feat(render): default JSON-model renderer + bright-white values (v1.4.0)

Render(v) treats JSON as the model: in -json mode it prints v as JSON;
otherwise it paints it as text — object scalars on one line as key=value
(keys blue, values bright white: a structural key/value distinction, not
semantic color), nested objects indented, arrays one block per element,
field order preserved via an order-keeping JSON decode. EmitJSON(v) is
the JSON-only arm for commands that paint their own text. Operates on
JSON only, so core stays protobuf-free (NFR-4).

Adds White to the palette. The example gains an `inspect` command
demoing Render (text vs -json). design.md FR-4.5/4.6 document the
renderer and the "JSON is always the full record; synopsis-vs-detail is
text-only" principle.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app.go

@@ -55,6 +55,10 @@ type App[C any] struct {
 	Root *Node[C]
 	// Greeting, if set, is printed after the version banner in interactive mode.
 	Greeting string
+	// JSON, when true, registers a -json flag that switches Emit to JSON output.
+	// Leave it false for CLIs whose commands do not (yet) use cli.Emit, so they
+	// never advertise a flag they cannot honor.
+	JSON bool
 
 	// DefaultServer is the -server default. If both it and ServerEnv are empty,
 	// no -server flag is registered (local CLI).
@@ -109,7 +113,10 @@ func (a *App[C]) Run(ctx context.Context, argv []string) error {
 		serverFlag = fs.String("server", defaultServer, usage)
 	}
 	color := fs.Bool("color", true, "colorize output (default: on in the shell, off one-shot)")
-	jsonOut := fs.Bool("json", false, "emit JSON instead of text")
+	var jsonOut *bool
+	if a.JSON {
+		jsonOut = fs.Bool("json", false, "emit JSON instead of text")
+	}
 	showVersion := fs.Bool("version", false, "print version and exit")
 	if a.RegisterFlags != nil {
 		a.RegisterFlags(fs)
@@ -125,7 +132,8 @@ func (a *App[C]) Run(ctx context.Context, argv []string) error {
 		fmt.Println(a.versionLine())
 		return nil
 	}
-	if *jsonOut {
+	jsonMode := jsonOut != nil && *jsonOut
+	if jsonMode {
 		SetFormat(FormatJSON)
 	}
 
@@ -145,7 +153,7 @@ func (a *App[C]) Run(ctx context.Context, argv []string) error {
 	if colorExplicit {
 		colorOn = *color
 	}
-	if *jsonOut {
+	if jsonMode {
 		colorOn = false
 	}
 	SetColor(colorOn)

color.go

@@ -13,6 +13,7 @@ const (
 	Blue   = "\x1b[94m" // bright blue
 	Yellow = "\x1b[93m" // bright yellow
 	Cyan   = "\x1b[96m" // bright cyan
+	White  = "\x1b[97m" // bright white
 )
 
 // colorEnabled is process-global, toggled once at startup via SetColor. It

docs/design.md

@@ -7,7 +7,7 @@ SPDX-License-Identifier: Apache-2.0
 
 | | |
 | --- | --- |
-| **Status** | Describes shipped behavior as of `v1.2.0` |
+| **Status** | Describes shipped behavior as of `v1.4.0` |
 | **Author** | Pim van Pelt `<pim@ipng.ch>` |
 | **Last updated** | 2026-06-05 |
 | **Audience** | Contributors, and authors of CLIs built on this library |
@@ -92,11 +92,21 @@ or JSON from the same code.
 - **FR-4.3** With color off or JSON selected, no ANSI escapes MUST be emitted.
 - **FR-4.4** Color MUST default on in the shell, off one-shot; `-color` overrides;
   `-json` forces it off.
+- **FR-4.5** `Render` MUST treat JSON as the model: in JSON mode it prints the
+  value as JSON; otherwise it paints it as text — object scalars on one line as
+  `key=value` (keys blue, values bright white, a purely *structural* distinction),
+  nested objects indented, arrays one block per element, source field order
+  preserved. It MUST NOT apply semantic (red/green) color; that is the caller's
+  job via its own printer (EmitJSON in the JSON arm, a painter otherwise).
+- **FR-4.6** JSON MUST always be the full record. The synopsis-vs-detail choice
+  (a one-line overview list vs an expanded section) is a text-only concern; the
+  same command emits complete JSON in either case.
 
