README.md

@@ -1,32 +1,17 @@
 # fejkdata
 
-A Go library and CLI for generating locale-aware fake data from JSON templates.
-Forked from [github.com/Timewave-AB/fakes](https://github.com/Timewave-AB/fakes).
+Locale-aware fake data for tests and fixtures, generated from JSON templates. Use
+it from Go or the CLI — no data on disk, no dependencies, and a seed makes output
+reproducible.
 
-## Goals
-
-1. **Valid by construction** — every value passes the check its real consumer
-   applies; facts that belong together come from one draw, within a value and
-   across categories.
-2. **Text means what it says** — a format renders as written; only `{…}` varies,
-   random characters included (`{digits(3)}`). One spelling per result; the wrong
-   one is a load error naming the right one.
-3. **Every mistake is a load error** — `New` rejects; `Fake` on a loaded generator
-   fails only for an unknown path.
-4. **Zero to a value in one command** — `go install`, then `fejkdata sv_SE.person`:
-   no checkout, no flag. Flags are GNU-form (`--seed 42`, `-n 3`) in any position;
-   the first custom template needs no escape and no option.
-5. **Data lives in JSON** — a builtin only for what data can't express.
-6. **Reproducible** — seed in, same stream out; no builtin reads a clock.
-7. **Zero dependencies** — standard library only.
-8. **Docs index the grammar** — every syntax feature is a heading; every example
-   runs under test and shows its output; a rule is stated once.
+```sh
+go install gitea.larvit.se/larvit/fejkdata/cmd/fejkdata@latest
+fejkdata sv_SE.person          # Sara Eriksson
+```
 
 ## CLI
 
 ```sh
-go install gitea.larvit.se/larvit/fejkdata/cmd/fejkdata@latest
-
 fejkdata sv_SE.person                          # Sara Eriksson
 fejkdata sv_SE.person.last                     # Eriksson
 fejkdata --seed 42 sv_SE.address               # the same address every run
@@ -34,17 +19,27 @@ fejkdata -n 3 --separator ', ' sv_SE.word      # nät, barn, sol
 fejkdata --list                                # every path the data offers
 fejkdata --data-path ./mydata sv_SE.word       # layer a directory over the shipped data
 fejkdata --no-shipped-data -d ./mydata --list  # only your data
+fejkdata 'name: {/sv_SE.person.last}'          # name: <a surname> — an inline template
+fejkdata '{"format":"name: {x}","x":["bosse","lina"]}'  # name: bosse or name: lina
 ```
 
 A path names a category, or a field inside one: each dot segment descends one
-level — folders, then the category (a JSON file), then fields.
+level — folders, then the category (a JSON file), then fields. An argument that is
+a JSON object, array or string, or that carries a `{` token, is instead an
+**inline template**: a format string or a JSON value compiled and rendered on the
+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. A path never contains a
+brace, a bracket or a quote, so the two cannot collide (see
+[Decisions](#decisions)).
 
 | Flag | |
 |------|--|
 | `-d`, `--data-path D` | a directory to layer over the shipped data; repeatable, the last wins a name clash |
 | `--no-shipped-data` | load only the `--data-path` directories |
 | `-s`, `--seed N` | reproducible output |
-| `-n`, `--repeat N` | render the path N times (up to 1048576), each an independent draw, streamed |
+| `-n`, `--repeat N` | render the value N times (up to 1048576), each an independent draw, streamed |
 | `--separator S` | between repeated values (default a newline) |
 | `--list` | print every path, then exit |
 | `--version`, `-h`, `--help` | print, then exit |
@@ -52,7 +47,9 @@ level — folders, then the category (a JSON file), then fields.
 `--name value` and `--name=value` both work, a short flag's value attaches or
 follows (`-n3`, `-n 3`) and short flags bundle (`-hn 3`) — see
 [Decisions](#decisions); flags go anywhere, `--` ends them. Exit codes: `0` success, `1` runtime error (missing
-dir, unknown path), `2` misuse. From a checkout: `go run ./cmd/fejkdata …`.
+dir, unknown path), `2` misuse — a bad flag, an argument that names neither a
+template nor a path, or an inline template that does not compile. From a checkout:
+`go run ./cmd/fejkdata …`.
 
 ### Your own data
 
@@ -89,6 +86,9 @@ if err != nil {
 }
 v, err := f.Fake("sv_SE.address") // "Kungsvägen 68\n379 17 Stockholm"
 paths := f.List()                  // every path Fake accepts, sorted
+v, err = f.FakeTemplate("name: {/sv_SE.person.last}")      // compile + render in one call
+t, err := f.NewTemplate(`{"format":"name: {x}","x":["bosse","lina"]}`) // compile once
+v = t.Fake()                                              // render many times, no re-parse
 ```
 
 | Option | |
@@ -110,8 +110,8 @@ 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 `/`;
+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`,
@@ -341,6 +341,26 @@ then costs about what its output costs: an unweighted pick is O(1) whatever the
 list's length, a weighted one O(log n), and long formats, deep nesting and many
 tokens add cost in proportion to the output.
 
+## Goals
+
+1. **Valid by construction** — every value passes the check its real consumer
+   applies; facts that belong together come from one draw, within a value and
+   across categories.
