showLinesStdOut and friends could keep the elements unchanged

[kubukoz]

Currently showLinesStdOut uses evalMap to print each element, discarding its value. I think these operators would be much more useful if they kept the original elements (so that there's a nice and easy way to debug the values of a stream at a stage without having to reimplement showLinesStdOut with evalTap).

[ChristopherDavenport]

.observe lets you have this semantic rather than passing through what is effectively an outlet of values.

[kubukoz]

Right, .observe(_.showLinesStdOut) does the job. It would be nice to have a faster way of doing this, especially repeatedly, like .through(debug)... we could have a pipe like that in the package object or Stream/Pipe's companion.

[mpilquist]

I like making debugging simpler. Signature of debug could be tricky. Where would it write elements? Does it need to take a String => F[Unit]?

[kubukoz]

I'd say we could add a specific one for starters (just debug(tag: String) that prints to stdout) and that'll be it... and maybe debugToString that won't require Show, and just toString the elements before printing.

A function for debugging that takes String => F[Unit] would be pretty similar to evalTap, so I'm not sure it'd be worth the hassle... but anything that allows skipping s => Sync[F].delay(println("tag: " + s)) will be a welcome change.

[kubukoz]

The greatest problem with observe is that it requires Concurrent, IMO. I think we could start with def debug(tag: String): Stream[F, O] that outputs $tag: $elem to stdout.