 **FR-5 `App` entry point**
 
-- **FR-5.1** `App` MUST register `-color`, `-json`, `-version`, and `-server`
+- **FR-5.1** `App` MUST register `-color` and `-version`; `-server` only when a
-  (only when a server is configured).
+  server is configured; and `-json` only when the app opts in (its commands use
+  `Emit`), so a CLI never advertises a flag it cannot honor.
 - **FR-5.2** `App` MUST NOT dial anything; the client is built in a caller's
   `Connect` callback.
 - **FR-5.3** `-version` MUST print and exit without connecting.

example/main.go

@@ -177,6 +177,25 @@ func runWatch(ctx context.Context, _ inventory, _ []string) error {
 	return nil
 }
 
+// runInspect demonstrates the default renderer: it builds one value (the model)
+// and hands it to cli.Render, which prints it as JSON under -json or as painted
+// text (blue keys, bright-white values, nested objects indented) otherwise — no
+// per-command text code.
+func runInspect(_ context.Context, inv inventory, _ []string) error {
+	type host struct {
+		Name     string   `json:"name"`
+		Services []string `json:"services"`
+	}
+	f := struct {
+		Count int    `json:"count"`
+		Hosts []host `json:"hosts"`
+	}{Count: len(inv.servers)}
+	for _, n := range inv.names() {
+		f.Hosts = append(f.Hosts, host{Name: n, Services: inv.servers[n]})
+	}
+	return cli.Render(f)
+}
+
 func runQuit(context.Context, inventory, []string) error { return cli.ErrQuit }
 
 // buildTree is the single source of truth for the command set, built with the
@@ -192,6 +211,7 @@ func buildTree() *cli.Node[inventory] {
 		b.Dir("ping", "ping a server",
 			b.Slot("<name>", "ping this server", dynServers, runPing)),
 		b.Cmd("watch", "stream a few events (any key stops it)", runWatch),
+		b.Cmd("inspect", "render the fleet via the default renderer (try -json / -color)", runInspect),
 		b.Cmd("colors", "show the ANSI color palette", runColors),
 		b.Cmd("quit", "exit the shell", runQuit),
 		b.Cmd("exit", "exit the shell", runQuit),
@@ -201,9 +221,10 @@ func buildTree() *cli.Node[inventory] {
 func main() {
 	(&cli.App[inventory]{
 		Name:     "example",
-		Version:  "1.1.0",
+		Version:  "1.3.0",
 		Prompt:   "inv> ",
 		Root:     buildTree(),
+		JSON:     true, // commands use cli.Emit, so advertise -json
 		Greeting: "golang-cli example — try: show server, ping db1, colors, '?' for help, TAB to complete",
 		// Local CLI: no -server flag. Connect just hands over the in-memory data,
 		// proving App is transport-agnostic (it never dials anything itself).

