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**
@@ -365,9 +366,12 @@ the reference sits, including in a field the format renders.
 
 A `{calc()}` operand is held on its own terms too, so the same fence guards it:
 `{..cat.net} x 2 = {calc(net * 2, 2)}` names one field two ways and is a load
-error, with no path token anywhere. What an operand's draw fixes is the value it
-renders, so a reference reaching *through* it is caught as well, while a sibling
-its format never renders is free — that sibling is no part of the value.
+error, with no path token anywhere. A calc renders its operand whole, so the hold
+pins what that render settled, following the operand's plain `{field}` tokens —
+`{net.v}` and `{..cat.net.v}` are rejected alike. It stops at a `{..path}`, where
+the operand's own value ends and a shared source begins: two names drawing from
+one referenced category are two draws, as `{word} {word}` is, so two dice over
+one `{..die}` are fine.
 
 ```text
 token {p} renders a level that {p.first} reads a path into; name the fields you want instead

bound_test.go

@@ -148,25 +148,77 @@ func TestALevelRenderedOnlyByAPathTokenIsHeld(t *testing.T) {
 // shown is not the value computed. Two names that merely draw from one source are
 // two draws, as {word} {word} is — the rule everywhere else in the engine.
 func TestACalcOperandIsHeldAgainstEveryRoute(t *testing.T) {
-	rejected := map[string]map[string]string{
+	rejected := map[string]struct {
+		files map[string]string
+		want  string // the route the error names, spelled as the author wrote it
+	}{
 		"a reference beside the operand": {
-			"cat": `{"format":"{..cat.net} x 2 = {calc(net * 2, 2)}","net":["10.00","20.00"]}`,
+			map[string]string{
+				"cat": `{"format":"{..cat.net} x 2 = {calc(net * 2, 2)}","net":["10.00","20.00"]}`,
+			},
+			`{..cat.net} renders "net"`,
 		},
 		"a reference one level down": {
-			"cat": `{"format":"{calc(net * 2, 2)} {q}","net":["10.00","20.00"],` +
-				`"q":{"format":"{..cat.net}"}}`,
+			map[string]string{
+				"cat": `{"format":"{calc(net * 2, 2)} {q}","net":["10.00","20.00"],` +
+					`"q":{"format":"{..cat.net}"}}`,
+			},
+			`{q} renders "net"`,
 		},
 		// A reference ending at the operand binds the choice itself, not a variant,
 		// so wrapping the operand in one changes nothing.
 		"a reference to an operand wrapped in a choice": {
-			"cat": `{"format":"{calc(n * 2, 2)} {q}","n":[{"format":"{v}","v":["1","2"]}],` +
-				`"q":{"format":"{..cat.n}"}}`,
+			map[string]string{
+				"cat": `{"format":"{calc(n * 2, 2)} {q}","n":[{"format":"{v}","v":["1","2"]}],` +
+					`"q":{"format":"{..cat.n}"}}`,
+			},
+			`{q} renders "n"`,
+		},
+		// The reaching route can be another operand, which is not a token — so the
+		// error must name it as one rather than invent a {b} the format never wrote.
+		"an operand reaching another operand": {
+			map[string]string{
+				"cat": `{"format":"{calc(a + b, 0)}","a":{"format":"{x}","x":["1","2"]},` +
+					`"b":{"format":"{..cat.a}"}}`,
+			},
+			`calc operand "b" renders "a"`,
+		},
+		// Reaching *into* the operand is the same shape as naming it: the draw the
+		// calc holds settled that value too. The token spelling {net.v} is already
+		// rejected, so the reference spelling has to be.
+		"a reference into the operand": {
+			map[string]string{
+				"cat": `{"format":"{calc(net * 2, 2)}|{..cat.net.v}",` +
+					`"net":{"format":"{v}","v":["10.00","20.00"]}}`,
+			},
+			`{..cat.net.v} renders "net"`,
+		},
+		"a reference into the operand one level down": {
+			map[string]string{
+				"cat": `{"format":"{calc(net * 2, 2)}|{q}","net":{"format":"{v}","v":["10.00","20.00"]},` +
+					`"q":{"format":"{..cat.net.v}"}}`,
+			},
+			`{q} renders "net"`,
+		},
+		// Each head is walked against its own seen set. Sharing one across heads
+		// would let the first head's walk mark the only route to the second's draw,
+		// and this violation sits on the later head in sorted order.