+2. **Text means what it says** — a format renders as written; only `{…}` varies,
+   random characters included (`{digits(3)}`). One spelling per result; the wrong
+   one is a load error naming the right one.
+3. **Every mistake is a load error** — `New` rejects the data and `NewTemplate`
+   the inline template; on a loaded generator `Fake` fails only for an unknown
+   path, and `Template.Fake` cannot fail at all.
+4. **Zero to a value in one command** — `go install`, then `fejkdata sv_SE.person`:
+   no checkout, no flag. Flags are GNU-form (`--seed 42`, `-n 3`) in any position;
+   the first custom template needs no escape and no option.
+5. **Data lives in JSON** — a builtin only for what data can't express.
+6. **Reproducible** — seed in, same stream out; no builtin reads a clock.
+7. **Zero dependencies** — standard library only.
+8. **Docs index the grammar** — every syntax feature is a heading; every example
+   runs under test and shows its output; a rule is stated once.
+
 ## Decisions
 
 - **Options and fields share one namespace.** `format`, `weight`, `repeat` and
@@ -356,6 +376,33 @@ tokens add cost in proportion to the output.
   naming the double-dash spelling, and `-s=42` is rejected naming both short
   spellings: `=` belongs to the long form, and reading `=42` as the value would
   make `-d=./x` a directory named `=./x`.
+- **An argument is a template by its shape, not by a flag.** A JSON object, array
+  or string, or a string carrying a `{` token, is an inline template; anything else
+  is a path. A name may not contain a brace, a bracket or a quote, so a path can
+  never collide with any of those spellings, and the leading `[` or `"` is gated on
+  valid JSON so a stray copied bracket never swallows an argument — it names
+  nothing, and says so. No `--template` flag is needed. Reserving the characters
+  whole — though only a leading one could collide — keeps one simple name rule
+  instead of a leading-position special case. The JSON string is what makes the
+  library's own advice reachable: the error for an object holding only a format
+  names `"…"`, and that spelling has to work where it is printed.
+- **An inline template skips the cycle fence, and only that one.** `New` proves the
+  loaded tree acyclic, an inline node is a finite tree of its own, and nothing in
+  the tree can reference it, so no render of it reaches itself. Every other fence
+  runs over both, from one `checkScope`.
+- **An inline template that does not compile is misuse (exit 2), including a
+  reference that resolves to nothing** — the whole argument is the spelling under
+  test, and `NewTemplate` compiles, links and validates as one step. An unknown
+  *path* stays a runtime error (exit 1): there the argument is well-formed and only
+  the data is absent.
+- **A padded JSON argument is rejected, not trimmed.** Padding is the one place the
+  two readings disagree — a format string renders it, JSON drops it — so the
+  spelling that renders is named rather than silently chosen.
+- **`FakeTemplate` and `NewTemplate` both stay.** They reach the same value but not
+  at the same cost: `NewTemplate` pays the compile and validation once and renders
+  many times, `FakeTemplate` is the one-shot call, and `--repeat` is exactly the
+  case that needs the first. The pair is `regexp.MustCompile` and `regexp.Match`,
+  not two spellings of one result.
 - **The shipped data is embedded, not discovered.** A directory a machine happens
   to have would make `--seed 42` machine-dependent. Data still lives in `data/`
   as JSON; `--data-path` layers over it.
@@ -444,6 +491,7 @@ fejkdata.go     Generator, New, options, the embedded data set, List
 node.go         the node model and JSON -> node compilation
 path.go         the dotted-path walk, and proving a path resolves
 render.go       Fake and the recursive renderer (choices, format strings, expansions)
+inline.go       inline templates: Template, NewTemplate, FakeTemplate, and their compile and link
 template.go     the {token} grammar: scanning, tokens, operands, validation, compiling a format
 hold.go         the hold: one draw per expansion for paths and operands, and its fences
 reference.go    reference sigils, and binding references across the tree
@@ -457,4 +505,4 @@ data/           shipped data (JSON), embedded at build: locale folders + a misc
 
 ## License
 
-MIT — see [LICENSE](LICENSE).
+MIT — see [LICENSE](LICENSE). Forked from [github.com/Timewave-AB/fakes](https://github.com/Timewave-AB/fakes).

builtins.go

@@ -133,7 +133,7 @@ func transformArg(fields map[string]node, a []string) error {
 		_, _, err := refShape(leaf)
 		return err
 	}
-	return checkArm(leaf, fields)
+	return checkArm(leaf, fields, false)
 }
 
 func transformOperand(a []string) []string {

cmd/fejkdata/main.go

@@ -9,6 +9,7 @@ package main
 
 import (
 	"bufio"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"io"
@@ -20,21 +21,30 @@ import (
 	"gitea.larvit.se/larvit/fejkdata"
 )
 
-const usage = `Usage: fejkdata [flags] <path>
+const usage = `Usage: fejkdata [flags] <path|template>
 
   <path>                 a category, or a dotted path into one (person, person.last)
+  <template>             a format string or JSON value to render inline, e.g.
