Fill a preference's ${setting:} from its manifest default (hq ADR 0262)

Without a default, a running module could never gain a setting: the file asking for it
failed to compose until set, and the key was refused as stray until a file asked for it.
Defaults sit under the mesh's and the node's settings, never merge into a JSON file, are
refused for the operator's own values, and settings shows each value's source.

cmd/mesh-controller/modules.go

@@ -502,13 +502,38 @@ func settingsCommand(ctx context.Context, args []string) error {
 		}
 		if !has {
 			fmt.Printf("%s on %s: no layer — the module's definition says\n", positionals[0], where)
-			return nil
+		} else {
+			shown, err := json.MarshalIndent(values, "", "  ")
+			if err != nil {
+				return err
+			}
+			fmt.Println(string(shown))
 		}
-		shown, err := json.MarshalIndent(values, "", "  ")
+		// Every value the module gives a default or a layer sets, and where it came from (novox/hq
+		// ADR 0262): the default, the mesh's layer, or this node's. Said after the layer, which stays
+		// the first thing printed because a caller reads it before replacing it (ADR 0217).
+		known, err := inv.Catalogue(ctx)
 		if err != nil {
 			return err
 		}
-		fmt.Println(string(shown))
+		m, ok := known[positionals[0]]
+		if !ok || len(m.Settings) == 0 {
+			return nil
+		}
+		var layers []catalogue.Layer
+		if *node != "" {
+			if mesh, has, err := inv.Layer(ctx, "", positionals[0]); err != nil {
+				return err
+			} else if has {
+				layers = append(layers, catalogue.Layer{From: catalogue.MeshWideLayer, Values: mesh})
+			}
+			if has {
+				layers = append(layers, catalogue.Layer{From: *node, Values: values})
+			}
+		} else if has {
+			layers = append(layers, catalogue.Layer{From: catalogue.MeshWideLayer, Values: values})
+		}
+		fmt.Print(describeEffective(positionals[0], where, catalogue.Effective(m, layers)))
 		return nil
 
 	case "clear":
@@ -526,6 +551,23 @@ func settingsCommand(ctx context.Context, args []string) error {
 	}
 }
 
+// describeEffective says each setting's value on a machine or the whole mesh, where it came from, and
+// the module's default when a layer overrides it.
+func describeEffective(module, where string, values []catalogue.SettingSource) string {
+	var b strings.Builder
+	fmt.Fprintf(&b, "%s on %s, every value and where it comes from:\n", module, where)
+	for _, v := range values {
+		value, _ := json.Marshal(v.Value)
+		fmt.Fprintf(&b, "  %s = %s  (%s", v.Key, value, v.From)
+		if v.HasDefault && v.From != catalogue.DefaultLayer {
+			d, _ := json.Marshal(v.Default)
+			fmt.Fprintf(&b, "; the default is %s", d)
+		}
+		b.WriteString(")\n")
+	}
+	return b.String()
+}
+
 // nodeFlag is ` --node <node>` for a machine's layer, nothing for the whole mesh's.
 func nodeFlag(node string) string {
 	if node == "" {

cmd/mesh-controller/plan.go

@@ -1425,7 +1425,7 @@ func servedOnNode(ctx context.Context, inv *inventory.Inventory, node string,
 	}
 	serves := catalogue.ServedOn(m, provision, ports)
 	if len(serves) > 0 {
-		serves, err = catalogue.Settle(serves, layers)
+		serves, err = catalogue.Settle(serves, catalogue.WithDefaults(m, layers))
 		if err != nil {
 			return nil, err
 		}

cmd/mesh-controller/settings_effective_test.go

@@ -0,0 +1,21 @@
+package main
+
+import (
+	"testing"
+
+	"github.com/novox/mesh-controller/internal/catalogue"
+)
+
+// `settings` says each value and where it came from, and the default a layer overrides (novox/hq ADR 0262).
+func TestSettingsSayWhereEachValueComesFrom(t *testing.T) {
+	got := describeEffective("dunst", "laptop", []catalogue.SettingSource{
+		{Key: "font-size", Value: float64(13), From: "laptop", Default: float64(10), HasDefault: true},
+		{Key: "width", Value: float64(250), From: catalogue.DefaultLayer, Default: float64(250), HasDefault: true},
+	})
+	want := "dunst on laptop, every value and where it comes from:\n" +
+		"  font-size = 13  (laptop; the default is 10)\n" +
+		"  width = 250  (default)\n"
+	if got != want {
+		t.Fatalf("said\n%s\nwant\n%s", got, want)
+	}
+}

