Prompt for input and cache during template execution

main.go

package main

import (
	"bufio"
	"bytes"
	"fmt"
	"log"
	"os"
	"strings"
	"text/template"
)

func main() {
	cache := make(map[string]string)
	for i := 1; i < len(os.Args); i++ {
		tokens := strings.SplitN(os.Args[i], "=", 2)
		if len(tokens) != 2 {
			log.Fatal("parameters must be passed as name=value")
		}

		name, value := tokens[0], tokens[1]
		cache[name] = value
	}

	funcs := template.FuncMap{
		"param": func(name string, params ...string) (value string, err error) {
			var ok bool
			if value, ok = cache[name]; !ok {
				defaultValue := ""
				if len(params) > 0 {
					defaultValue = params[0]
				}

				prompt := name
				if len(params) > 1 {
					prompt = fmt.Sprintf("%s (%s)", params[1], name)
				}

				reader := bufio.NewReader(os.Stdin)

				fmt.Fprintf(os.Stderr, "\033[32m%s? \033[90m[%s]\033[0m: ", prompt, defaultValue)
				value, err = reader.ReadString('\n')
				if err != nil {
					return
				}

				value = strings.TrimSpace(value)
				if value == "" {
					value = defaultValue
				}

				cache[name] = value
			}

			return
		},
	}

	templ, err := template.New("").Funcs(funcs).Parse(`Hello, {{param "name" "world" "Who should I greet"}}! It's nice to meet you, {{param "name"}}.`)
	if err != nil {
		log.Fatalf("failed to parse template: %v", err)
	}

	buffer := &bytes.Buffer{}
	err = templ.Execute(buffer, nil)
	if err != nil {
		log.Fatalf("failed to execute template: %v", err)
	}

	fmt.Println(buffer.String())
}