+                         'name: {/sv_SE.person.last}' or '{"format":"{x}","x":["bosse","lina"]}'
+
+An argument containing a { token, or a JSON object, array or string, is a
+template; any other argument is a path (a path never contains a brace, a bracket
+or a quote). Templates reach the data by reference from the root —
+{/sv_SE.person.last} — whether the data is shipped or layered with --data-path. An
+argument carrying a bracket, a closing brace or a quote but no valid JSON names
+neither.
 
   -d, --data-path D      a data directory to layer over the shipped data (repeatable; last wins on a clash)
   -h, --help             print this help, then exit
       --list             list the paths the data offers, then exit
       --no-shipped-data  load only the --data-path directories
-  -n, --repeat N         render the path N times, 1..1048576 (default 1)
+  -n, --repeat N         render the value N times, 1..1048576 (default 1)
   -s, --seed N           seed for reproducible output
       --separator S      string between repeated values (default newline)
       --version          print the version, then exit
 
-Flags may come before or after <path>; -- ends the flags. A short flag's value
-attaches or follows (-n3, -n 3); short flags bundle (-hn 3).
+Flags may come before or after <path|template>; -- ends the flags. A short flag's
+value attaches or follows (-n3, -n 3); short flags bundle (-hn 3).
 `
 
 type invocation struct {
@@ -186,18 +196,22 @@ func parseArgs(argv []string) (invocation, error) {
 	return in, nil
 }
 
-// check rejects a flag combination that cannot run.
-func (in invocation) check() error {
+// check rejects a flag combination or an argument that cannot run, and reports
+// what the argument names, so its shape is settled before any data is read.
+func (in invocation) check() (argKind, error) {
 	if in.list && len(in.paths) > 0 {
-		return errors.New("--list takes no path")
+		return argPath, errors.New("--list takes no path")
 	}
 	if in.list && (in.repeatSet || in.separatorSet) {
-		return errors.New("--list takes no --repeat or --separator")
+		return argPath, errors.New("--list takes no --repeat or --separator")
 	}
-	if !in.list && len(in.paths) != 1 {
-		return fmt.Errorf("expected one path, got %d", len(in.paths))
+	if in.list {
+		return argPath, nil
 	}
-	return nil
+	if len(in.paths) != 1 {
+		return argPath, fmt.Errorf("expected one path or template, got %d", len(in.paths))
+	}
+	return classify(in.paths[0])
 }
 
 func (in invocation) options() []fejkdata.Option {
@@ -214,14 +228,23 @@ func (in invocation) options() []fejkdata.Option {
 	return opts
 }
 
-// write streams the path's renders to w, repeat of them joined by the separator
-// and ended by a newline. A path that renders once renders every time, so a Fake
-// failure comes before anything is written; a write failure surfaces from Flush,
-// bufio keeping the first one.
-func (in invocation) write(f *fejkdata.Generator, w io.Writer) error {
+// write streams the argument's renders to w, repeat of them joined by the
+// separator and ended by a newline. A value that renders once renders every time,
+// so a render failure comes before anything is written; a write failure surfaces
+// from Flush, bufio keeping the first one.
+func (in invocation) write(f *fejkdata.Generator, kind argKind, w io.Writer) error {
+	arg := in.paths[0]
+	draw := func() (string, error) { return f.Fake(arg) }
+	if kind == argTemplate {
+		t, err := f.NewTemplate(arg)
+		if err != nil {
+			return templateError{err}
+		}
+		draw = func() (string, error) { return t.Fake(), nil }
+	}
 	out := bufio.NewWriter(w)
 	for i := 0; i < in.repeat; i++ {
-		v, err := f.Fake(in.paths[0])
+		v, err := draw()
 		if err != nil {
 			return err
 		}
@@ -234,6 +257,36 @@ func (in invocation) write(f *fejkdata.Generator, w io.Writer) error {
 	return out.Flush()
 }
 
+// templateError marks a render failure that is the argument's own fault — an
+// inline template that does not compile. run reports it as misuse (exit 2, with a
+// pointer to --help), unlike an unknown path, which is a runtime error (exit 1).
+type templateError struct{ error }
+
+func (e templateError) Unwrap() error { return e.error }
+
+type argKind int
+
+const (
+	argPath argKind = iota
+	argTemplate
+)
+
+// classify reads what a positional argument names by its shape: a { token, or a
+// JSON object, array or string, is an inline template; anything else is a path.
+func classify(arg string) (argKind, error) {
+	if strings.ContainsRune(arg, '{') || (isJSONStart(strings.TrimSpace(arg)) && json.Valid([]byte(arg))) {
+		return argTemplate, nil
+	}
+	if i := strings.IndexAny(arg, `[]}"`); i >= 0 {
+		return argPath, 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])
+	}
+	return argPath, nil
+}
+
+func isJSONStart(arg string) bool {
+	return strings.HasPrefix(arg, "[") || strings.HasPrefix(arg, `"`)
+}
+
 func main() { os.Exit(run(os.Args[1:], os.Stdout, os.Stderr)) }
 
 // run returns the exit code: 0 ok, 1 runtime error, 2 misuse.
