Say why an empty name is rejected, and finish what that removed

The reason given was wrong. A dot path did reach an empty-named field:
Fake("a.") returned it, Fake("a.b.") returned a nested one, and {..a.} bound
it. What was true is narrower — List never offered it, because an empty name
is no path segment — so the engine accepted spellings it would not advertise.
The message, the test comment and the README say that instead.

Two things the rejection finished off:

A {} token still reported "no field \"\"", pointing at a fix the loader now
rejects — two errors for one rule. It is told the name can never exist, like
an option token already is.

addressable is dead: both halves of "not empty and no dot" are now rejected
where a name is authored. Mutating it to panic leaves the suite green here
and panics on main, so it was live and is not. Keeping it would preserve the
silent-hiding this change removes — List quietly omitting a name rather than
New refusing it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -238,8 +238,9 @@ option stays unset. `New` rejects an option that cannot take effect — a
 `separator` without a `repeat` above 1, a `weight` outside a choice — and a name
 using a character the grammars reserve: `.` separates the segments of a path, `|`
 the arms of a token, `(` opens a function call and `}` ends the token, so a name
-carrying one is a name no format could ever spell. That holds for a category, a
-folder and a field alike.
+carrying one is a name no format could ever spell. An empty name goes the same
+way — it is no path segment at all, so `List` never offers it. That holds for a
+category, a folder and a field alike.
 
 **Functions.** A `{name()}` token calls a built-in function instead of rendering
 a field. `{luhn()}` appends a Luhn check digit over the digits emitted **so far**

edge_test.go

@@ -120,9 +120,9 @@ func TestNewErrors(t *testing.T) {
 			map[string]string{"a|b": `["1"]`},
 			`category "a|b" contains "|"`,
 		},
-		// An empty name is reachable by no dot path, so List cannot advertise it and
-		// Fake cannot ask for it — {} is the one spelling that reaches it, which is
-		// the same trap {a.} is already rejected for.
+		// An empty name is not a path segment, so List never offered it — while a
+		// bare {}, a trailing dot in Fake("a.") and a {..a.} reference all reached
+		// it. The engine accepted spellings it would never advertise.
 		"empty field name": {
 			map[string]string{"a": `{"format":"[{}]","":"VALUE"}`},
 			`field "" is empty`,
@@ -156,9 +156,15 @@ func TestNewErrors(t *testing.T) {
 			map[string]string{"a": `{"format":"{..|..}"}`},
 			"reference has no path",
 		},
+		// No field can be named "", so the token is told that rather than sent to
+		// name one — the fix "no field" points at is itself a load error.
 		"repeated empty arm": {
 			map[string]string{"a": `{"format":"{|}"}`},
-			`no field ""`,
+			"a name is never empty",
+		},
+		"bare empty token": {
+			map[string]string{"a": `{"format":"[{}]","x":["1"]}`},
+			"a name is never empty",
 		},
 	}
 	for name, c := range rejected {

fakes.go

@@ -22,7 +22,6 @@ import (
 	"fmt"
 	"math/rand/v2"
 	"sort"
-	"strings"
 )
 
 // Fakes generates fake data from a loaded namespace tree. Create one with [New].
@@ -93,14 +92,6 @@ func (f *Fakes) List() []string {
 	return out
 }
 
-// addressable reports whether a name can be one segment of a dot path. A dot would
-// split it into two segments and an empty name into none, so a path through such a
-// name cannot be spelled — List must not offer one, and it must not count towards
-// what a choice's variants share.
-func addressable(name string) bool {
-	return name != "" && !strings.Contains(name, ".")
-}
-
 // paths lists the dot paths addressable from n, relative to it, where "" is n
 // itself. A group has no value of its own, so it contributes only its children's.
 func paths(n node) []string {
@@ -108,9 +99,6 @@ func paths(n node) []string {
 	case *group:
 		var out []string
 		for _, name := range sortedNames(n.children) {
-			if !addressable(name) {
-				continue
-			}
 			for _, p := range paths(n.children[name]) {
 				out = append(out, join(name, p))
 			}
@@ -119,7 +107,7 @@ func paths(n node) []string {
 	case *template:
 		out := []string{""}
 		for _, name := range sortedNames(n.fields) {
-			if isRef(name) || !addressable(name) { // a binding, or unreachable by path
+			if isRef(name) { // a binding, not a path segment
 				continue
 			}
 			for _, p := range paths(n.fields[name]) {

node.go

@@ -267,7 +267,7 @@ var reservedList = strings.Join(strings.Split(reservedInName, ""), " ")
 // name may contain.
 func checkName(name string) error {
 	if name == "" {
-		return fmt.Errorf("%q is empty, so no dot path can reach it", name)
+		return fmt.Errorf("%q is empty, which is not a path segment, so List never offers it", name)
 	}
 	if i := strings.IndexAny(name, reservedInName); i >= 0 {
 		return fmt.Errorf("%q contains %q; a name may not use %s, which the dot path and {token} grammars reserve",

template.go

@@ -147,6 +147,9 @@ func checkTokens(format string, fields map[string]node) error {
 			}
 			head, ok := fields[a.key]
 			if !ok {
+				if a.key == "" {
+					return fmt.Errorf("token {%s}: a name is never empty, so this token can name no field", t.body)
+				}
 				if isOption(a.key) {
 					return fmt.Errorf("token {%s}: %q is an option and can never be a field", t.body, a.key)
 				}