The Phobos Put

[Paul Backus]

On Friday, 31 March 2023 at 02:23:29 UTC, Steven Schveighoffer 
wrote:
> There's a certain attempt in phobos in some places to try and 
> ensure code that is going to confuse will not compile. I think 
> this is one of those attempts.
>
> Consider that if you pass a slice into `put`, then it returns 
> nothing. There is no indication of what actually was written. 
> It's essentially an inconclusive call, because the "result" is 
> the output range itself. How many elements were written? You 
> can't tell.

This is a general issue with the `put` interface—it has no 
standardized way to report failure or partial success to the 
caller. And the problem applies to both lvalue and rvalue output 
ranges. For example:

```d
void main()
{
     import std.stdio, std.range;

     auto sink = stdout.lockingTextWriter;
     put(sink, "hello");
}
```

How many characters were written? Nobody knows! Maybe you can 
find out with `File.tell`, if the file is seekable, but there's 
no guarantee it is. And if you're working in a generic context, 
where you don't know the specific type of output range you're 
dealing with? Forget it.

> I'd argue that the way input ranges are used as output ranges 
> today is extremely confusing. It makes sort of a logical sense, 
> but the fact that you need to store your "original" range, and 
> then do some length math to figure out what was written makes 
> such code very awkward all around. The output is decipherable, 
> but not obvious.
>
> I stand by my assertion that probably lvalue input ranges 
> should never have been treated as output ranges implicitly. 
> They should have had to go through some sort of wrapper.

IMO this is downstream of the issue with `put`. If `put` itself 
returned information about what was written, then there would be 
no need to try and recreate that information via side channels 
like this (or like `File.tell`).

I agree that there are ways we could change things to make the 
side channels easier to use, but ultimately that's treating the 
symptom, not the disease.