@@ -250,7 +303,8 @@ func run(args []string, stdout, stderr io.Writer) int {
 		fmt.Fprintln(stdout, "fejkdata "+buildVersion())
 		return 0
 	}
-	if err := in.check(); err != nil {
+	kind, err := in.check()
+	if err != nil {
 		return misuse(stderr, err)
 	}
 	f, err := fejkdata.New(in.options()...)
@@ -267,7 +321,11 @@ func run(args []string, stdout, stderr io.Writer) int {
 		}
 		return 0
 	}
-	if err := in.write(f, stdout); err != nil {
+	if err := in.write(f, kind, stdout); err != nil {
+		var te templateError
+		if errors.As(err, &te) {
+			return misuse(stderr, te.error)
+		}
 		fmt.Fprintln(stderr, err)
 		return 1
 	}
@@ -275,7 +333,8 @@ func run(args []string, stdout, stderr io.Writer) int {
 }
 
 func misuse(stderr io.Writer, err error) int {
-	fmt.Fprintf(stderr, "fejkdata: %v\ntry 'fejkdata --help'\n", err)
+	// A library error already names the program, so the prefix is not doubled.
+	fmt.Fprintf(stderr, "fejkdata: %s\ntry 'fejkdata --help'\n", strings.TrimPrefix(err.Error(), "fejkdata: "))
 	return 2
 }
 

cmd/fejkdata/main_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"os"
 	"path/filepath"
+	"regexp"
 	"strings"
 	"testing"
 )
@@ -301,6 +302,92 @@ func TestRunShippedDataByDefault(t *testing.T) {
 	}
 }
 
+func TestClassify(t *testing.T) {
+	for arg, want := range map[string]argKind{
+		"sv_SE.person":   argPath,
+		"person.last":    argPath,
+		"name: {x}":      argTemplate, // a { token: a path can never carry a brace
+		`{"format":"x"}`: argTemplate,
+		`["a","b"]`:      argTemplate, // a JSON array carries no brace
+		`[1, 2]`:         argTemplate,
+		` ["a","b"]`:     argTemplate, // padding is the template's own error, not a shape verdict
+		`"hello"`:        argTemplate, // a JSON string, the spelling a format-only object names
+	} {
+		got, err := classify(arg)
+		if err != nil || got != want {
+			t.Errorf("classify(%q) = %v, %v; want %v", arg, got, err, want)
+		}
+	}
+	for arg, want := range map[string]string{
+		"[abc]":       `holds a "["`,
+		"[abc].field": `holds a "["`,
+		"x[1]":        `holds a "["`,
+		"a]b":         `holds a "]"`,
+		"a}b":         `holds a "}"`,
+		`"abc`:        `holds a "\""`,
+		`"a]b`:        `holds a "\""`, // the opener the reader typed, not the bracket behind it
+	} {
+		_, err := classify(arg)
+		if err == nil || !strings.Contains(err.Error(), want) {
+			t.Errorf("classify(%q) = %v; want it rejected naming %s", arg, err, want)
+		}
+	}
+}
+
+func TestUsageReferencesResolve(t *testing.T) {
+	for _, token := range regexp.MustCompile(`\{/[^}]+\}`).FindAllString(usage, -1) {
+		code, out, errb := runOut("--seed", "1", token)
+		if code != 0 || strings.TrimSpace(out) == "" {
+			t.Errorf("usage advertises %s: run = %d, %q, stderr %q", token, code, out, errb)
+		}
+	}
+}
+
+func TestRunShapeMisuseBeforeLoad(t *testing.T) {
+	code, _, errb := runOut("--no-shipped-data", "[abc]")
+	if code != 2 || !strings.Contains(errb, "[abc]") || strings.Contains(errb, "--data-path") {
+		t.Fatalf("shape misuse with no data = %d, %q; want the shape error before any load", code, errb)
+	}
+}
+
+func TestRunInlineTemplate(t *testing.T) {
+	code, out, errb := runOut("--seed", "1", "name: {/sv_SE.person.last}")
+	if code != 0 || !strings.HasPrefix(out, "name: ") || strings.Contains(out, "{") {
+		t.Fatalf("inline format string = %d, %q, stderr %q", code, out, errb)
+	}
+	code, out, errb = runOut("--seed", "1", `{"format":"name: {x}","x":["bosse","lina"]}`)
+	if code != 0 || (out != "name: bosse\n" && out != "name: lina\n") {
+		t.Fatalf("inline JSON template = %d, %q, want one name, stderr %q", code, out, errb)
+	}
+	code, out, errb = runOut("--seed", "1", `"name: {/sv_SE.person.last}"`)
+	if code != 0 || !strings.HasPrefix(out, "name: ") || strings.Contains(out, "{") {
+		t.Fatalf("inline JSON string = %d, %q, stderr %q", code, out, errb)
+	}
+	code, out, errb = runOut("--seed", "1", "-n", "2", `{digits(1)}`)
+	if code != 0 || len(strings.Split(strings.TrimRight(out, "\n"), "\n")) != 2 {
+		t.Fatalf("inline template with --repeat = %d, %q, stderr %q", code, out, errb)
+	}
+}
+
+func TestRunTemplateMisuse(t *testing.T) {
+	for arg, want := range map[string]string{
+		"{bad":            "unterminated",
+		"[red,green]":     "names no template either",
+		`{"format":"x"}`:  "is a string",
