We're long overdue for a "D is awesome" post

[Steven Schveighoffer]

On 5/30/23 5:42 PM, H. S. Teoh wrote:
> So here's one.
> 
> Today, I was writing some code that iterates over a data structure and
> writes output to bunch of different files. It looks something like this:
> 
> void genSplitHtml(Data data, ...) {
> 	auto outputTemplate = File("template.html", "r");
> 	foreach (...) {
> 		auto filename = generateFilename(...);
> 		auto sink = File(filename, "w").lockingTextWriter;
> 		...
> 		while (line; outputTemplate.byLine) {
> 			if (line.canFind("MAGIC_TOKEN")) {
> 				generateOutput(...);
> 			} else {
> 				sink.put(line);
> 			}
> 		}
> 	}
> }

> 
> Since the whole point of this function was to write output to different
> files (with automatically determined names), I wanted to write a
> unittest that tests whether it creates the correct files with the
> correct contents.  But I didn't want unittests to touch the actual
> filesystem either -- didn't want to have to clean up the mess during
> development where the code might sometimes break and leave detritus
> behind in a temporary directory, or interact badly with other unittests
> running in parallel, etc..

All of iopipe is 100% unittestable, because all strings are treated as 
input pipes.

Roughly speaking, if this were iopipe, I would do it like:

```d
// returns an iopipe with the translated data
auto genSplitHtml(Input)(Input inputPipe, Data data, ...)
{
    ...
}

// and then you can call it like:

File("template.html", "r").genSplitHtml(data).writeTo(File(filename, 
"w")).process();
// in unittest
auto result = "contents of template".genSplitHtml(data);
result.process(); // oof, would be nice if I could do this in one line
assert(result.window == expectedData);
```

I need to work on it more, that `writeTo` function is not there yet 
(https://github.com/schveiguy/iopipe/issues/39) and the file opening 
stuff isn't as solid as I'd like.

> D not only r0x0rs, D boulders!!

Indeed it does!

-Steve