internal/catalogue/consumer_into_serves.go

@@ -238,7 +238,7 @@ func (r Resolution) derivedFor(provision, as, consumer, local string, settings S
 					"different things and nothing would compare them (novox/hq ADR 0202)",
 				consumer, local, provision, m.Module, orNothing(sortedAnyKeys(names)))
 		}
-		settled, err := Settle(names, settings[m.Module])
+		settled, err := Settle(names, WithDefaults(m, settings[m.Module]))
 		if err != nil {
 			return nil, fmt.Errorf("%s serving %s: %w", m.Module, provision, err)
 		}

internal/catalogue/declaration.go

@@ -907,7 +907,7 @@ func (r Resolution) compose(with Rendering, owner map[string]string,
 			// **An operator's value, from the assignment** (novox/hq ADR 0112, ADR 0155): what a
 			// definition may not carry because it is true of one installation only. Filled from
 			// the same layers a mergeable file takes, and refused when no layer set it.
-			if err := settingInto(copied, with.Settings[m.Module], m.Module); err != nil {
+			if err := settingInto(copied, WithDefaults(m, with.Settings[m.Module]), m.Module); err != nil {
 				return nil, err
 			}
 			// **Placed before anything reads a path.** A pathless directory receives the path
@@ -1495,7 +1495,7 @@ func (r Resolution) composed(m Manifest, to string, raw map[string]any, layers [
 	// Overridden, not merged: a setting changes a key the contribution declares and adds none.
 	// The provider reads the contribution as a contract, and a setting made for one of this
 	// module's files is no part of it (novox/hq 04-ISSUES/173).
-	values, err := overridden(raw, layers, what)
+	values, err := overridden(raw, WithDefaults(m, layers), what)
 	if err != nil {
 		return nil, fmt.Errorf("%s: %w", what, err)
 	}
@@ -1926,7 +1926,7 @@ func (r Resolution) servedOnThisMachine(provision string, with Rendering) (map[s
 			// one, so keep looking rather than concluding from the first.
 			continue
 		}
-		settled, err := Settle(serves, with.Settings[m.Module])
+		settled, err := Settle(serves, WithDefaults(m, with.Settings[m.Module]))
 		if err != nil {
 			return nil, false, fmt.Errorf("%s serving %s: %w", m.Module, provision, err)
 		}

internal/catalogue/manifest.go

@@ -458,6 +458,13 @@ type Manifest struct {
 	// (novox/hq ADR 0201). Not history — that is an event — and never a secret, sealed or not.
 	State []StateDeclaration `json:"state,omitempty"`
 
+	// Settings are the defaults this module gives its settings (novox/hq ADR 0262): each key a file,
+	// a contribution or a served fact asks for as `${setting:<key>}`, its default, and why that
+	// default. Only a preference has one — a font size, a width, a number of workers — and a value
+	// that is inherently the operator's (a domain, an identity, a secret) is declared nowhere here and
+	// stays refused by name until a layer sets it. The mesh's layer, then the node's, override it.
+	Settings map[string]SettingDeclaration `json:"settings,omitempty"`
+
 	// Data is every kind of data this module keeps — its own, by directory, and what it keeps for
 	// its consumers, by provision — each with a class the mesh protects and watches it by (novox/hq
 	// ADR 0233). One list: the backup holder's lines, the bindings that do not move, what an
@@ -1541,6 +1548,8 @@ func ParseManifest(raw []byte) (Manifest, error) {
 	problems = append(problems, EventProblems(m)...)
 	// And what it may call its state, and whose it may read (state.go, novox/hq ADR 0201).
 	problems = append(problems, StateProblems(m)...)
+	// And the defaults it gives its settings (setting_defaults.go, novox/hq ADR 0262).
+	problems = append(problems, SettingProblems(m)...)
 	wellFormed := true
 	for _, c := range m.Claims {
 		if !name.MatchString(c.Name) {