+		"{/no.such.path}": "no entry",
+		"x[1]":            "names no template either",
+		` ["a","b"] `:     "may not be padded",
+	} {
+		code, out, errb := runOut("--seed", "1", arg)
+		if code != 2 || out != "" || !strings.Contains(errb, "try 'fejkdata --help'") || !strings.Contains(errb, want) {
+			t.Errorf("run(%q) = %d, %q, %q; want misuse naming %q and --help", arg, code, out, errb, want)
+		}
+		if strings.Contains(errb, "fejkdata: fejkdata:") {
+			t.Errorf("run(%q) doubled the program prefix: %q", arg, errb)
+		}
+	}
+}
+
 func TestRunNoShippedData(t *testing.T) {
 	code, list, errb := runOut("--no-shipped-data", "-d", svSE, "--list")
 	if code != 0 {

data.go

@@ -68,10 +68,7 @@ func loadData(sources []dataSource) (map[string]node, error) {
 	if err := checkNoCycles(root); err != nil {
 		return nil, err
 	}
-	if err := checkRepeatReach(root); err != nil {
-		return nil, err
-	}
-	if err := checkBoundLevelsHeld(root); err != nil {
+	if err := checkScope(treeScope(root)); err != nil {
 		return nil, err
 	}
 	return root, nil

fejkdata.go

@@ -39,7 +39,8 @@ var ErrNoData = errors.New("no data: WithoutShippedData needs at least one WithD
 
 // Generator generates fake data from a loaded namespace tree. Create one with [New].
 // It is safe for concurrent use; a seeded sequence is reproducible only when drawn
-// from one goroutine.
+// from one goroutine. The compiled tree is immutable after [New], and Fake,
+// NewTemplate and List read it concurrently without a lock.
 type Generator struct {
 	mu         sync.Mutex
 	rand       *session

graph.go

@@ -10,29 +10,36 @@ import (
 // visiting keys in sorted order so which of several broken nodes gets reported does
 // not depend on map iteration.
 func walkNodes(root map[string]node, fn func(path string, n node) error) error {
-	seen := map[node]bool{}
-	var visit func(string, node) error
-	visit = func(path string, n node) error {
-		if n == nil || seen[n] {
-			return nil
-		}
-		seen[n] = true
-		if err := fn(path, n); err != nil {
+	for _, name := range sortedNames(root) {
+		if err := eachNode(root[name], name, fn); err != nil {
 			return err
 		}
-		for _, c := range contained(n) {
+	}
+	return nil
+}
+
+// eachNode visits n and every node contained within it once, passing the dot path
+// that reaches each. It never crosses a reference edge — a bound {/path} field is
+// skipped — so a single inline node is walked on its own.
+func eachNode(n node, path string, fn func(path string, n node) error) error {
+	seen := map[node]bool{}
+	var visit func(string, node) error
+	visit = func(path string, m node) error {
+		if m == nil || seen[m] {
+			return nil
+		}
+		seen[m] = true
+		if err := fn(path, m); err != nil {
+			return err
+		}
+		for _, c := range contained(m) {
 			if err := visit(join(path, c.name), c.node); err != nil {
 				return err
 			}
 		}
 		return nil
 	}
-	for _, name := range sortedNames(root) {
-		if err := visit(name, root[name]); err != nil {
-			return err
-		}
-	}
-	return nil
+	return visit(path, n)
 }
 
 // namedNode is a contained child and the segment reaching it; a choice's items carry
@@ -158,34 +165,56 @@ func pathLeaves(n node, tail []string) []node {
 	return out
 }
 
-// checkRepeatReach bounds the renders a repeat multiplies to along any root-to-leaf
-// path, so nested repeats cannot build what one repeat may not. It runs after
-// checkNoCycles, whose guarantee is what lets the walk terminate.
-func checkRepeatReach(root map[string]node) error {
-	reach := map[node]int{}
-	var of func(n node) int
-	of = func(n node) int {
-		if r, done := reach[n]; done {
+// nodeScope is the set of nodes one validation pass covers: a whole loaded tree,
+// or a single inline node.
+type nodeScope func(fn func(path string, n node) error) error
+
+func treeScope(root map[string]node) nodeScope {
+	return func(fn func(path string, n node) error) error { return walkNodes(root, fn) }
+}
+
+func inlineScope(n node) nodeScope {
+	return func(fn func(path string, m node) error) error { return eachNode(n, "template", fn) }
+}
+
+// checkScope runs the per-node fences over a scope, each over the whole scope
+// before the next, so which of several broken nodes is reported does not depend on
+// the walk. It runs after checkNoCycles, whose guarantee is what lets the walks
+// terminate.