+		"a violation on the second of two held heads": {
+			map[string]string{
+				"cat": `{"format":"{calc(a + b, 0)} {w}","a":{"format":"{x}","x":["1","2"]},` +
+					`"b":{"format":"{y}","y":["3","4"]},"w":{"format":"{..cat.b}"}}`,
+			},
+			`{w} renders "b"`,
 		},
 	}
-	for name, files := range rejected {
-		_, err := New([]string{writeData(t, files)})
+	for name, c := range rejected {
+		_, err := New([]string{writeData(t, c.files)})
 		if err == nil || !strings.Contains(err.Error(), "a {calc()} also reads") {
 			t.Errorf("%s: New = %v, want the second route to the operand rejected", name, err)
+			continue
+		}
+		if !strings.Contains(err.Error(), c.want) {
+			t.Errorf("%s: New = %v, want it to name %q", name, err, c.want)
 		}
 	}
 
@@ -200,6 +252,11 @@ func TestACalcOperandIsHeldAgainstEveryRoute(t *testing.T) {
 		"a literal operand named twice": {
 			"cat": `{"format":"{calc(n * 2, 0)} {..cat.n}","n":"5"}`,
 		},
+		// One node reached twice while walking the operand: the walk must not
+		// revisit it, and the repeat is not a second route to anything.
+		"an operand that renders one field twice": {
+			"cat": `{"format":"{calc(v * 2, 0)}","v":{"format":"{a}{a}","a":["1","2"]}}`,
+		},
 	}
 	for name, files := range accepted {
 		if _, err := New([]string{writeData(t, files)}); err != nil {
@@ -599,12 +656,13 @@ func TestDeepPathsUnderOneHeadStayIndependentWhereTheyDiverge(t *testing.T) {
 }
 
 func TestEmptyPathSegmentIsRejected(t *testing.T) {
-	// "{a.}", "{.b}" and "{a..b}" are unfinished paths. A field really named ""
-	// would otherwise make them resolve, which reads as a typo that worked.
+	// "{a.}", "{.b}" and "{a..b}" are unfinished paths, and the segment naming
+	// nothing is reported as that rather than as a missing field. An empty name is
+	// rejected where it is authored, so no data can make these resolve.
 	rejected := map[string]string{
-		"trailing dot": `{"format":"[{a.}]","a":{"format":"x","":["V"]}}`,
-		"leading dot":  `{"format":"[{.b}]","":{"format":"{b}","b":["V"]}}`,
-		"double dot":   `{"format":"[{a..b}]","a":{"format":"x","":{"format":"{b}","b":["V"]}}}`,
+		"trailing dot": `{"format":"[{a.}]","a":{"format":"x"}}`,
+		"leading dot":  `{"format":"[{.b}]","a":{"format":"{b}","b":["V"]}}`,
+		"double dot":   `{"format":"[{a..b}]","a":{"format":"x"}}`,
 	}
 	for name, file := range rejected {
 		_, err := New([]string{writeData(t, map[string]string{"cat": file})})

builtins.go

@@ -48,9 +48,7 @@ var builtins = map[string]builtin{
 		return func(s *session, _ string, _ []string) string { return strconv.Itoa(lo + s.IntN(span)) }
 	}},
 	"float": {arity: 3, check: floatArgs, prep: func(a []string) callFn {
-		lo, _ := strconv.ParseFloat(a[0], 64)
-		hi, _ := strconv.ParseFloat(a[1], 64)
-		dp := atoi(a[2])
+		lo, hi, dp := atof(a[0]), atof(a[1]), atoi(a[2])
 		return func(s *session, _ string, _ []string) string {
 			return strconv.FormatFloat(lo+s.Float64()*(hi-lo), 'f', dp, 64)
 		}
@@ -104,6 +102,15 @@ func atoi(s string) int {
 	return n
 }
 
+// atof is atoi for a float arg, and reports an unvalidated one the same way.