internal/catalogue/setting_defaults.go

@@ -0,0 +1,172 @@
+package catalogue
+
+import (
+	"fmt"
+	"sort"
+	"strings"
+)
+
+// A preference has a default; the operator's own value has none (novox/hq ADR 0262, extending ADR
+// 0112 and taking the default half of ADR 0164).
+//
+// `${setting:<key>}` was refused whenever no layer set the key, and `settings set` refused a key no
+// file asked for yet. Together they meant a running module could never gain a setting: the file that
+// asks for it fails to compose until somebody sets it, and nobody can set it until the file asks.
+// A definition may now give a key a default, with why, when the value is a **preference** — a font
+// size, a width, a number of workers: something true of the software that a machine may tune. A
+// value that is inherently the operator's — a domain, a public name, an identity, a secret — gets no
+// default and stays refused by name until a layer sets it, which is what ADR 0112 and ADR 0155 were
+// written for.
+//
+// The default is the lowest layer. The mesh's layer, then the node's, override it. It fills only
+// `${setting:<key>}`: a mergeable file's content already is its defaults, and laying a default over
+// it would reach every mergeable file of the module (novox/hq issue 168).
+
+// SettingDeclaration is one key's default, as the definition gives it.
+type SettingDeclaration struct {
+	// Kind says what sort of value this is. `preference` is the only kind with a default; it is
+	// stated rather than assumed so a reviewer sees the claim being made.
+	Kind string `json:"kind"`
+	// Default is the value when no layer sets the key: a string, a number or a boolean.
+	Default any `json:"default"`
+	// Why this default: one sentence for whoever wonders whether to change it.
+	Why string `json:"why"`
+}
+
+// KindPreference is the kind of a setting that may have a default.
+const KindPreference = "preference"
+
+// DefaultLayer is what the layer of a module's own defaults is called where a value's source is said.
+const DefaultLayer = "default"
+
+// meshWords are the settings keys the mesh reads itself; a module declares none of them.
+var meshWords = map[string]bool{
+	PortsSetting: true, ExposeSetting: true, ReachSetting: true, EndpointsSetting: true,
+	PlacesSetting: true, AccessesSetting: true, NetworksSetting: true,
+}
+
+// operatorsOwn are words that, in a key's name, say its value is the operator's and never a
+// preference: a default for one would be the literal ADR 0112 removed from definitions.
+var operatorsOwn = []string{
+	"domain", "host", "issuer", "url", "email", "mail", "address", "identity", "login", "user",
+	"account", "password", "secret", "token", "credential",
+}
+
+// SettingProblems is every way a definition's setting defaults are wrong, in its own words.
+func SettingProblems(m Manifest) []string {
+	if len(m.Settings) == 0 {
+		return nil
+	}
+	used := settingKeysUsedBy(m)
+	var problems []string
+	for _, key := range sortedSettingKeys(m.Settings) {
+		d := m.Settings[key]
+		say := func(format string, args ...any) {
+			problems = append(problems, fmt.Sprintf("%s's setting %q: ", m.Module, key)+fmt.Sprintf(format, args...))
+		}
+		if !settingRef.MatchString("${setting:" + key + "}") {
