convy/internal/cli/prompt.go

package cli

import (
	"bufio"
	"errors"
	"fmt"
	"io"
	"strings"
)

// Commands that change something run in two modes, and which one is meant is
// read off the command line: bare means interactive, any flag means automatic.
//
// The two modes address two different callers. A person types the command and
// is walked through the fields with a hint for each; an agent or a script
// passes every field at once and must never be blocked waiting on a terminal
// that is not there. Neither mode guesses: a field that is required and absent
// stops the run in both.

// Field is one thing a command asks for.
type Field struct {
	// Flag is the name the field carries on the command line.
	Flag string
	// Ask is the question put to a person.
	Ask string
	// Hint is the one line explaining what belongs here and why.
	Hint string
	// Default is offered as the answer when the person just presses enter.
	Default string
	// Optional fields may stay empty.
	Optional bool
	// Check validates an answer; it is applied in both modes.
	Check func(string) error
}

// errStop ends the dialogue on end of input.
var errStop = errors.New("input ended")

// dialogue asks the fields one by one, re-asking what did not pass validation.
type dialogue struct {
	in  *bufio.Reader
	out io.Writer
}

func newDialogue(env Env) *dialogue {
	return &dialogue{in: bufio.NewReader(env.In), out: env.Out}
}

// ask puts one question and returns the answer, looping until it validates.
func (d *dialogue) ask(f Field) (string, error) {
	for {
		if f.Hint != "" {
			fmt.Fprintf(d.out, "\n%s\n", f.Hint)
		}
		prompt := f.Ask
		switch {
		case f.Default != "":
			prompt += fmt.Sprintf(" [%s]", f.Default)
		case f.Optional:
			prompt += " [may stay empty]"
		}
		fmt.Fprintf(d.out, "%s: ", prompt)

		line, err := d.in.ReadString('\n')
		answer := strings.TrimSpace(line)
		if answer == "" && err != nil {
			return "", errStop
		}
		if answer == "" {
			answer = f.Default
		}
		if answer == "" && !f.Optional {
			fmt.Fprintln(d.out, "  the field is required")
			continue
		}
		if err := validate(f, answer); err != nil {
			fmt.Fprintf(d.out, "  %s\n", err)
			continue
		}
		return answer, nil
	}
}

// validate applies a field's check to a value that is not empty.
func validate(f Field, value string) error {
	if value == "" || f.Check == nil {
		return nil
	}
	return f.Check(value)
}

// resolve settles the fields in automatic mode: what came on the command line
// is validated, what is required and missing is named. Every missing field is
// reported at once — being sent back one flag at a time is the worst way to
// learn what a command wants.
func resolve(fields []Field, given map[string]string) error {
	var missing []string
	var problems []string
	for _, f := range fields {
		value := given[f.Flag]
		if value == "" {
			value = f.Default
			given[f.Flag] = value
		}
		if value == "" {
			if !f.Optional {
				missing = append(missing, "--"+f.Flag)
			}
			continue
		}
		if err := validate(f, value); err != nil {
			problems = append(problems, fmt.Sprintf("--%s: %s", f.Flag, err))
		}
	}
	if len(missing) > 0 {
		problems = append(problems, "required and missing: "+strings.Join(missing, ", "))
	}
	if len(problems) > 0 {
		return errors.New(strings.Join(problems, "\n"))
	}
	return nil
}