countUntil to print all the index of a given string.

[Jonathan M Davis]

On Sunday, February 25, 2018 01:49:05 Seb via Digitalmars-d-learn wrote:
> On Tuesday, 20 February 2018 at 08:44:37 UTC, aberba wrote:
> > On Sunday, 18 February 2018 at 15:23:14 UTC, Cym13 wrote:
> >> On Sunday, 18 February 2018 at 14:48:59 UTC, Cym13 wrote:
> >>> [...]
> >>
> >> Just thought of a much better/simpler solution for that last
> >> case that also doesn't force you to read all data (which might
> >>
> >> be impossible when dealing with infinite ranges):
> >>     import std.range;
> >>     import std.algorithm;
> >>
> >>     a[]
> >>
> >>      .enumerate                       // get tuples (index,
> >>
> >> value)
> >>
> >>      .filter!(t => t[1] == "Test2")   // keep only if value ==
> >>
> >> "Test2"
> >>
> >>      .map!(t => t[0])                 // keep only the index
> >>
> >> part
> >>
> >>      .writeln;
> >>
> >> Completely lazy.
> >
> > How does one detect an operation as lazy or not?  Is the some
> > compile-time or runtime check for that?
> >
> > My guess is by referring to the docs function signature.
>
> While it's not a replacement for checking the code manually,
> @nogc helps a lot:

That doesn't actually tell you whether the range is lazy. It just tells you
whether any allocations may occur. If any exceptions could be thrown, then a
lazy solution can't be @nogc (something that's often the case with strings
thanks to auto-decoding and UTFExceptions), and a solution could be eager
without allocating if the result doesn't require any allocation. Also, you
could have a lazy range that involves a lambda that allocates a closure.

So, yeah, a lot of the time, @nogc means that the range is lazy, but it
doesn't guarantee it, and not being able to be @nogc doesn't mean that it's
eager. So, I'd argue that while @nogc gives you a clue, it's ultimately a
pretty poor way to try and figure out whether a range is lazy or not. All it
really tells you is whether it's guaranteed that no allocations will occur
on the GC heap.

- Jonathan M Davis