+			say("not a usable key: lower-case letters, digits, dots, dashes and underscores")
+			continue
+		}
+		if meshWords[key] {
+			say("the mesh reads %q itself, and a module gives it no default", key)
+			continue
+		}
+		if d.Kind != KindPreference {
+			say("kind is %q, and only a %q has a default (novox/hq ADR 0262)", d.Kind, KindPreference)
+		}
+		for _, word := range operatorsOwn {
+			if strings.Contains(key, word) {
+				say("a key naming %q is the operator's value, and has no default — it is the "+
+					"assignment's, never the definition's (novox/hq ADR 0112, ADR 0262)", word)
+				break
+			}
+		}
+		switch v := d.Default.(type) {
+		case string:
+			if strings.TrimSpace(v) == "" {
+				say("an empty default is no default; give the value, or declare nothing")
+			}
+		case float64, bool:
+		case nil:
+			say("no default: a key without one is the operator's, and is not declared here")
+		default:
+			say("a default is a string, a number or a boolean, and this is %T", d.Default)
+		}
+		if strings.TrimSpace(d.Why) == "" {
+			say("no why: say in one sentence why this default")
+		}
+		if !used[key] {
+			say("nothing asks for ${setting:%s}, so the default reaches nothing", key)
+		}
+	}
+	return problems
+}
+
+// Defaults is the layer a module's own defaults make, or nothing when it gives none.
+func Defaults(m Manifest) (Layer, bool) {
+	if len(m.Settings) == 0 {
+		return Layer{}, false
+	}
+	values := map[string]any{}
+	for key, d := range m.Settings {
+		if d.Default != nil {
+			values[key] = d.Default
+		}
+	}
+	return Layer{From: DefaultLayer, Values: values}, len(values) > 0
+}
+
+// WithDefaults is a module's layers with its defaults under them, for filling `${setting:<key>}`.
+func WithDefaults(m Manifest, layers []Layer) []Layer {
+	d, has := Defaults(m)
+	if !has {
+		return layers
+	}
+	return append([]Layer{d}, layers...)
+}
+
+// SettingSource is one key's effective value and the layer it came from.
+type SettingSource struct {
+	Key   string
+	Value any
+	// From is DefaultLayer, MeshWideLayer or the node's name.
+	From string
+	// Default is the module's default, when it gives one.
+	Default    any
+	HasDefault bool
+}
+
+// Effective is every key a module gives a default or a layer sets, with its value and where it came
+// from: the default, then the mesh's layer, then the node's — later wins.
+func Effective(m Manifest, layers []Layer) []SettingSource {
+	byKey := map[string]*SettingSource{}
+	for key, d := range m.Settings {
+		byKey[key] = &SettingSource{Key: key, Value: d.Default, From: DefaultLayer, Default: d.Default, HasDefault: true}
+	}
+	for _, layer := range layers {
+		for key, v := range layer.Values {
+			s, ok := byKey[key]
+			if !ok {
+				s = &SettingSource{Key: key}
+				byKey[key] = s
+			}
+			s.Value, s.From = v, layer.From
+		}
+	}
+	out := make([]SettingSource, 0, len(byKey))
+	for _, s := range byKey {
+		out = append(out, *s)
+	}
+	sort.Slice(out, func(i, j int) bool { return out[i].Key < out[j].Key })
+	return out
+}
+
+func sortedSettingKeys(in map[string]SettingDeclaration) []string {
+	keys := make([]string, 0, len(in))
+	for k := range in {
+		keys = append(keys, k)
+	}
+	sort.Strings(keys)
+	return keys
+}

