0, 0).this(5), define
> opApply(), range primitives, or use .tupleof
>
> Thanks for your help!

It doesn't work unless the module using the range either contains the free
functions or imports them. So, I believe that that would mean that for
isInputRange to pass, std.range.primitives would have to have access to the
functions, which isn't going to happen for anything but dynamic arrays. It's
a limitation of UFCS in general (it's similar to why using string lambdas
with std.functional has become fairly rare - you can only use functions in
them that std.functional already knows about). The only way for the
functions to be associated with the type and thus always be available is if
they're member functions. So, if you want a type to be a range, you have to
declare the range functions as member functions.

- Jonathan M Davis