Parameterized nested field reference inside User Op

[philrz]

Repro is with Zed commit 2314053.

Let's start from this very simple example of outputting the value from a field in an embedded record.

$ zq -version
Version: v1.8.1-28-g23140538

$ echo '{foo: {bar: "baz"}}' | zq -Z 'yield foo.bar' -
"baz"

If we want to do the same when this is passed into a User Op, it's easy enough if we always know the precise field path and can express it in dotted notation or with indexing.

$ cat yield-in-op-1.zed 
op CallMe(...): (
  yield {dotted: foo.bar, indexed: this["foo"]["bar"]}
)
CallMe(this)

$ echo '{foo: {bar: "baz"}}' | zq -Z -I yield-in-op-1.zed -
{
    dotted: "baz",
    indexed: "baz"
}

However, we expect users will want to write User Ops when the field name/path is not known in advance and it's passed in from the main scope as a parameter. This currently works fine with a top-level field reference.

$ cat yield-in-op-2.zed 
op CallMe(..., const f): (
  yield this[f]
)
CallMe(this, "foo")

$ echo '{foo: {bar: "baz"}}' | zq -Z -I yield-in-op-2.zed -
{
    bar: "baz"
}

However, there's not currently a way to reference a nested field path this way. For instance, if I was a user hacking away, I might attempt:

$ cat yield-in-op-3.zed 
op CallMe(..., const f): (
  yield this[f]
)
CallMe(this, ["foo"]["bar"])

$ echo '{foo: {bar: "baz"}}' | zq -Z -I yield-in-op-3.zed -
zq: error parsing Zed in yield-in-op-3.zed at line 4, column 21:
CallMe(this, ["foo"]["bar"])
                === ^ ===

We've discussed as a team implementing an approach to this using "first class paths".

[philrz]

Verified in Zed commit e57e90e.

With the changes in the linked PR #4663, I can now pass in a dotted reference to a nested field or use field referencing with indexing to retrieve a field's value from inside the user op. Updating the repro steps to align with the revised syntax, we now see:

$ zq -version
Version: v1.8.1-37-ge57e90ee

$ cat yield-in-op-revised.zed
op CallMe(f): (
  yield f
)

$ echo '{foo: {bar: "baz"}}' | zq -Z -I yield-in-op-revised.zed 'CallMe(foo.bar)' -
"baz"

$ echo '{foo: {bar: "baz"}}' | zq -Z -I yield-in-op-revised.zed 'CallMe(this["foo"]["bar"])' -
"baz"

Thanks @mattnibs!