internal/catalogue/setting_defaults_test.go

@@ -0,0 +1,195 @@
+package catalogue
+
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+)
+
+// A preference has a default in the definition; the mesh's layer, then the node's, override it; a key
+// with a default is not stray; and a value that is the operator's still has none (novox/hq ADR 0262).
+
+func notifier() Manifest {
+	return Manifest{Module: "notifier",
+		Settings: map[string]SettingDeclaration{
+			"font-size": {Kind: KindPreference, Default: float64(10), Why: "readable at a scale of one"},
+			"width":     {Kind: KindPreference, Default: float64(250), Why: "fits a title of forty characters"},
+		},
+		Resources: []map[string]any{{"id": "configuration", "type": "file", "path": "/x/notifierrc",
+			"content": "font = Inter ${setting:font-size}\nwidth = ${setting:width}\n"}},
+	}
+}
+
+func parsed(t *testing.T, m Manifest) error {
+	t.Helper()
+	raw, err := json.Marshal(m)
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = ParseManifest(raw)
+	return err
+}
+
+func TestAnUnsetPreferenceTakesItsDefault(t *testing.T) {
+	m := notifier()
+	for _, layers := range [][]Layer{nil, {{From: MeshWideLayer, Values: map[string]any{}}}} {
+		file := map[string]any{}
+		for k, v := range m.Resources[0] {
+			file[k] = v
+		}
+		if err := settingInto(file, WithDefaults(m, layers), m.Module); err != nil {
+			t.Fatal(err)
+		}
+		if file["content"] != "font = Inter 10\nwidth = 250\n" {
+			t.Fatalf("filled as %q", file["content"])
+		}
+	}
+	if err := JudgeSettings(m, nil, false); err != nil {
+		t.Fatalf("a module whose every key has a default does not compose with no layer: %v", err)
+	}
+}
+
+func TestTheNodeOverTheMeshOverTheDefault(t *testing.T) {
+	m := notifier()
+	layers := []Layer{
+		{From: MeshWideLayer, Values: map[string]any{"width": float64(300)}},
+		{From: "laptop", Values: map[string]any{"font-size": float64(13), "width": float64(340)}},
+	}
+	file := map[string]any{"type": "file", "content": m.Resources[0]["content"]}
+	if err := settingInto(file, WithDefaults(m, layers), m.Module); err != nil {
+		t.Fatal(err)
+	}
+	if file["content"] != "font = Inter 13\nwidth = 340\n" {
+		t.Fatalf("filled as %q", file["content"])
+	}
+	file = map[string]any{"type": "file", "content": m.Resources[0]["content"]}
+	if err := settingInto(file, WithDefaults(m, layers[:1]), m.Module); err != nil {
+		t.Fatal(err)
+	}
+	if file["content"] != "font = Inter 10\nwidth = 300\n" {
+		t.Fatalf("the mesh's layer over the default filled as %q", file["content"])
+	}
+}
+
+// Composed for a machine, the way a push writes it: the default reaches the file, and the node's
+// layer overrides it.
+func TestAComposedMachineGetsTheDefaultAndTheNodesValue(t *testing.T) {
+	r := anAdoptedAnchor()
+	r.Modules = append(r.Modules, notifier())
+	with := anchorRendering(false)
+	composed, err := r.Compose(with)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if why, left := composed.LeftOut["notifier"]; left {
+		t.Fatalf("the notifier was left out: %s", why)
+	}
+	if c := byID(composed.Resources)["notifier.configuration"]["content"]; c != "font = Inter 10\nwidth = 250\n" {
+		t.Fatalf("composed with no layer as %q", c)
+	}
+	with.Settings["notifier"] = []Layer{{From: "anchor", Values: map[string]any{"font-size": float64(13)}}}
+	if composed, err = r.Compose(with); err != nil {
+		t.Fatal(err)
+	}
+	if c := byID(composed.Resources)["notifier.configuration"]["content"]; c != "font = Inter 13\nwidth = 250\n" {
+		t.Fatalf("composed with the node's font size as %q", c)
+	}
+}
+
+// Setting a key the module gives a default is not refused as reaching nothing: it overrides the
+// default, which is how a running module gains a setting with no gap between.
+func TestAKeyWithADefaultIsNotStray(t *testing.T) {
+	m := notifier()
+	m.Resources[0]["content"] = "font = Inter ${setting:font-size}\nwidth = ${setting:width}\n"
+	stray := UnusedSettings(m, []Layer{{From: "laptop", Values: map[string]any{"font-size": float64(13), "colour": "red"}}})
+	joined := strings.Join(stray, "; ")
+	if strings.Contains(joined, `"font-size"`) || !strings.Contains(joined, `"colour"`) {
