Support limited variable interpolation.

Kong supports limited variable interpolation into help strings, enum lists and
default values.

Variables are in the form:

    ${<name>}

Variables are set with the `Vars(map[string]string)` option. Undefined
variable references in the grammar will result in an error at construction
time.

README.md

@@ -18,6 +18,7 @@
 1. [Maps](#maps)
 1. [Custom named types](#custom-named-types)
 1. [Supported tags](#supported-tags)
+1. [Variable interpolation](#variable-interpolation)
 1. [Modifying Kong's behaviour](#modifying-kongs-behaviour)
     1. [`Name(help)` and `Description(help)` - set the application name description](#namehelp-and-descriptionhelp---set-the-application-name-description)
     1. [`Configuration(loader, paths...)` - load defaults from configuration files](#configurationloader-paths---load-defaults-from-configuration-files)
@@ -313,8 +314,8 @@ function `NamedMapper(name, mapper)`.
 
 | Name              | Description                                       |
 |-------------------|---------------------------------------------------|
-| `file`            | A path. ~ expansion is applied.                   |
+| `path`            | A path. ~ expansion is applied.                   |
-| `existingfile`    | An existing path. ~ expansion is applied.         |
+| `existingfile`    | An existing file. ~ expansion is applied.         |
 | `existingdir`     | An existing directory. ~ expansion is applied.    |
 
 
@@ -348,6 +349,41 @@ Both can coexist with standard Tag parsing.
 | `hidden`               | If present, command or flag is hidden.      |
 | `format:"X"`           | Format for parsing input, if supported.     |
 | `sep:"X"`              | Separator for sequences (defaults to ","). May be `none` to disable splitting. |
+| `enum:"X,Y,..."`       |
+
+## Variable interpolation
+
+Kong supports limited variable interpolation into help strings, enum lists and
+default values.
+
+Variables are in the form:
+
+    ${<name>}
+
+Variables are set with the `Vars(map[string]string)` option. Undefined
+variable references in the grammar will result in an error at construction
+time.
+
+When interpolating into flag or argument help strings, some extra variables
+are defined from the value itself:
+
+    ${default}
+    ${enum}
+
+eg.
+
+```go
+type cli struct {
+  Config string `type:"path" default:"${config_file}"`
+}
+
+func main() {
+  kong.Parse(&cli,
+    kong.Vars(map[string]string{
+      "config_file": "~/.app.conf",
+    }))
+}
+```
 
 ## Modifying Kong's behaviour
 

build.go

@@ -151,6 +151,7 @@ func buildField(k *Kong, node *Node, v reflect.Value, ft reflect.StructField, fv
 		Mapper:  mapper,
 		Tag:     tag,
 		Target:  fv,
+		Enum:    tag.Enum,
 
 		// Flags are optional by default, and args are required by default.
 		Required: (!tag.Arg && tag.Required) || (tag.Arg && !tag.Optional),

interpolate.go

@@ -0,0 +1,27 @@
+package kong
+
+import (
+	"fmt"
+	"regexp"
+)
+
+var interpolationRegex = regexp.MustCompile(`(\${[[:alpha:]_][[:word:]]*})|(\$)|([^$]+)`)
+
+// Interpolate variables from vars into s for substrings in the form ${var}.
+func interpolate(s string, vars map[string]string) (string, error) {
+	out := ""
+	matches := interpolationRegex.FindAllStringSubmatch(s, -1)
+	for _, match := range matches {
+		if match[1] != "" {
+			name := match[1][2 : len(match[1])-1]
+			value, ok := vars[name]
+			if !ok {
+				return "", fmt.Errorf("undefined variable ${%s}", name)
+			}
+			out += value
+		} else {
+			out += match[0]
+		}
+	}
+	return out, nil
+}

interpolate_test.go

@@ -0,0 +1,17 @@
+package kong
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestInterpolate(t *testing.T) {
+	vars := map[string]string{
+		"name": "Bobby Brown",
+		"age":  "35",
+	}
+	actual, err := interpolate("${name} is ${age} years old", vars)
+	require.NoError(t, err)
+	require.Equal(t, `Bobby Brown is 35 years old`, actual)
+}

kong.go

@@ -51,6 +51,7 @@ type Kong struct {
 	help          HelpPrinter
 	helpOptions   HelpOptions
 	helpFlag      *Flag
+	vars          map[string]string
 
 	// Set temporarily by Options. These are applied after build().
 	postBuildOptions []Option
@@ -67,6 +68,7 @@ func New(grammar interface{}, options ...Option) (*Kong, error) {
 		before:    map[reflect.Value]HookFunc{},
 		registry:  NewRegistry().RegisterDefaults(),
 		resolvers: []ResolverFunc{Envars()},
+		vars:      map[string]string{},
 	}
 
 	for _, option := range options {
@@ -88,15 +90,71 @@ func New(grammar interface{}, options ...Option) (*Kong, error) {
 	k.Model.HelpFlag = k.helpFlag
 
 	for _, option := range k.postBuildOptions {
-		if err := option(k); err != nil {
+		if err = option(k); err != nil {
 			return nil, err
 		}
 	}
 	k.postBuildOptions = nil
 
+	if err = k.interpolate(k.Model.Node); err != nil {
+		return nil, err
+	}
+
 	return k, nil
 }
 
