Convert resolvers to an interface with a Validate() method.

context.go

@@ -51,7 +51,7 @@ type Context struct {
 	Error error
 
 	values    map[*Value]reflect.Value // Temporary values during tracing.
-	resolvers []ResolverFunc           // Extra context-specific resolvers.
+	resolvers []Resolver               // Extra context-specific resolvers.
 	scan      *Scanner
 }
 
@@ -119,6 +119,11 @@ func (c *Context) Empty() bool {
 
 // Validate the current context.
 func (c *Context) Validate() error {
+	for _, resolver := range c.combineResolvers() {
+		if err := resolver.Validate(c.Model); err != nil {
+			return err
+		}
+	}
 	for _, path := range c.Path {
 		if err := checkMissingFlags(path.Flags); err != nil {
 			return err
@@ -183,7 +188,7 @@ func (c *Context) Command() string {
 // AddResolver adds a context-specific resolver.
 //
 // This is most useful in the BeforeResolve() hook.
-func (c *Context) AddResolver(resolver ResolverFunc) {
+func (c *Context) AddResolver(resolver Resolver) {
 	c.resolvers = append(c.resolvers, resolver)
 }
 
@@ -345,31 +350,9 @@ func (c *Context) trace(node *Node) (err error) { // nolint: gocyclo
 	return nil
 }
 
-func findPotentialCandidates(needle string, haystack []string, format string, args ...interface{}) error {
-	if len(haystack) == 0 {
-		return fmt.Errorf(format, args...)
-	}
-	closestCandidates := []string{}
-	for _, candidate := range haystack {
-		if strings.HasPrefix(candidate, needle) || levenshtein(candidate, needle) <= 2 {
-			closestCandidates = append(closestCandidates, fmt.Sprintf("%q", candidate))
-		}
-	}
-	prefix := fmt.Sprintf(format, args...)
-	if len(closestCandidates) == 1 {
-		return fmt.Errorf("%s, did you mean %s?", prefix, closestCandidates[0])
-	} else if len(closestCandidates) > 1 {
-		return fmt.Errorf("%s, did you mean one of %s?", prefix, strings.Join(closestCandidates, ", "))
-	}
-	return fmt.Errorf("%s", prefix)
-}
-
 // Resolve walks through the traced path, applying resolvers to any unset flags.
 func (c *Context) Resolve() error {
-	// Combine application-level resolvers and context resolvers.
+	resolvers := c.combineResolvers()
-	resolvers := []ResolverFunc{}
-	resolvers = append(resolvers, c.Kong.resolvers...)
-	resolvers = append(resolvers, c.resolvers...)
 	if len(resolvers) == 0 {
 		return nil
 	}
@@ -382,7 +365,7 @@ func (c *Context) Resolve() error {
 				continue
 			}
 			for _, resolver := range resolvers {
-				s, err := resolver(c, path, flag)
+				s, err := resolver.Resolve(c, path, flag)
 				if err != nil {
 					return err
 				}
@@ -407,6 +390,14 @@ func (c *Context) Resolve() error {
 	return nil
 }
 
+// Combine application-level resolvers and context resolvers.
+func (c *Context) combineResolvers() []Resolver {
+	resolvers := []Resolver{}
+	resolvers = append(resolvers, c.Kong.resolvers...)
+	resolvers = append(resolvers, c.resolvers...)
+	return resolvers
+}
+
 func (c *Context) getValue(value *Value) reflect.Value {
 	v, ok := c.values[value]
 	if !ok {
@@ -573,3 +564,22 @@ func checkMissingPositionals(positional int, values []*Value) error {
 	}
 	return fmt.Errorf("missing positional arguments %s", strings.Join(missing, " "))
 }
+
+func findPotentialCandidates(needle string, haystack []string, format string, args ...interface{}) error {
+	if len(haystack) == 0 {
+		return fmt.Errorf(format, args...)
+	}
+	closestCandidates := []string{}
+	for _, candidate := range haystack {
+		if strings.HasPrefix(candidate, needle) || levenshtein(candidate, needle) <= 2 {
+			closestCandidates = append(closestCandidates, fmt.Sprintf("%q", candidate))
+		}
+	}
+	prefix := fmt.Sprintf(format, args...)
+	if len(closestCandidates) == 1 {
+		return fmt.Errorf("%s, did you mean %s?", prefix, closestCandidates[0])
+	} else if len(closestCandidates) > 1 {
+		return fmt.Errorf("%s, did you mean one of %s?", prefix, strings.Join(closestCandidates, ", "))
+	}
+	return fmt.Errorf("%s", prefix)
+}

kong.go

@@ -44,7 +44,7 @@ type Kong struct {
 
 	bindings  bindings
 	loader    ConfigurationFunc
-	resolvers []ResolverFunc
+	resolvers []Resolver
 	registry  *Registry
 
 	noDefaultHelp bool

options.go

@@ -180,8 +180,8 @@ func ClearResolvers() OptionFunc {
 	}
 }
 
-// Resolver registers flag resolvers.
+// Resolvers registers flag resolvers.
-func Resolver(resolvers ...ResolverFunc) OptionFunc {
+func Resolvers(resolvers ...Resolver) OptionFunc {
 	return func(k *Kong) error {
 		k.resolvers = append(k.resolvers, resolvers...)
 		return nil

resolver.go

@@ -7,9 +7,27 @@ import (
 	"strings"
 )
 
