rapidly prototyped demo of thema and codegen for loki config file

lokicfg.cue

package lokicfg

import "github.com/grafana/thema"

// - MIGHT be helpful for them if there could be an init command that tries to start from Go structs
// - Probably need to generate YAML struct tags

thema.#Lineage
name: "lokicfg"
seqs: [
	{
		schemas: [
			// v0.0
			{
				server: {
					port: int32
				}
			},
			// v0.1
			{
				server: {
					port: int32
					tlsVersion: int32 & (*1 | 2)
				}
			}
		]
	},
]

lokicfg_gen.go

// WE'RE IMAGINING THIS FILE IS AUTOGENERATED

package lokicfg

import (
	"embed"
	"sync"

	"cuelang.org/go/cue/cuecontext"
	"github.com/grafana/thema"
	"github.com/grafana/thema/load"
	"github.com/grafana/thema/vmux"
)

type Lokicfg struct {
	Server struct {
		Port       int32 `json:"port"`
		TlsVersion int32 `json:"tlsVersion"`
	} `json:"server"`
}

// vars representing the state of the lineage cue file
var (
	//go:embed lokicfg.cue
	cueFS          embed.FS
	currentVersion = thema.SV(0, 1)
)

// vars for handling caching of the lineage factory output
var (
	lineageOnce sync.Once
	clin        thema.ConvergentLineage[*Lokicfg]
	clinerr     error
)

var allctx = cuecontext.New()
var allrt = thema.NewRuntime(allctx)

// type guard
var _ thema.ConvergentLineageFactory[*Lokicfg] = Lineage

func init() {
	// It's probably useful to trigger this on init, and panic right away if there's
	// a problem, as the only possible fix is updating lokicfg.cue.
	_, err := Lineage(allrt)
	if err != nil {
		panic(err)
	}
}

func Lineage(rt *thema.Runtime, opt ...thema.BindOption) (thema.ConvergentLineage[*Lokicfg], error) {
	if rt == nil || rt == allrt {
		lineageOnce.Do(func() {
			// This freezes the thema.BindOption set to whatever is passed in on the first
			// call. That could be problematic in the future, but for now the set is really
			// small.
			clin, clinerr = doLineage(allrt, opt...)
		})
		return clin, clinerr
	}
	return doLineage(rt, opt...)
}

func doLineage(rt *thema.Runtime, opt ...thema.BindOption) (thema.ConvergentLineage[*Lokicfg], error) {
	linval, err := load.InstancesWithThema(cueFS, ".")
	if err != nil {
		return nil, err
	}
	// this returns a thema.Lineage
	lin, err := thema.BindLineage(rt.Context().BuildInstance(linval), rt)
	if err != nil {
		// This would error out if you have a generally invalid lineage -
		// e.g., schemas don't follow backwards compatibility rules,
		// lenses are incomplete, etc.; all the general invariants Thema
		// aims to provide over all lineages
		return nil, err
	}
	sch, _ := lin.Schema(currentVersion)
	tsch, err := thema.BindType[*Lokicfg](sch, &Lokicfg{})
	if err != nil {
		// This would error out if the Lokicfg isn't assignable to the 0.1 schema.
		// This should be unreachable, barring a critical bug in Thema's Go generator.
		return nil, err
	}
	return tsch.ConvergentLineage(), nil
}

func GimmeValueMux(rt *thema.Runtime) vmux.ValueMux[*Lokicfg] {
	lin, err := Lineage(rt)
	if err != nil {
		panic(err)
	}
	return vmux.NewValueMux(lin.TypedSchema(), vmux.NewYAMLEndec("conf.yaml"))
}