`into` semantics as transform?

[max-sixty]

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Thanks to @broneill — full details at https://github.com/PRQL/prql/discussions/4277#discussioncomment-8648606

I had been thinking of into as a transform — so following a pipe (either a | or new line).

Currently, quoting from the discussion linked above

valid, correctly so

from y
into x

from x

valid, but incorrectly so — I think should be invalid

from y into x  # should require `|`

from x

invalid, but incorrectly so — I think should be valid

from y | into x  # the `|` should make it valid

from x

[snth]

Can you give an example of how 1 into a is legal? I tried to produce one myself but couldn't get it to work.

[max-sixty]

Can you give an example of how 1 into a is legal? I tried to produce one myself but couldn't get it to work.

[edited]

A better example is:

from y into x
from x

Will edit the original and hide these, thanks a lot @snth

[aljazerzen]

Discussion for adding into is here: #2427

#2427 mentions benefit of scanning only the first keyword in the line to find all variable declarations, which would be against this.

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I'm not sold on making into a normal function, because we want into to be a special keyword, because it cannot be used somewhere in the middle of a pipeline (or a function call tree):

let a = (
  from x
  into b
)

This feels very wrong.

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To get the proposed results, we could just add additional syntax that would allow top-level expression to have | into x appended so that would count as variable declaration.

I would be -0 on this compromise.

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It feels like the best solution here would be to just provide good errors when the into keyword is used where it shouldn't be.

[max-sixty]

My main point here is that saying "within a pipeline, a pipe character is the same as a newline — except in the case of into" seems complicated, and we get a much simpler result by avoiding that exception.

Ref these docs:

In almost all situations, a line break acts as a pipe. [though we go on:] But there are a few exceptions where a line break doesn’t create a pipeline:

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A level down:

let a = (
  from x
  into b
)

This feels very wrong.

Yes fair! Though:

• this could be an error because there's no value being assigned to a
• or even into could return nothing, a bit like let foo = {let x = 5;} returns nothing in rust, and so let a get assigned null (not saying this is necessary or even good, just that it's not obviously a compiler error)