-// ResolverFunc resolves a Flag value from an external source.
+// A Resolver resolves a Flag value from an external source.
+type Resolver interface {
+	// Validate configuration against Application.
+	//
+	// This can be used to validate that all provided configuration is valid within  this application.
+	Validate(app *Application) error
+
+	// Resolve the value for a Flag.
+	Resolve(context *Context, parent *Path, flag *Flag) (string, error)
+}
+
+// ResolverFunc is a convenience type for non-validating Resolvers.
 type ResolverFunc func(context *Context, parent *Path, flag *Flag) (string, error)
 
+var _ Resolver = ResolverFunc(nil)
+
+func (r ResolverFunc) Resolve(context *Context, parent *Path, flag *Flag) (string, error) { // nolint: golint
+	return r(context, parent, flag)
+}
+func (r ResolverFunc) Validate(app *Application) error { return nil } //  nolint: golint
+
 // JSON returns a Resolver that retrieves values from a JSON source.
 //
 // Hyphens in flag names are replaced with underscores.

resolver_test.go

@@ -1,6 +1,7 @@
 package kong_test
 
 import (
+	"errors"
 	"os"
 	"reflect"
 	"strings"
@@ -91,7 +92,7 @@ func TestJSONBasic(t *testing.T) {
 	r, err := kong.JSON(strings.NewReader(json))
 	require.NoError(t, err)
 
-	parser := mustNew(t, &cli, kong.Resolver(r))
+	parser := mustNew(t, &cli, kong.Resolvers(r))
 	_, err = parser.Parse([]string{})
 	require.NoError(t, err)
 	require.Equal(t, "🍕", cli.String)
@@ -129,14 +130,14 @@ func TestResolverWithBool(t *testing.T) {
 		Bool bool
 	}
 
-	resolver := func(context *kong.Context, parent *kong.Path, flag *kong.Flag) (string, error) {
+	var resolver kong.ResolverFunc = func(context *kong.Context, parent *kong.Path, flag *kong.Flag) (string, error) {
 		if flag.Name == "bool" {
 			return "true", nil
 		}
 		return "", nil
 	}
 
-	p := mustNew(t, &cli, kong.Resolver(resolver))
+	p := mustNew(t, &cli, kong.Resolvers(resolver))
 
 	_, err := p.Parse(nil)
 	require.NoError(t, err)
@@ -162,7 +163,7 @@ func TestLastResolverWins(t *testing.T) {
 		return "", nil
 	}
 
-	p := mustNew(t, &cli, kong.Resolver(first), kong.Resolver(second))
+	p := mustNew(t, &cli, kong.Resolvers(first, second))
 	_, err := p.Parse(nil)
 	require.NoError(t, err)
 	require.Equal(t, []int{2}, cli.Int)
@@ -173,13 +174,13 @@ func TestResolverSatisfiesRequired(t *testing.T) {
 	var cli struct {
 		Int int `required`
 	}
-	resolver := func(context *kong.Context, parent *kong.Path, flag *kong.Flag) (string, error) {
+	var resolver kong.ResolverFunc = func(context *kong.Context, parent *kong.Path, flag *kong.Flag) (string, error) {
 		if flag.Name == "int" {
 			return "1", nil
 		}
 		return "", nil
 	}
-	_, err := mustNew(t, &cli, kong.Resolver(resolver)).Parse(nil)
+	_, err := mustNew(t, &cli, kong.Resolvers(resolver)).Parse(nil)
 	require.NoError(t, err)
 	require.Equal(t, 1, cli.Int)
 }
@@ -198,9 +199,25 @@ func TestResolverTriggersHooks(t *testing.T) {
 		return "", nil
 	}
 
-	_, err := mustNew(t, &cli, kong.Bind(ctx), kong.Resolver(first)).Parse(nil)
+	_, err := mustNew(t, &cli, kong.Bind(ctx), kong.Resolvers(first)).Parse(nil)
 	require.NoError(t, err)
 
 	require.Equal(t, "1", string(cli.Flag))
 	require.Equal(t, []string{"before:", "after:1"}, ctx.values)
 }
+
+type validatingResolver struct {
+	err error
+}
+
+func (v *validatingResolver) Validate(app *kong.Application) error { return v.err }
+func (v *validatingResolver) Resolve(context *kong.Context, parent *kong.Path, flag *kong.Flag) (string, error) {
+	return "", nil
+}
+
+func TestValidatingResolverErrors(t *testing.T) {
+	resolver := &validatingResolver{err: errors.New("invalid")}
+	var cli struct{}
+	_, err := mustNew(t, &cli, kong.Resolvers(resolver)).Parse(nil)
+	require.EqualError(t, err, "invalid")
+}