Convert env management to a resolver.

This ensures that hooks correctly detect when values are set via
environment variables. Fixes #73.

env.go

@@ -0,0 +1,21 @@
+package kong
+
+import (
+	"os"
+)
+
+// EnvarResolver resolves values from environment variables.
+//
+// It is installed by default. Use ClearResolvers() to disable this.
+func EnvarResolver() Resolver {
+	return ResolverFunc(func(context *Context, parent *Path, flag *Flag) (interface{}, error) {
+		if flag.Tag.Env == "" {
+			return nil, nil
+		}
+		envar := os.Getenv(flag.Tag.Env)
+		if envar != "" {
+			return envar, nil
+		}
+		return nil, nil
+	})
+}

kong.go

@@ -71,6 +71,7 @@ func New(grammar interface{}, options ...Option) (*Kong, error) {
 		vars:          Vars{},
 		bindings:      bindings{},
 		helpFormatter: DefaultHelpValueFormatter,
+		resolvers:     []Resolver{EnvarResolver()},
 	}
 
 	options = append(options, Bind(k))

kong_test.go

@@ -675,6 +675,19 @@ func TestHooksCalledForDefault(t *testing.T) {
 	require.Equal(t, []string{"before:default", "after:default"}, ctx.values)
 }
 
+func TestHooksCalledForEnv(t *testing.T) {
+	var cli struct {
+		Flag hookValue `env:"FLAG"`
+	}
+	defer tempEnv(envMap{"FLAG": "flagged"})()
+
+	ctx := &hookContext{}
+	_, err := mustNew(t, &cli, kong.Bind(ctx)).Parse(nil)
+	require.NoError(t, err)
+	require.Equal(t, "flagged", string(cli.Flag))
+	require.Equal(t, []string{"before:", "after:flagged"}, ctx.values)
+}
+
 func TestEnum(t *testing.T) {
 	var cli struct {
 		Flag string `enum:"a,b,c"`

model.go

@@ -3,7 +3,6 @@ package kong
 import (
 	"fmt"
 	"math"
-	"os"
 	"reflect"
 	"strconv"
 	"strings"
@@ -326,16 +325,6 @@ func (v *Value) ApplyDefault() error {
 // Does not include resolvers.
 func (v *Value) Reset() error {
 	v.Target.Set(reflect.Zero(v.Target.Type()))
-	if v.Tag.Env != "" {
-		envar := os.Getenv(v.Tag.Env)
-		if envar != "" {
-			err := v.Parse(ScanFromTokens(Token{Type: FlagValueToken, Value: envar}), v.Target)
-			if err != nil {
-				return fmt.Errorf("%s (from envar %s=%q)", err, v.Tag.Env, envar)
-			}
-			return nil
-		}
-	}
 	if v.Default != "" {
 		return v.Parse(ScanFromTokens(Token{Type: FlagValueToken, Value: v.Default}), v.Target)
 	}