+func checkScope(s nodeScope) error {
+	mem := reachMemo{}
+	if err := s(func(path string, n node) error { return repeatCheck(path, n, mem) }); err != nil {
+		return err
+	}
+	return s(heldCheck)
+}
+
+type reachMemo map[node]int
+
+func (m reachMemo) of(n node) int {
+	if r, done := m[n]; done {
 		return r
 	}
 	r := 1
 	for _, e := range renderEdges(n) {
-			if c := of(e.to); c > r {
+		if c := m.of(e.to); c > r {
 			r = c
 		}
 	}
 	if t, ok := n.(*template); ok {
 		r *= t.repeat
 	}
-		reach[n] = r
+	m[n] = r
 	return r
-	}
-	return walkNodes(root, func(path string, n node) error {
-		if t, ok := n.(*template); ok && t.repeat > 1 && of(n) > MaxRepeat {
-			return fmt.Errorf("%s: repeat %d multiplies to %d renders along one path, above the maximum %d", path, t.repeat, of(n), MaxRepeat)
+}
+
+// repeatCheck bounds the renders a repeat multiplies to along any root-to-leaf
+// path, so nested repeats cannot build what one repeat may not.
+func repeatCheck(path string, n node, mem reachMemo) error {
+	if t, ok := n.(*template); ok && t.repeat > 1 && mem.of(n) > MaxRepeat {
+		return fmt.Errorf("%s: repeat %d multiplies to %d renders along one path, above the maximum %d", path, t.repeat, mem.of(n), MaxRepeat)
 	}
 	return nil
-	})
 }
 
 // checkNoCycles rejects a reference cycle: a node whose rendering can reach itself

hold.go

@@ -6,15 +6,12 @@ import (
 	"strings"
 )
 
-// checkBoundLevelsHeld rejects every route to a held name except the ones that read
-// its draw. An expansion holds one draw of that name; anything else that renders it
-// draws again, and the two disagree. checkNoOverlap settles the spellings within one
+// heldCheck rejects every route to a held name except the ones that read its draw.
+// An expansion holds one draw of that name; anything else that renders it draws
+// again, and the two disagree. checkNoOverlap settles the spellings within one
 // format (a token, an operand); this settles the rest — a reference, whether it
 // sits in that format or in anything the format renders, however deep.
-//
-// It runs after checkNoCycles, whose guarantee is what lets the walk terminate.
-func checkBoundLevelsHeld(root map[string]node) error {
-	return walkNodes(root, func(path string, n node) error {
+func heldCheck(path string, n node) error {
 	t, ok := n.(*template)
 	if !ok || len(t.held) == 0 {
 		return nil
@@ -26,7 +23,6 @@ func checkBoundLevelsHeld(root map[string]node) error {
 		}
 	}
 	return nil
-	})
 }
 
 // heldHeads lists a template's held names, operand heads first, then paths, each

inline.go

@@ -0,0 +1,88 @@
+package fejkdata
+
+import (
+	"encoding/json"
+	"fmt"
+	"strings"
+)
+
+// Template is an inline template compiled, referenced and validated against a
+// generator's loaded data once, ready to render many times with [Template.Fake].
+// It is safe for concurrent use: Fake serializes on its generator's lock, so a
+// seeded sequence is reproducible only when a generator — and its templates — are
+// drawn from one goroutine.
+type Template struct {
+	g *Generator
+	n node
+}
+
+// Fake renders the template with one draw.
+func (t *Template) Fake() string {
+	t.g.mu.Lock()
+	defer t.g.mu.Unlock()
+	return render(t.g.rand, t.n)
+}
+
+// NewTemplate compiles an inline template — a format string or a JSON value — and
+// binds its references against the loaded tree, so repeated renders pay the
+// compile and validation once. It shares [New]'s guarantees: a bad template errors
+// here, and rendering cannot fail.
+func (f *Generator) NewTemplate(input string) (*Template, error) {
+	n, err := compileInput(input)
+	if err != nil {
+		return nil, fmt.Errorf("fejkdata: %w", err)
+	}
+	scope := inlineScope(n)
+	if err := linkNodeRefs(scope, f.categories); err != nil {
+		return nil, fmt.Errorf("fejkdata: %w", err)
+	}
+	if err := checkScope(scope); err != nil {
+		return nil, fmt.Errorf("fejkdata: %w", err)
+	}
+	return &Template{g: f, n: n}, nil
+}
+
+// FakeTemplate compiles and renders an inline template in one call. It is
+// [NewTemplate] then [Template.Fake]; to render the same template many times, hold
+// the *Template and call its Fake.
+func (f *Generator) FakeTemplate(input string) (string, error) {
+	t, err := f.NewTemplate(input)
+	if err != nil {
+		return "", err
+	}
+	return t.Fake(), nil
+}
+
+// compileInput compiles an inline template: a JSON value, or a bare format string
+// when the input is not JSON.
+func compileInput(input string) (node, error) {
+	var raw any
+	if err := json.Unmarshal([]byte(input), &raw); err != nil {
+		return compile(input)
+	}
+	if trimmed := strings.TrimSpace(input); trimmed != input {
+		return nil, fmt.Errorf("a JSON template may not be padded with spaces, which a format string would render; write %s", trimmed)
+	}
+	return compile(raw)
+}
+
+// linkNodeRefs binds the references in an inline node's templates against the
+// loaded tree.
