How can I merge two yaml files using yq library ?

[mikefarah]

Discussed in #1966

^{Originally posted by GeorgeDuckman March 7, 2024}
Hello,

I am attempting to use yq as a library in my Golang application. I want to merge two yaml contents that are contained in two different string variables.

I am aware that the yq command can be used to merge two YAML files :

yq -n 'load("file1.yaml") * load("file2.yaml")'

To use yqlib, I am using the StringEvaluator which only takes one input. I am concatenating my two variables into one as follows:

---
a: Hello
---
a: Goodbye

And I want the result to be :

---
a: Goodbye

Is there a way to do this ? I would prefer not to create temporary files if possible :)
I've also tried merging by select but it returns nothing.

Here is my minimal code sample if you want to try yourself :

package main

import (
	"fmt"
	"log"

	"github.com/mikefarah/yq/v4/pkg/yqlib"
)

func main() {
	yamlA := "---\n" +
		"a: Hello"

	yamlB := "---\n" +
		"a: Goodbye"

	yaml := yamlA + "\n" + yamlB

	se := yqlib.NewStringEvaluator()
	expression := "select(document_index == 0) * select(document_index == 1)"
	strung, err := se.Evaluate(expression, yaml, yqlib.NewYamlEncoder(2, true, yqlib.YamlPreferences{}), yqlib.NewGoccyYAMLDecoder())
	if err != nil {
		log.Fatal(err)
	}
	fmt.Println(strung)
}

Thank you in advance !

[mikefarah]

There actually isn't a great way to do that at the moment, the Evaluate function runs through the documents inside the string one by one as a stream. I've added a new "EvaluateAll" function to StringEvaluator that you can use in the next release.