Should IO ports be first class?

[tesujimath]

What new feature should Elvish have?

I am wondering whether IO ports should be first class, so they could be assigned to variables, passed to functions, etc.

My use-case motivating this is processing stream input, where there is a header to be processed first. So I envisage being able to do from-lines | take 1 for the header, but still subsequently have access to the IO port to read the rest of the stream.

I haven't thought at all about syntax yet, it's the concept I would first like to understand.

This would seem to be an answer also for #741

Thoughts?

Output of "elvish -version"

0.21.0

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[tesujimath]

Here's a bit more about what this could look like.

Let's suppose we have a new builtin function make-io-port. And here, obviously, echo is standing in for an external program.

var p = (make-io-port)
echo "1\n2\n3\n4\n5" | from-lines | $p
$p | take 2
▶ 1
▶ 2
$p | take 1
▶ 3
$p
▶ 4
▶ 5

Perhaps we could also or instead have a builtin function capture, which creates an IO port and attaches its input.

var p = (echo "1\n2\n3\n4\n5" | from-lines | capture)
$p | take 2
▶ 1
▶ 2
$p | take 1
▶ 3
$p
▶ 4
▶ 5

This should work for streaming input, so imagine instead of 5 lines, an unending stream from some external source.

I hope that gives the flavour of what I am envisaging. I am sure there is an interesting design space for such a feature!