+func linkNodeRefs(scope nodeScope, root map[string]node) error {
+	return scope(func(path string, m node) error {
+		t, ok := m.(*template)
+		if !ok {
+			return nil
+		}
+		for _, name := range refTokens(t.format) {
+			sigil, rest, err := refShape(name)
+			if err != nil {
+				return fmt.Errorf("%s: reference {%s}: %w", path, name, err)
+			}
+			if sigil != "/" {
+				return fmt.Errorf("%s: reference {%s}: an inline template has no folder; write {/%s}", path, name, rest)
+			}
+		}
+		return linkTemplateRefs(nil, path, t, root)
+	})
+}

inline_test.go

@@ -0,0 +1,159 @@
+package fejkdata
+
+import (
+	"strings"
+	"testing"
+)
+
+func tmpl(t *testing.T, f *Generator, input string) string {
+	t.Helper()
+	s, err := f.FakeTemplate(input)
+	if err != nil {
+		t.Fatalf("FakeTemplate(%q): %v", input, err)
+	}
+	return s
+}
+
+func shipped(t *testing.T, opts ...Option) *Generator {
+	t.Helper()
+	f, err := New(append([]Option{WithSeed(1)}, opts...)...)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return f
+}
+
+func TestFakeTemplateFormatString(t *testing.T) {
+	f := shipped(t)
+	got := tmpl(t, f, "name: {/sv_SE.person.last}")
+	if !strings.HasPrefix(got, "name: ") || strings.HasSuffix(got, " ") || strings.Contains(got, "{") {
+		t.Fatalf("FakeTemplate = %q, want a rendered last name after the prefix", got)
+	}
+}
+
+func TestFakeTemplateJSONObject(t *testing.T) {
+	f := shipped(t)
+	seen := map[string]bool{}
+	for i := 0; i < 50; i++ {
+		seen[tmpl(t, f, `{"format":"name: {x}","x":["bosse","lina"]}`)] = true
+	}
+	if !seen["name: bosse"] || !seen["name: lina"] || len(seen) != 2 {
+		t.Fatalf("JSON template produced %v, want both names", seen)
+	}
+}
+
+func TestFakeTemplateJSONArray(t *testing.T) {
+	f := shipped(t)
+	seen := map[string]bool{}
+	for i := 0; i < 50; i++ {
+		seen[tmpl(t, f, `["foo","bar","baz"]`)] = true
+	}
+	if len(seen) != 3 {
+		t.Fatalf("JSON array choice produced %v, want three items", seen)
+	}
+}
+
+func TestFakeTemplateCorrelatedReferences(t *testing.T) {
+	dir := writeData(t, map[string]string{
+		"person": `[{"format":"{first} {last}","first":"Ada","last":"Lovelace"},{"format":"{first} {last}","first":"Bo","last":"Ek"}]`,
+	})
+	f := newGenerator(t, dir, WithSeed(1))
+	for i := 0; i < 100; i++ {
+		got := tmpl(t, f, "{/person.first} {/person.last}")
+		if got != "Ada Lovelace" && got != "Bo Ek" {
+			t.Fatalf("correlated references = %q, want one person's first and last", got)
+		}
+	}
+}
+
+func TestFakeTemplateDeterministic(t *testing.T) {
+	a, b := shipped(t), shipped(t)
+	for i := 0; i < 20; i++ {
+		in := "row: {/misc.uuid} {digits(3)}"
+		if x, y := tmpl(t, a, in), tmpl(t, b, in); x != y {
+			t.Fatalf("same seed diverged: %q != %q", x, y)
+		}
+	}
+}
+
+func TestFieldlessTokenHint(t *testing.T) {
+	f := shipped(t)
+	_, err := f.FakeTemplate(`{sv_SE.person.last}`)
+	if err == nil || !strings.Contains(err.Error(), "write {/sv_SE.person.last}") {
+		t.Fatalf("FakeTemplate(bare token) = %v, want a hint naming {/sv_SE.person.last}", err)
+	}
+	_, err = f.FakeTemplate(`{"format":"{x}","repeat":2}`)
+	if err == nil || !strings.Contains(err.Error(), `this template has none — write {/x}`) {
+		t.Errorf("FakeTemplate(fieldless object) = %v, want the hint without calling it a bare string", err)
+	}
+	// A hint is only a drop-in where the name is the whole token: {/x} inside a
+	// transform or an alternation renders a different value, so none is offered.