+		t.Fatalf("stray: %s", joined)
+	}
+}
+
+// A default fills ${setting:…} only: it never becomes a key of a mergeable file (issue 168).
+func TestADefaultIsNotMergedIntoAJSONFile(t *testing.T) {
+	m := notifier()
+	m.Resources = append(m.Resources, map[string]any{"id": "other", "type": "file", "path": "/x/other.json",
+		"merge": MergeJSON, "content": `{"keep": 1}`})
+	out, err := ApplySettings(m.Resources[1], WithDefaults(m, nil))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(out["content"].(string), "font-size") {
+		t.Fatalf("a default reached a mergeable file: %s", out["content"])
+	}
+}
+
+// A value that is the operator's has no default: no layer setting it is refused by name, as before.
+func TestAnOperatorsValueWithoutADefaultIsStillRefused(t *testing.T) {
+	m := notifier()
+	m.Resources[0]["content"] = "font = Inter ${setting:font-size}\nwidth = ${setting:width}\nfrom = ${setting:domain}\n"
+	err := JudgeSettings(m, nil, false)
+	if err == nil || !strings.Contains(err.Error(), "${setting:domain}") {
+		t.Fatalf("judged %v", err)
+	}
+	if strings.Contains(err.Error(), "font-size") {
+		t.Fatalf("a default was named as set today: %v", err)
+	}
+}
+
+func TestTheParserTakesAPreferenceAndRefusesTheRest(t *testing.T) {
+	if err := parsed(t, notifier()); err != nil {
+		t.Fatalf("a preference with a default was refused: %v", err)
+	}
+	for _, c := range []struct {
+		name   string
+		key    string
+		d      SettingDeclaration
+		refuse string
+	}{
+		{"no kind", "font-size", SettingDeclaration{Default: float64(10), Why: "x"}, `only a "preference" has a default`},
+		{"another kind", "font-size", SettingDeclaration{Kind: "operator", Default: float64(10), Why: "x"}, `only a "preference"`},
+		{"no default", "font-size", SettingDeclaration{Kind: KindPreference, Why: "x"}, "no default"},
+		{"an empty default", "font-size", SettingDeclaration{Kind: KindPreference, Default: " ", Why: "x"}, "an empty default"},
+		{"an object", "font-size", SettingDeclaration{Kind: KindPreference, Default: map[string]any{"a": 1.0}, Why: "x"}, "a string, a number or a boolean"},
+		{"no why", "font-size", SettingDeclaration{Kind: KindPreference, Default: float64(10)}, "no why"},
+		{"the operator's", "mail-domain", SettingDeclaration{Kind: KindPreference, Default: "example.tld", Why: "x"}, "is the operator's value"},
+		{"a secret", "api-token", SettingDeclaration{Kind: KindPreference, Default: "x", Why: "x"}, "is the operator's value"},
+		{"the mesh's word", PortsSetting, SettingDeclaration{Kind: KindPreference, Default: float64(1), Why: "x"}, "the mesh reads"},
+		{"read by nothing", "height", SettingDeclaration{Kind: KindPreference, Default: float64(300), Why: "x"}, "reaches nothing"},
+	} {
+		m := notifier()
+		m.Resources[0]["content"] = m.Resources[0]["content"].(string) + "x = ${setting:" + c.key + "}\n"
+		if c.name == "read by nothing" {
+			m.Resources[0]["content"] = "font = Inter ${setting:font-size}\nwidth = ${setting:width}\n"
+		}
+		m.Settings[c.key] = c.d
+		err := parsed(t, m)
+		if err == nil || !strings.Contains(err.Error(), c.refuse) {
+			t.Errorf("%s: parsed %v, want %q", c.name, err, c.refuse)
+		}
+	}
+}
+
+func TestEveryValueSaysWhereItCameFrom(t *testing.T) {
+	m := notifier()
+	m.Resources[0]["content"] = m.Resources[0]["content"].(string) + "x = ${setting:position}\n"
+	got := Effective(m, []Layer{
+		{From: MeshWideLayer, Values: map[string]any{"width": float64(300), "position": "top-right"}},
+		{From: "laptop", Values: map[string]any{"font-size": float64(13)}},
+	})
+	want := map[string]string{"font-size": "laptop", "position": MeshWideLayer, "width": MeshWideLayer}
+	if len(got) != 3 {
+		t.Fatalf("effective: %+v", got)
+	}
+	for _, s := range got {
+		if s.From != want[s.Key] {
+			t.Errorf("%s from %q, want %q", s.Key, s.From, want[s.Key])
+		}
+	}
+	if got[0].Key != "font-size" || got[0].Value != float64(13) || got[0].Default != float64(10) {
+		t.Fatalf("font-size: %+v", got[0])
+	}
+	only := Effective(m, nil)
+	if only[0].From != DefaultLayer || only[0].Value != float64(10) {
+		t.Fatalf("with no layer: %+v", only)
+	}
+}

