templatefile/main.go

package main

import (
	encjson "encoding/json"
	"fmt"
	"os"
	"text/template"

	"github.com/hashicorp/hcl/v2"
	"github.com/hashicorp/hcl/v2/hclsyntax"
	"github.com/zclconf/go-cty/cty"
	ctyjson "github.com/zclconf/go-cty/cty/json"
)

func checkErr(e error) {
	if e != nil {
		panic(e)
	}
}

func checkDiags(diag hcl.Diagnostics) {
	if diag.HasErrors() {
		panic(diag.Error())
	}
}

func processGoTemplate(file string, config []byte) {

	// The JSON configuration
	var confData map[string]interface{}
	err := encjson.Unmarshal(config, &confData)
	checkErr(err)

	// Read the template
	data, err := os.ReadFile(file)
	checkErr(err)

	tpl, err := template.New("conf").Parse(string(data))
	checkErr(err)

	checkErr(tpl.Execute(os.Stdout, confData))

}

func processHCLTemplate(file string, config []byte) {

	fct, err := os.ReadFile(file)
	checkErr(err)

	expr, diags := hclsyntax.ParseTemplate(fct, file, hcl.Pos{Line: 1, Column: 1})
	checkDiags(diags)

	// Retrieve values from JSON
	var varsVal cty.Value
	ctyType, err := ctyjson.ImpliedType(config)
	if err != nil {
		panic(err)
		/* Maybe one day
		cexpr, diags := hclsyntax.ParseExpression(config, "", hcl.Pos{Line: 1, Column: 1})
		if diags.HasErrors() {
			panic(diags.Error())
		}
		varsVal, diags = cexpr.Value(&hcl.EvalContext{})
		fmt.Println(cexpr.Variables())
		checkDiags(diags)
		*/
	} else {
		varsVal, err = ctyjson.Unmarshal(config, ctyType)
		checkErr(err)
	}

	ctx := &hcl.EvalContext{
		Variables: varsVal.AsValueMap(),
	}

	for n := range ctx.Variables {
		if !hclsyntax.ValidIdentifier(n) {
			panic(fmt.Errorf("invalid template variable name %q: must start with a letter, followed by zero or more letters, digits, and underscores", n))
		}
	}

	for _, traversal := range expr.Variables() {
		root := traversal.RootName()
		if _, ok := ctx.Variables[root]; !ok {
			panic(fmt.Errorf("vars map does not contain key %q, referenced at %s", root, traversal[0].SourceRange()))
		}
	}

	val, diags := expr.Value(ctx)
	if diags.HasErrors() {
		panic(diags.Error())
	}

	fmt.Printf("%s", val.AsString())
}

func main() {
	// The template to process
	templateType := os.Args[1]
	templateFile := os.Args[2]
	config := []byte(os.Args[3])

	if templateType == "go" {
		processGoTemplate(templateFile, config)
	} else if templateType == "hcl" {
		processHCLTemplate(templateFile, config)
	} else {
		panic(fmt.Errorf("Unsupported template type"))
	}
}