+func atof(s string) float64 {
+	f, err := strconv.ParseFloat(s, 64)
+	if err != nil {
+		panic(fmt.Sprintf("fakes: builtin arg %q reached prep unvalidated: %v", s, err))
+	}
+	return f
+}
+
 func randBytes(r rng, n int) []byte {
 	b := make([]byte, n)
 	for i := range b {

builtins_test.go

@@ -193,3 +193,27 @@ func TestRegistryShapes(t *testing.T) {
 		}
 	}
 }
+
+// TestArgGuardsPanic pins the guards that report a builtin arg its check should
+// have rejected. No data reaches them — checkFunc runs a builtin's check before
+// compileOps ever calls prep — so they are exercised directly.
+func TestArgGuardsPanic(t *testing.T) {
+	for name, call := range map[string]func(){
+		"atoi on an unvalidated arg": func() { atoi("nope") },
+		"atof on an unvalidated arg": func() { atof("nope") },
+	} {
+		mustPanic(t, name, call)
+	}
+}
+
+// mustPanic fails unless call panics, which is what separates a reported
+// invariant break from a silently wrong value.
+func mustPanic(t *testing.T, name string, call func()) {
+	t.Helper()
+	defer func() {
+		if recover() == nil {
+			t.Errorf("%s: no panic, want the invariant reported", name)
+		}
+	}()
+	call()
+}

calc_test.go

@@ -103,6 +103,19 @@ func TestCalcTokenOperandsReadsOnlyACalc(t *testing.T) {
 	}
 }
 
+// TestCalcGuardsPanic pins the guards on calc's own invariants: checkCalc parsed
+// the expression before prep sees it, and indexVars places every name calcVars
+// read, so a break in either is reported rather than computed around.
+func TestCalcGuardsPanic(t *testing.T) {
+	for name, call := range map[string]func(){
+		"prep on an expression that does not parse": func() { calcPrep([]string{"1 +"}) },
+		"an operand name never placed":              func() { calcVar("n").eval(nil) },
+		"indexVars on a name it did not read":       func() { indexVars(calcVar("n"), map[string]int{}) },
+	} {
+		mustPanic(t, name, call)
+	}
+}
+
 // --- one draw, one value: a calc operand reads the expansion's draw ---
 
 // TestCalcOperandReadsTheExpansionsDraw pins the one-draw rule for a calc operand.

edge_test.go

@@ -120,6 +120,13 @@ func TestNewErrors(t *testing.T) {
 			map[string]string{"a|b": `["1"]`},
 			`category "a|b" contains "|"`,
 		},
+		// 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`,
+		},
 		"folder name with a paren": {
 			map[string]string{"a(b/cat": `["1"]`},
 			`folder "a(b" contains "("`,
@@ -149,9 +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

@@ -266,6 +266,9 @@ var reservedList = strings.Join(strings.Split(reservedInName, ""), " ")
 // category or folder and a field go through it, so there is one answer to what a
 // name may contain.
 func checkName(name string) error {
+	if 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",
 			name, name[i:i+1], reservedList)

reference.go

@@ -65,10 +65,10 @@ func checkBoundLevelsHeld(root map[string]node) error {
 			if isPath {
 				cover(t.fields[head], held)
 			} else {
-				coverRendered(t.fields[head], held)
+				operandDraw(t.fields[head], held)
 			}
 			if len(held) == 0 {
-				continue // a fixed string, which cannot disagree with itself
+				continue // an early out: a literal head holds nothing to reach
 			}
 			// One seen set across the edges: a node that cannot reach the level
 			// cannot reach it by another route either, so it is walked once here.
@@ -79,9 +79,9 @@ func checkBoundLevelsHeld(root map[string]node) error {
 				}
 				if renders(e.to, held, seen) {
 					if isPath {
-						return fmt.Errorf("%s: {%s} renders %q, which {%s} reads a path into; name the fields you want instead", path, e.label, head, reader)
+						return fmt.Errorf("%s: %s renders %q, which {%s} reads a path into; name the fields you want instead", path, e.reached(), head, reader)
 					}
-					return fmt.Errorf("%s: {%s} renders %q, which a {calc()} also reads; reach it one way so it is drawn once", path, e.label, head)
+					return fmt.Errorf("%s: %s renders %q, which a {calc()} also reads; reach it one way so it is drawn once", path, e.reached(), head)
 				}
 			}
 		}
@@ -103,14 +103,17 @@ func cover(n node, into map[node]bool) {
 	}
 }
 
-// coverRendered collects what one held draw of a calc operand answers for: what
-// rendering it reaches, following the same edges expand does. An operand is only
-// ever rendered — checkNoOverlap rejects a path into one — so its draw fixes that
-// value and nothing else. That is narrower than containment where a sibling the
-// operand's format never renders cannot disagree with it, and wider where the
-// operand renders through a reference, which is part of the value it produces.