render.go

@@ -0,0 +1,210 @@
+// SPDX-FileCopyrightText: (C) Copyright 2026 Pim van Pelt <pim@ipng.ch>
+// SPDX-License-Identifier: Apache-2.0
+
+package cli
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"os"
+	"strings"
+)
+
+// Render emits v in the current output format, treating JSON as the model. In
+// JSON mode it prints v as indented JSON. Otherwise it paints v as text: each
+// object's scalar fields render on one line as blue "key=value" pairs (keys
+// blue, values bright white, so the data stands out from the labels), nested
+// objects indent under a blue "key:" header, and arrays render as one block per
+// element.
+//
+// Pass a json.RawMessage (e.g. from protojson) to control the exact JSON and
+// preserve field order; pass a struct or map and encoding/json handles it
+// (structs keep field order, maps sort keys).
+//
+// Render is the default presentation, equivalent to "no printer". A command that
+// wants bespoke text — semantic color, tables, custom layout — should instead
+// branch on IsJSON(), calling EmitJSON in the JSON arm and its own painter
+// otherwise. The library never applies semantic (red/green) color; that is the
+// command's choice, because only it knows what a value means.
+func Render(v any) error {
+	raw, err := toRawJSON(v)
+	if err != nil {
+		return err
+	}
+	if IsJSON() {
+		return printIndentedJSON(raw)
+	}
+	s, err := renderText(raw)
+	if err != nil {
+		return err
+	}
+	_, _ = fmt.Fprint(os.Stdout, s)
+	return nil
+}
+
+// renderText paints raw JSON as text (blue keys, white values). Split out so it
+// can be tested without capturing stdout.
+func renderText(raw json.RawMessage) (string, error) {
+	val, err := parseOrdered(raw)
+	if err != nil {
+		return "", err
+	}
+	var b strings.Builder
+	paintValue(&b, val, "")
+	return b.String(), nil
+}
+
+// EmitJSON prints v as indented JSON to stdout, regardless of the output format.
+// Use it in the JSON arm of a command that paints its own text.
+func EmitJSON(v any) error {
+	raw, err := toRawJSON(v)
+	if err != nil {
+		return err
+	}
+	return printIndentedJSON(raw)
+}
+
+func toRawJSON(v any) (json.RawMessage, error) {
+	if rm, ok := v.(json.RawMessage); ok {
+		return rm, nil
+	}
+	b, err := json.Marshal(v)
+	if err != nil {
+		return nil, fmt.Errorf("marshal json: %w", err)
+	}
+	return b, nil
+}
+
+func printIndentedJSON(raw json.RawMessage) error {
+	var buf bytes.Buffer
+	if err := json.Indent(&buf, raw, "", "  "); err != nil {
+		_, _ = fmt.Fprintln(os.Stdout, string(raw)) // not indentable JSON — print as-is
+		return nil
+	}
+	_, _ = fmt.Fprintln(os.Stdout, buf.String())
+	return nil
+}
+
+// orderedField is one key/value of a JSON object, preserving source order so the
+// painted text follows the order of the (proto) JSON rather than Go map order.
+type orderedField struct {
+	key string
+	val any // string, json.Number, bool, nil, []any, or []orderedField
+}
+
+func parseOrdered(raw json.RawMessage) (any, error) {
+	dec := json.NewDecoder(bytes.NewReader(raw))
+	dec.UseNumber()
+	v, err := parseOrderedValue(dec)
+	if err != nil {
+		return nil, fmt.Errorf("parse json: %w", err)
+	}
+	return v, nil
+}
+
+func parseOrderedValue(dec *json.Decoder) (any, error) {
+	t, err := dec.Token()
+	if err != nil {
+		return nil, err
+	}
+	if d, ok := t.(json.Delim); ok {
+		switch d {
+		case '{':
+			return parseOrderedObject(dec)
+		case '[':
+			return parseOrderedArray(dec)
+		}
+	}
+	return t, nil // string, json.Number, bool, or nil
+}
+
+func parseOrderedObject(dec *json.Decoder) ([]orderedField, error) {
+	var fields []orderedField
+	for dec.More() {
+		keyTok, err := dec.Token()
+		if err != nil {
+			return nil, err
+		}
+		val, err := parseOrderedValue(dec)
+		if err != nil {
+			return nil, err
+		}
+		fields = append(fields, orderedField{key: keyTok.(string), val: val})
+	}
+	_, err := dec.Token() // closing '}'
+	return fields, err
+}
+
+func parseOrderedArray(dec *json.Decoder) ([]any, error) {
+	var arr []any
+	for dec.More() {
+		v, err := parseOrderedValue(dec)
+		if err != nil {
+			return nil, err
+		}
+		arr = append(arr, v)
+	}
+	_, err := dec.Token() // closing ']'
+	return arr, err
+}
+
+func paintValue(b *strings.Builder, v any, indent string) {
+	switch t := v.(type) {
+	case []orderedField:
+		paintObject(b, t, indent)
+	case []any:
+		paintArray(b, t, indent)
+	default:
+		fmt.Fprintf(b, "%s%s\n", indent, Paint(scalarString(v), White))
+	}
+}
+
+func paintObject(b *strings.Builder, fields []orderedField, indent string) {
+	var scalars []string
+	var nested []orderedField
+	for _, f := range fields {
+		switch f.val.(type) {
+		case []orderedField, []any:
+			nested = append(nested, f)
+		default:
+			scalars = append(scalars, Label(f.key)+"="+Paint(scalarString(f.val), White))
+		}
+	}
+	if len(scalars) > 0 {
+		fmt.Fprintf(b, "%s%s\n", indent, strings.Join(scalars, "  "))
+	}
+	for _, f := range nested {
+		fmt.Fprintf(b, "%s%s:\n", indent, Label(f.key))
+		paintValue(b, f.val, indent+"  ")
+	}
+}
+
+func paintArray(b *strings.Builder, arr []any, indent string) {
+	for _, el := range arr {
+		switch el.(type) {
+		case []orderedField, []any:
+			paintValue(b, el, indent)
+		default:
+			fmt.Fprintf(b, "%s%s\n", indent, Paint(scalarString(el), White))
+		}
+	}
+}
+
+func scalarString(v any) string {
+	switch x := v.(type) {
+	case nil:
+		return "null"
+	case bool:
+		if x {
+			return "true"
+		}
+		return "false"
+	case json.Number:
+		return x.String()
+	case string:
+		return x
+	default:
+		return fmt.Sprintf("%v", x)
+	}
+}

