Fold the range check into columnKind.past, refuse a leading / in IsTemplate, reserve the name -, and document fake:"-" on an embedded struct

README.md

@@ -31,9 +31,9 @@ spot. Its tokens reach the data by reference from the root —
 `{/sv_SE.person.last}`, so shipped and `--data-path` categories are alike
 available. An inline template sits in no folder, so the folder-relative `{.name}`
 and `{..name}` are rejected naming the root spelling, and one reference alone —
-`{/sv_SE.person}` — is the path written as a template, rejected naming the path. A
-path never contains a brace, a bracket or a quote, so the two cannot collide (see
-[Decisions](#decisions)).
+`{/sv_SE.person}` — is the path written as a template, rejected naming the path, as is
+a path written `/sv_SE.person`. A path never contains a brace, a bracket or a quote,
+so the two cannot collide (see [Decisions](#decisions)).
 
 | Flag | |
 |------|--|
@@ -191,9 +191,9 @@ which a [`null`](#null) item leaves nil. An integer stays within int64 whatever
 kind, and a value the kind cannot hold, such as `{int(0,300)}` in a `uint8`, is refused
 naming a kind that holds it. The fields an embedded struct promotes are columns of the
 same record; a named struct field, or a pointer to one, fills from its own tags as a
-record of its own, so its references draw apart from its parent's, and `fake:"-"`
-leaves it unfilled. Untagged fields keep their values, and so does a pointer back to a
-struct already being filled. The first call for a type compiles its tags and reports
+record of its own, so its references draw apart from its parent's. `fake:"-"` leaves a
+struct field, embedded or named, or a pointer to one, unfilled. Untagged fields keep
+their values, and so does a pointer back to a struct already being filled. The first call for a type compiles its tags and reports
 what they get wrong, with the same error on every later call; a `datatype` in a tag
 names the Go type that already sets it.
 
@@ -209,8 +209,8 @@ work with no data on disk. A directory is a namespace: each JSON file is a
 category named after the file, each subdirectory a dot-path segment, so
 `mydata/sv_SE/person.json` is `sv_SE.person` and replaces the shipped one.
 Sources merge in order; matching folders combine, any other clash is won by the
-last loaded. Names may not use `.`, `|`, `(`, `{`, `}`, `[`, `]`, `"` or `/`;
-dot-prefixed entries are skipped, so a data directory can also be a checkout.
+last loaded. Names may not use `.`, `|`, `(`, `{`, `}`, `[`, `]`, `"` or `/`, nor be
+`-`, which a struct tag reserves; dot-prefixed entries are skipped, so a data directory can also be a checkout.
 
 Each locale carries `address`, `color`, `company`, `date`, `email`, `ip`,
 `person`, `phone`, `price`, `sentence`, `ssn`, `time`, `url`, `username`,
@@ -625,9 +625,9 @@ tokens add cost in proportion to the output.
   fields an embedded struct promotes are the struct's own — `e.First`, as
   `encoding/json` and SQL mappers read them — so they are columns of its record and
   share its draws; a tagged field that another field hides is refused, not dropped. A
-  named struct field is another entity and a record of its own; `fake:"-"` leaves it
-  unfilled, whatever a category named `-` holds, and a pointer back to a struct
-  already being filled is left alone, since filling it would never end. `New` cannot
+  named struct field is another entity and a record of its own. `fake:"-"` leaves a
+  struct field, embedded or named, unfilled, so no name may be `-`; a pointer back to
+  a struct already being filled is left alone, since filling it would never end. `New` cannot
   see a caller's types, so the first `FakeStruct` for a type compiles its tags and the
   answer, error included, is kept per type: a test's first call is its load, and no
   `NewStruct` handle is needed, as the cache already compiles once.