+// Interpolate variables into model.
+func (k *Kong) interpolate(node *Node) (err error) {
+	node.Help, err = interpolate(node.Help, k.vars)
+	if err != nil {
+		return fmt.Errorf("help for %s: %s", node.Path(), err)
+	}
+	for _, flag := range node.Flags {
+		if err = k.interpolateValue(flag.Value); err != nil {
+			return err
+		}
+	}
+	for _, pos := range node.Positional {
+		if err = k.interpolateValue(pos); err != nil {
+			return err
+		}
+	}
+	for _, child := range node.Children {
+		if err = k.interpolate(child); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func (k *Kong) interpolateValue(value *Value) (err error) {
+	if value.Default, err = interpolate(value.Default, k.vars); err != nil {
+		return fmt.Errorf("default value for %s: %s", value.Summary(), err)
+	}
+	if value.Enum, err = interpolate(value.Enum, k.vars); err != nil {
+		return fmt.Errorf("enum value for %s: %s", value.Summary(), err)
+	}
+	vars := mergeVars(k.vars, map[string]string{
+		"default": value.Default,
+		"enum":    value.Enum,
+	})
+	if value.Help, err = interpolate(value.Help, vars); err != nil {
+		return fmt.Errorf("help for %s: %s", value.Summary(), err)
+	}
+	return nil
+}
+
+func mergeVars(base, extra map[string]string) map[string]string {
+	out := make(map[string]string, len(base)+len(extra))
+	for k, v := range base {
+		out[k] = v
+	}
+	for k, v := range extra {
+		out[k] = v
+	}
+	return out
+}
+
 // Provide additional builtin flags, if any.
 func (k *Kong) extraFlags() []*Flag {
 	if k.noDefaultHelp {

kong_test.go

@@ -512,3 +512,24 @@ func TestRun(t *testing.T) {
 	err = ctx.Run("ERROR")
 	require.Error(t, err)
 }
+
+func TestInterpolationIntoModel(t *testing.T) {
+	var cli struct {
+		Flag    string `default:"${default}" help:"Help, I need ${somebody}" enum:"${enum}"`
+		EnumRef string `enum:"a,b" help:"One of ${enum}"`
+	}
+	_, err := kong.New(&cli)
+	require.Error(t, err)
+	p, err := kong.New(&cli, kong.Vars(map[string]string{
+		"default":  "Some default value.",
+		"somebody": "chickens!",
+		"enum":     "a,b,c,d",
+	}))
+	require.NoError(t, err)
+	flag := p.Model.Flags[1]
+	flag2 := p.Model.Flags[2]
+	require.Equal(t, "Some default value.", flag.Default)
+	require.Equal(t, "Help, I need chickens!", flag.Help)
+	require.Equal(t, map[string]bool{"a": true, "b": true, "c": true, "d": true}, flag.EnumMap())
+	require.Equal(t, "One of a,b", flag2.Help)
+}

model.go

@@ -185,10 +185,11 @@ func (n *Node) Path() (out string) {
 
 // A Value is either a flag or a variable positional argument.
 type Value struct {
-	Flag     *Flag
+	Flag     *Flag // Nil if positional argument.
 	Name     string
 	Help     string
 	Default  string
+	Enum     string
 	Mapper   Mapper
 	Tag      *Tag
 	Target   reflect.Value
@@ -198,6 +199,16 @@ type Value struct {
 	Position int    // Position (for positional arguments).
 }
 
+// EnumMap returns a map of the enums in this value.
+func (v *Value) EnumMap() map[string]bool {
+	parts := strings.Split(v.Enum, ",")
+	out := make(map[string]bool, len(parts))
+	for _, part := range parts {
+		out[strings.TrimSpace(part)] = true
+	}
+	return out
+}
+
 // Summary returns a human-readable summary of the value.
 func (v *Value) Summary() string {
 	if v.Flag != nil {

options.go

@@ -12,6 +12,16 @@ import (
 // An Option applies optional changes to the Kong application.
 type Option func(k *Kong) error
 
+// Vars sets the variables to use for interpolation into help strings and default values.
+//
+// See README for details.
+func Vars(vars map[string]string) Option {
+	return func(k *Kong) error {
+		k.vars = vars
+		return nil
+	}
+}
+
 // Exit overrides the function used to terminate. This is useful for testing or interactive use.
 func Exit(exit func(int)) Option {
 	return func(k *Kong) error {

tag.go

@@ -24,7 +24,7 @@ type Tag struct {
 	Short       rune
 	Hidden      bool
 	Sep         rune
-	Enum        map[string]bool
+	Enum        string
 
 	// Storage for all tag keys for arbitrary lookups.
 	items map[string]string
@@ -114,7 +114,6 @@ func parseTag(fv reflect.Value, ft reflect.StructField) *Tag {
 	s, chars := getTagInfo(ft)
 	t := &Tag{
 		items: parseTagItems(s, chars),
-		Enum:  map[string]bool{},
 	}
 	t.Cmd = t.Has("cmd")
 	t.Arg = t.Has("arg")
@@ -149,10 +148,7 @@ func parseTag(fv reflect.Value, ft reflect.StructField) *Tag {
 	if t.PlaceHolder == "" {
 		t.PlaceHolder = strings.ToUpper(dashedString(fv.Type().Name()))
 	}
-	for _, part := range strings.Split(t.Get("enum"), ",") {
+	t.Enum = t.Get("enum")
-		t.Enum[part] = true
-	}
-
 	return t
 }