Automatically add envar to help.

See #57.

README.md

@@ -117,6 +117,10 @@ eg.
       -f, --force        Force removal.
       -r, --recursive    Recursively remove files.
 
+For flags with associated environment variables, the variable `${env}` can be
+interpolated into the help string. In the absence of this variable in the help, 
+
+
 <a id="markdown-command-handling" name="command-handling"></a>
 ## Command handling
 
@@ -459,6 +463,7 @@ are defined from the value itself:
 
     ${default}
     ${enum}
+    ${env}
 
 eg.
 

interpolate.go

@@ -7,6 +7,17 @@ import (
 
 var interpolationRegex = regexp.MustCompile(`((?:\${([[:alpha:]_][[:word:]]*))(?:=([^}]+))?})|(\$)|([^$]+)`)
 
+// Returns true if the variable "v" is interpolated in "s".
+func interpolationHasVar(s string, v string) bool {
+	matches := interpolationRegex.FindAllStringSubmatch(s, -1)
+	for _, match := range matches {
+		if name := match[2]; name == v {
+			return true
+		}
+	}
+	return false
+}
+
 // Interpolate variables from vars into s for substrings in the form ${var} or ${var=default}.
 func interpolate(s string, vars map[string]string) (string, error) {
 	out := ""

kong.go

@@ -155,6 +155,19 @@ func (k *Kong) interpolateValue(value *Value, vars Vars) (err error) {
 	})
 	if value.Tag.Env != "" {
 		vars["env"] = value.Tag.Env
+		if !interpolationHasVar(value.Help, "env") {
+			suffix := "($" + value.Tag.Env + ")"
+			switch {
+			case strings.HasSuffix(value.Help, "."):
+				value.Help = value.Help[:len(value.Help)-1] + " " + suffix + "."
+
+			case value.Help == "":
+				value.Help += suffix
+
+			default:
+				value.Help += " " + suffix
+			}
+		}
 	}
 	if value.Help, err = interpolate(value.Help, vars); err != nil {
 		return fmt.Errorf("help for %s: %s", value.Summary(), err)

kong_test.go

@@ -750,6 +750,17 @@ func TestEnvarEnumValidated(t *testing.T) {
 	require.EqualError(t, err, "--flag must be one of \"valid\" but got \"invalid\"")
 }
 
+func TestEnvarAutoHelp(t *testing.T) {
+	var cli struct {
+		Flag string `env:"FLAG" help:"A flag."`
+	}
+	w := &strings.Builder{}
+	p := mustNew(t, &cli, kong.Writers(w, w), kong.Exit(func(int) {}))
+	_, err := p.Parse([]string{"--help"})
+	require.NoError(t, err)
+	require.Contains(t, w.String(), "A flag ($FLAG).")
+}
+
 func TestXor(t *testing.T) {
 	var cli struct {
 		Hello bool   `xor:"another"`