inline.go

@@ -52,8 +52,8 @@ func (f *Generator) FakeTemplate(input string) (string, error) {
 // IsTemplate reports whether arg is an inline template rather than a path, by its shape: a {
 // token, or a JSON object, array or string, is a template, and anything else is a path. A
 // name never holds a bracket, a brace or a quote, so an arg holding one that is not valid
-// JSON names neither, and errors; so does a template of one reference alone, which is a path
-// written as a template.
+// JSON names neither, and errors; so do a template of one reference alone, which is a path
+// written as a template, and a path written with a leading /.
 func IsTemplate(arg string) (bool, error) {
 	inline, err := isTemplate(arg)
 	if err != nil {
@@ -72,6 +72,9 @@ func isTemplate(arg string) (bool, error) {
 	if i := strings.IndexAny(arg, `[]}"`); i >= 0 {
 		return false, fmt.Errorf("%q holds a %q, which no path may, and it is not valid JSON, so it names no template either", arg, arg[i:i+1])
 	}
+	if len(arg) > 1 && strings.HasPrefix(arg, "/") {
+		return false, fmt.Errorf("path %s starts with /, and every path starts at the root already; write %s", arg, arg[1:])
+	}
 	return false, nil
 }
 

node.go

@@ -378,13 +378,17 @@ const reservedInName = ".|({}/[]\""
 // reservedList spells reservedInName for an error message, so the two cannot drift.
 var reservedList = strings.Join(strings.Split(reservedInName, ""), " ")
 
-// checkName rejects a name the dot path, {token} and JSON grammars cannot spell.
+// checkName rejects a name the dot path, {token} and JSON grammars cannot spell, or a struct
+// tag cannot read.
 // Both a 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 name == "-" {
+		return fmt.Errorf(`%q is reserved: the struct tag fake:"-" leaves a field unfilled, so no tag could read it; rename 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, {token} and JSON grammars reserve",
 			name, name[i:i+1], reservedList)

struct.go

@@ -206,8 +206,6 @@ func tagValue(sf reflect.StructField, tag string) (any, error) {
 		return nil, err
 	case inline:
 		return inputValue(tag)
-	case strings.HasPrefix(tag, "/"):
-		return nil, fmt.Errorf("path %q starts with /, and every path starts at the root already; write %s", tag, tag[1:])
 	}
 	for _, seg := range strings.Split(tag, ".") {
 		if err := checkName(seg); err != nil {
@@ -290,13 +288,23 @@ var columnKinds = map[reflect.Kind]columnKind{
 	reflect.Uint8:   {DataTypeInteger, 0, math.MaxUint8, reflect.Int64},
 }
 
-// reach is the least and greatest value v proves a field of this kind receives. An integer
-// prints whole, so its bounds round inward.
-func (k columnKind) reach(v proven) (lo, hi float64) {
-	if k.datatype == DataTypeInteger {
-		return math.Ceil(v.lo), math.Floor(v.hi)
+// past is the bound of v a field of this kind cannot hold, if either is. An integer prints
+// whole, so its bounds round inward first.
+func (k columnKind) past(v proven) (float64, bool) {
+	lo, hi := v.lo, v.hi
+	switch k.datatype {
+	case DataTypeString, DataTypeBoolean:
+		return 0, false
+	case DataTypeInteger:
+		lo, hi = math.Ceil(lo), math.Floor(hi)
 	}
-	return v.lo, v.hi
+	switch {
+	case lo < k.lo:
+		return lo, true
+	case hi > k.hi:
+		return hi, true
+	}
+	return 0, false
 }
 
 // checkField rejects a column some render of which a field of Go type ft cannot hold: a null
@@ -318,15 +326,8 @@ func (p *valueProof) checkField(label string, ft reflect.Type, column node) erro
 		if reason := v.not[kind.datatype]; reason != "" {
 			return fmt.Errorf("%s (%s): %s", label, ft, reason)
 		}
-		if kind.datatype == DataTypeBoolean {
-			continue
-		}
-		if lo, hi := kind.reach(v); lo < kind.lo || hi > kind.hi {
-			past := hi
-			if lo < kind.lo {
-				past = lo
-			}
-			return fmt.Errorf("%s (%s): %q can reach %s, past %s; make it %s", label, ft, it.format, strconv.FormatFloat(past, 'g', -1, 64), elem.Kind(), kind.wider)
+		if bound, over := kind.past(v); over {
+			return fmt.Errorf("%s (%s): %q can reach %s, past %s; make it %s", label, ft, it.format, strconv.FormatFloat(bound, 'g', -1, 64), elem.Kind(), kind.wider)
 		}
 	}
 	return nil