internal/catalogue/setting_into.go

@@ -40,8 +40,9 @@ func settingsUsed(content string) []string {
 
 // settingInto fills a file's ${setting:…} placeholders from the layers over a module.
 //
-// The last layer setting a key wins, which is the node's over the mesh's — the same order settle
-// applies to a mergeable file. A value that is not a string is written the way a program would read
+// The last layer setting a key wins, which is the node's over the mesh's over the module's own
+// default (novox/hq ADR 0262) — the same order settle applies to a mergeable file. The caller lays
+// the defaults under the layers with WithDefaults. A value that is not a string is written the way a program would read
 // it (a number without a trailing .000000, a boolean as true/false).
 func settingInto(resource map[string]any, layers []Layer, module string) error {
 	if fmt.Sprint(resource["type"]) != "file" {
@@ -56,7 +57,8 @@ func settingInto(resource map[string]any, layers []Layer, module string) error {
 		if !set {
 			return fmt.Errorf(
 				"%s has a file that says ${setting:%s}, and nothing sets %q for it — an operator's "+
-					"value is the assignment's, never the definition's (novox/hq ADR 0112): "+
+					"value is the assignment's, never the definition's (novox/hq ADR 0112), and only a "+
+					"preference has a default in the definition (ADR 0262): "+
 					"`settings set %s <file>` with {%q: …}%s",
 				module, key, key, module, key, orNoSettings(layers))
 		}
@@ -80,6 +82,9 @@ func settingValue(layers []Layer, key string) (any, bool) {
 func orNoSettings(layers []Layer) string {
 	var keys []string
 	for _, l := range layers {
+		if l.From == DefaultLayer {
+			continue
+		}
 		for k := range l.Values {
 			keys = append(keys, k)
 		}

internal/catalogue/settings.go

@@ -151,6 +151,12 @@ func settle(base map[string]any, layers []Layer, protected map[string]bool, what
 	map[string]any, error) {
 	merged := deepCopy(base)
 	for _, layer := range layers {
+		if layer.From == DefaultLayer {
+			// A module's own defaults fill ${setting:<key>} only (novox/hq ADR 0262). A mergeable
+			// file's content already is its defaults, and a contribution or served fact declares its
+			// own; laid on here, a default would reach every mergeable file of its module (issue 168).
+			continue
+		}
 		for key, value := range layer.Values {
 			if key == PortsSetting {
 				// Where the machine puts a port is the mesh's to apply, not a value for a file or
@@ -224,6 +230,11 @@ func UnusedSettings(m Manifest, layers []Layer) []string {
 	}
 
 	lands := settingKeysUsedBy(m)
+	// A key the module gives a default reaches what asks for it (novox/hq ADR 0262): setting it
+	// overrides the default, which is how a running module gains a setting without a gap between.
+	for key := range m.Settings {
+		lands[key] = true
+	}
 	for _, values := range m.Contributes {
 		for key := range values {
 			lands[key] = true
@@ -420,7 +431,7 @@ func JudgeSettings(m Manifest, layers []Layer, adopted bool) error {
 		for k, v := range settled {
 			copied[k] = v
 		}
-		if err := settingInto(copied, layers, m.Module); err != nil {
+		if err := settingInto(copied, WithDefaults(m, layers), m.Module); err != nil {
 			return err
 		}
 	}

internal/catalogue/verbs.go

@@ -249,7 +249,9 @@ var ControllerVerbs = []Verb{
 		}, nil, "adopted"),
 		Replaces: []string{"mesh-controller token issue", "wg set"}},
 	{Name: "settings", Description: "Read or set what an assignment is configured with: a module's settings for the whole " +
-		"mesh, or for one machine. Without values or clear, answers the layer as it stands — read it before setting it; " +
+		"mesh, or for one machine. Without values or clear, answers the layer as it stands — read it before setting it — " +
+		"then every value the module gives a default or a layer sets, with where it comes from: the module's default, " +
+		"the mesh, or the machine (novox/hq ADR 0262); " +
 		"with history, the layers it replaced. Setting replaces that layer whole and answers each key it adds (+), " +
 		"changes (~) and removes (-); a set that would remove a key is refused unless replace says it is meant " +
 		"(novox/hq ADR 0217). Takes effect at the next push. With clear, removes the layer and the module is back to " +