Unifying incomplete CUE and concrete data, using the cue command and the Go API

[jpluscplusm]

Slack thread: https://cuelang.slack.com/archives/CLT3ULF6C/p1718013879866749

Original Slack thread conclusion

myitcv

In general, CUE can deal with references to fields that don't exist (yet)
e.g.

x: _
y: x.f

This is referred to as incomplete CUE.
Such CUE cannot be exported, but it can be evaluated.
It can also be made complete by unifying it with a value that makes it complete.

step1: {
  x: _
  y: x.f
}

step2: {
  x: f: 5
}

res: step1 & step2

Here, step1 could be the result of a first load, for example
step2 could come from anywhere else
res is the result of unifying the two.

Here's a quick demo

go mod tidy
go run .
cmp stdout stdout.golden

-- go.mod --
module mod.example

go 1.22.3

require cuelang.org/go v0.9.2

-- main.go --
package main

import (
	"fmt"
	"log"

	"cuelang.org/go/cue"
	"cuelang.org/go/cue/cuecontext"
	"cuelang.org/go/cue/load"
	"cuelang.org/go/encoding/yaml"
)

func main() {
	ctx := cuecontext.New()

	// Load the CUE package in the current directory.
	// It contains only one file, x.cue. This is step1.
	bis := load.Instances([]string{"."}, nil)
	step1 := ctx.BuildInstance(bis[0])
	fmt.Printf("step1: %v\n", step1)

	// Load x.yaml as step2.
	step2File, err := yaml.Extract("x.yaml", nil)
	if err != nil {
		log.Fatal(err)
	}
	step2 := ctx.BuildFile(step2File)
	fmt.Printf("step2: %v\n", step2)

	// Ensure the result of unifying the two steps is
	// valid and concrete, i.e. can be exported as data.
	res := step1.Unify(step2)
	if err := res.Validate(cue.Concrete(true)); err != nil {
		log.Fatal(err)
	}
	fmt.Printf("res: %v\n", res)
}

-- x.cue --
package x

x: _
y: x.f

-- x.yaml --
x:
  f:
    5

-- stdout.golden --
step1: {
	x: _
	y: x.f
}
step2: {
	x: {
		f: 5
	}
}
res: {
	x: {
		f: 5
	}
	y: 5
}

The idea being that we can show how to work with incomplete values and unification (the simplest of pipelines) to make things concrete.

[myitcv]

WIP CL https://review.gerrithub.io/c/cue-lang/cuelang.org/+/1203092