Add configuration path explanations

internal/adapter/cli/config.go

@@ -23,16 +23,19 @@ func configCommand(args []string, stdout, stderr io.Writer) int {
 		return ExitRuntimeError
 	}
 	mode := args[0]
-	if mode != "validate" && mode != "show" && mode != "sources" {
+	if mode != "validate" && mode != "show" && mode != "sources" && mode != "explain" {
 		fmt.Fprintln(stderr, "config parse error: unknown configuration mode")
 		return ExitRuntimeError
 	}
 
 	fs := flag.NewFlagSet("config "+mode, flag.ContinueOnError)
 	fs.SetOutput(io.Discard)
-	var path, overlay string
+	var path, overlay, pointer string
 	registerConfigSelectionFlags(fs, &path, &overlay)
 	registerConfigurationOverrides(fs)
+	if mode == "explain" {
+		fs.StringVar(&pointer, "path", "", "configuration path as a JSON Pointer")
+	}
 	if err := fs.Parse(args[1:]); err != nil {
 		writeApplicationError(stderr, "config parse", err)
 		return ExitRuntimeError
@@ -41,6 +44,19 @@ func configCommand(args []string, stdout, stderr io.Writer) int {
 		fmt.Fprintln(stderr, "config parse error: positional arguments are not accepted")
 		return ExitRuntimeError
 	}
+	var explanationPath configkit.Path
+	if mode == "explain" {
+		if !flagWasSet(fs, "path") {
+			fmt.Fprintln(stderr, "config parse error: --path is required")
+			return ExitRuntimeError
+		}
+		var err error
+		explanationPath, err = configkit.ParsePath(pointer)
+		if err != nil {
+			fmt.Fprintln(stderr, "config parse error: --path must be a JSON Pointer")
+			return ExitRuntimeError
+		}
+	}
 
 	result, err := appconfig.Load(appconfig.LoadOptions{
 		Path: path, Explicit: flagWasSet(fs, "config"), Overlay: overlay,
@@ -66,6 +82,8 @@ func configCommand(args []string, stdout, stderr io.Writer) int {
 			})
 		}
 		err = json.NewEncoder(stdout).Encode(sources)
+	case "explain":
+		err = result.WriteExplanation(stdout, explanationPath)
 	}
 	if err != nil {
 		fmt.Fprintf(stderr, "config output error: %v\n", err)