-// Literals are left out for the reason cover leaves them out.
-func coverRendered(n node, into map[node]bool) {
+// operandDraw collects what one held draw of a {calc()} operand answers for: the
+// operand and what rendering it settles inside itself. A calc renders its operand
+// whole, so that draw fixes every value the render produced, and a second route to
+// any of them disagrees with it.
+//
+// The walk stops at a {..path} edge, which is where the operand's own value ends
+// and a shared source begins: two names referencing one category are two draws, the
+// same rule {word} {word} follows. cover stops there too, by way of named, so both
+// halves of the fence end at the same boundary. A literal is left out for the
+// reason cover leaves one out.
+func operandDraw(n node, into map[node]bool) {
 	if _, fixed := n.(literal); fixed {
 		return
 	}
@@ -119,7 +122,10 @@ func coverRendered(n node, into map[node]bool) {
 	}
 	into[n] = true
 	for _, e := range renderEdges(n) {
-		coverRendered(e.to, into)
+		if isRef(e.label) {
+			continue
+		}
+		operandDraw(e.to, into)
 	}
 }
 
@@ -267,11 +273,23 @@ func refTokens(format string) []string {
 	return refs
 }
 
-// renderEdge is a child a node renders into, labelled by the token that reaches it
-// (a field name, reference, or choice index) for a readable cycle report.
+// renderEdge is a child a node renders into, labelled by what reaches it (a field
+// name, reference, or choice index) for a readable cycle report. operand marks a
+// label that is a {calc()} operand name rather than a token, so an error can name
+// it the way the author wrote it.
 type renderEdge struct {
-	to    node
-	label string
+	to      node
+	label   string
+	operand bool
+}
+
+// reached names an edge as the author spelled it, the vocabulary boundReaders uses
+// for the sibling fence.
+func (e renderEdge) reached() string {
+	if e.operand {
+		return fmt.Sprintf("calc operand %q", e.label)
+	}
+	return "{" + e.label + "}"
 }
 
 // renderEdges lists the children rendering n recurses into, mirroring expand: a
@@ -282,21 +300,27 @@ func renderEdges(n node) []renderEdge {
 	case *choice:
 		es := make([]renderEdge, len(n.items))
 		for i, it := range n.items {
-			es[i] = renderEdge{it, fmt.Sprintf("[%d]", i)}
+			es[i] = renderEdge{to: it, label: fmt.Sprintf("[%d]", i)}
 		}
 		return es
 	case *template:
 		var es []renderEdge
-		for _, name := range append(fieldTokens(n.format), calcOperands(n.format)...) {
+		add := func(name string, operand bool) {
 			a := splitArm(name)
 			c, ok := n.fields[a.key]
 			if !ok {
-				continue
+				return
 			}
 			for _, leaf := range pathLeaves(c, a.tail) {
-				es = append(es, renderEdge{leaf, name})
+				es = append(es, renderEdge{leaf, name, operand})
 			}
 		}
+		for _, name := range fieldTokens(n.format) {
+			add(name, false)
+		}
+		for _, name := range calcOperands(n.format) {
+			add(name, true)
+		}
 		return es
 	default:
 		return nil

render.go

@@ -156,7 +156,8 @@ func expand(s *session, t *template) string {
 		case 'f':
 			b.WriteString(readField(s, t, held, o.arms[s.IntN(len(o.arms))]))
 		case 'b':
-			// Read in the order calcVars fixed, which is what op.operands holds.
+			// Read before the call, so the value a calc computes is the value the
+			// format showed. calcVars fixed the order op.operands holds.
 			var operands []string
 			if len(o.operands) > 0 {
 				operands = make([]string, len(o.operands))

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)
 				}