Silly idea I had

environment.go

@@ -35,9 +35,16 @@ type Environment struct {
 func (this *Environment) Init (ctx context.Context) error {
 	this.documents = usync.NewLocker(make(map[string] *Document))
 	this.funcMap = make(template.FuncMap)
+	for _, provider := range this.Providers {
+		if provider, ok := provider.(ConfigProcessor); ok {
+			err := provider.ProcessConfig(this.Conf)
+			if err != nil { return err }
+		}
+	}
 	for _, provider := range this.Providers {
 		if provider, ok := provider.(Configurable); ok {
-			provider.Configure(this.Conf)
+			err := provider.Configure(this.Conf)
+			if err != nil { return err }
 		}
 		if provider, ok := provider.(FuncProvider); ok {
 			funcMap := provider.FuncMap()

provider.go

@@ -5,6 +5,23 @@ import "html/template"
 // Provider is an object which provides extra functionality to an environment.
 type Provider any
 
+// Configurable is an object that can be configured according to metadata.
+type Configurable interface {
+	Provider
+	// Configure uses config to configure the object. It must not modify
+	// config. Keys are namespaced with a '.' and are written in kebab case.
+	Configure (config Meta) error
+}
+
+// ConfigProcessor is an object that can alter the environment configuration
+// before other providers are configured in order to provide a sort of macro
+// system.
+type ConfigProcessor interface {
+	// ProcessConfig analyzes and makes necessary changes to config. Unlike
+	// Configurable.Configure, it can modify config.
+	ProcessConfig (config Meta) error
+}
+
 // FuncProvider provides a template.FuncMap.
 type FuncProvider interface {
 	Provider
@@ -21,11 +38,3 @@ type FuncProviderFor interface {
 	// may return nil, in which case its result is simply not considered.
 	FuncMapFor (*Document) template.FuncMap
 }
-
-// Configurable is an object that can be configured according to metadata.
-type Configurable interface {
-	Provider
-	// Configure uses config to configure the object. It must not modify
-	// config. Keys are namespaced with a '.' and are written in kebab case.
-	Configure (config Meta) error
-}