render_test.go

@@ -0,0 +1,92 @@
+// SPDX-FileCopyrightText: (C) Copyright 2026 Pim van Pelt <pim@ipng.ch>
+// SPDX-License-Identifier: Apache-2.0
+
+package cli
+
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+)
+
+// TestRenderTextNoColor checks the plain text shape: object scalars on one line
+// as key=value, nested objects indented under "key:", arrays as one block per
+// element, and source field order preserved (not sorted).
+func TestRenderTextNoColor(t *testing.T) {
+	SetColor(false)
+	raw := json.RawMessage(`{
+		"instanceId": "host1",
+		"connected": true,
+		"version": "1.2.3",
+		"labels": {"site": "ams", "rack": "b3"},
+		"bvis": [
+			{"evpnId": "blue", "installed": true},
+			{"evpnId": "red", "installed": false}
+		]
+	}`)
+	got, err := renderText(raw)
+	if err != nil {
+		t.Fatalf("renderText: %v", err)
+	}
+	want := "instanceId=host1  connected=true  version=1.2.3\n" +
+		"labels:\n" +
+		"  site=ams  rack=b3\n" +
+		"bvis:\n" +
+		"  evpnId=blue  installed=true\n" +
+		"  evpnId=red  installed=false\n"
+	if got != want {
+		t.Errorf("renderText mismatch:\n--- got ---\n%s\n--- want ---\n%s", got, want)
+	}
+}
+
+// TestRenderTextColor checks keys are blue and values bright white when color is
+// on.
+func TestRenderTextColor(t *testing.T) {
+	SetColor(true)
+	defer SetColor(false)
+	got, err := renderText(json.RawMessage(`{"a": "x"}`))
+	if err != nil {
+		t.Fatalf("renderText: %v", err)
+	}
+	want := Blue + "a" + Reset + "=" + White + "x" + Reset + "\n"
+	if got != want {
+		t.Errorf("colored render = %q, want %q", got, want)
+	}
+}
+
+// TestRenderPreservesNumberAndNull checks json.Number passes through verbatim
+// (no float reformatting) and null renders as "null".
+func TestRenderPreservesNumberAndNull(t *testing.T) {
+	SetColor(false)
+	got, err := renderText(json.RawMessage(`{"vni": 10000000, "primary": null}`))
+	if err != nil {
+		t.Fatalf("renderText: %v", err)
+	}
+	if !strings.Contains(got, "vni=10000000") {
+		t.Errorf("number not verbatim: %q", got)
+	}
+	if !strings.Contains(got, "primary=null") {
+		t.Errorf("null not rendered: %q", got)
+	}
+}
+
+// TestRenderTextFromStruct checks a Go struct (not RawMessage) is accepted and
+// keeps struct field order.
+func TestRenderTextFromStruct(t *testing.T) {
+	SetColor(false)
+	v := struct {
+		Name  string `json:"name"`
+		Count int    `json:"count"`
+	}{Name: "web1", Count: 3}
+	raw, err := toRawJSON(v)
+	if err != nil {
+		t.Fatalf("toRawJSON: %v", err)
+	}
+	got, err := renderText(raw)
+	if err != nil {
+		t.Fatalf("renderText: %v", err)
+	}
+	if got != "name=web1  count=3\n" {
+		t.Errorf("struct render = %q", got)
+	}
+}