+	for _, input := range []string{`{ /sv_SE.person.last }`, "{lowercase(x)}", "{x|y}"} {
+		_, err := f.FakeTemplate(input)
+		if err == nil || strings.Contains(err.Error(), "write {") {
+			t.Errorf("FakeTemplate(%q) = %v, want no hint naming a spelling that means something else", input, err)
+		}
+	}
+}
+
+func TestFakeTemplateErrors(t *testing.T) {
+	f := shipped(t)
+	for _, c := range []struct {
+		input string
+		want  string
+	}{
+		{`{"x":"Q"}`, "missing string \"format\""},
+		{`"{x}"`, `no field "x"`},
+		{`"{digits(0)}"`, "must be positive"},
+		{`name: {/no.such.path}`, "no entry"},
+		{`name: {..nope}`, "write {/nope}"},
+		{`{"format":"x"}`, "is a string"},
+		{`{/misc.country} {/misc.country.alpha2}`, "renders a level"},
+		{`{"format":"{/misc.country.alpha2} {x}","x":"{/misc.country}"}`, "reads a path into"},
+	} {
+		_, err := f.FakeTemplate(c.input)
+		if err == nil || !strings.Contains(err.Error(), c.want) {
+			t.Errorf("FakeTemplate(%q) = %v, want an error containing %q", c.input, err, c.want)
+		}
+	}
+}
+
+func TestFakeTemplateJSONString(t *testing.T) {
+	f := shipped(t)
+	got := tmpl(t, f, `"name: {/sv_SE.person.last}"`)
+	if !strings.HasPrefix(got, "name: ") || strings.Contains(got, "{") {
+		t.Fatalf("FakeTemplate(JSON string) = %q, want a rendered last name after the prefix", got)
+	}
+}
+
+func TestPaddedJSONIsRejected(t *testing.T) {
+	f := shipped(t)
+	in := `{"format":"{x}","x":["a","b"]}`
+	_, err := f.NewTemplate("  " + in + "  ")
+	if err == nil || !strings.Contains(err.Error(), "write "+in) {
+		t.Fatalf("NewTemplate(padded JSON) = %v, want an error naming the unpadded spelling", err)
+	}
+}
+
+func TestNewTemplateReusable(t *testing.T) {
+	f := shipped(t)
+	reusable, err := f.NewTemplate(`{digits(2)}`)
+	if err != nil {
+		t.Fatalf("NewTemplate: %v", err)
+	}
+	seen := map[string]bool{}
+	for i := 0; i < 50; i++ {
+		seen[reusable.Fake()] = true
+	}
+	if len(seen) < 2 {
+		t.Fatalf("Template.Fake() repeated %v, want varied draws from one compile", seen)
+	}
+}
+
+func TestFakeTemplateRepeatBound(t *testing.T) {
+	f := shipped(t)
+	_, err := f.FakeTemplate(`{"format":"{x}","repeat":200,"x":{"format":"{y}","repeat":200,"y":{"format":"z","repeat":200}}}`)
+	if err == nil || !strings.Contains(err.Error(), "maximum") {
+		t.Errorf("nested repeat over the cap = %v, want it rejected naming the maximum", err)
+	}
+}

loading_test.go

@@ -275,6 +275,18 @@ func TestNewErrors(t *testing.T) {
 			map[string]string{"a|b": `"1"`},
 			`category "a|b" contains "|"`,
 		},
+		"field name with a bracket": {
+			map[string]string{"a": `{"format":"{x}","x":"1","b[c":"2"}`},
+			`field "b[c" contains "["`,
+		},
+		"category name with a bracket": {
+			map[string]string{"[abc]": `"1"`},
+			`category "[abc]" contains "["`,
+		},
+		"field name with a quote": {
+			map[string]string{"a": `{"format":"{x}","x":"1","b\"c":"2"}`},
+			`field "b\"c" 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.

node.go

@@ -87,10 +87,24 @@ func compileItem(v any) (node, error) {
 	case map[string]any:
 		return compileTemplate(v)
 	default:
-		return nil, fmt.Errorf("unsupported node type %T", v)
+		return nil, fmt.Errorf("a template value must be a string, a list or an object, not %s", jsonKind(v))
 	}
 }
 
+// jsonKind names a JSON value a template cannot hold, in the data format's own
+// terms rather than the decoding library's.
+func jsonKind(v any) string {
+	switch v.(type) {
+	case float64:
+		return "a number"
+	case bool:
+		return "a boolean"
+	case nil:
+		return "null"
+	}
+	return fmt.Sprintf("%T", v)
+}
+
 func compileString(s string) (node, error) {
 	if err := checkTokens(s, nil); err != nil {
 		return nil, err
@@ -288,7 +302,7 @@ func repeatOf(m map[string]any) (int, error) {
 	if r == 1 {
 		return 0, fmt.Errorf("repeat 1 is the default, so it has no effect; drop it")
 	}
-	if r > MaxRepeat { // caps the renders one repeat asks for; checkRepeatReach bounds what nested ones multiply to
+	if r > MaxRepeat { // caps the renders one repeat asks for; repeatCheck bounds what nested ones multiply to
 		return 0, fmt.Errorf("repeat %v exceeds the maximum %d", rv, MaxRepeat)
 	}
 	return int(r), nil
@@ -323,23 +337,23 @@ func weightOf(raw any) (float64, error) {
 
 // reservedInName is what a category, folder or field name may not contain: a dot
 // separates the segments of a path, '|' the arms of a token, '(' opens a function
-// call, braces delimit the token and '/' starts a reference. A name carrying one is
-// reachable by no format, so it is rejected where it is authored rather than at
-// the token that cannot reach it.
-const reservedInName = ".|({}/"
+// call, braces delimit the token, '/' starts a reference, and brackets and a quote
+// open a JSON value. A name carrying one is rejected where it is authored rather
+// than where it would be unreachable.
+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 and {token} grammars cannot spell. Both a
-// category or folder and a field go through it, so there is one answer to what a
-// name may contain.
+// checkName rejects a name the dot path, {token} and JSON grammars cannot spell.
+// 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 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",
+		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)
 	}
 	return nil

one_spelling_test.go

@@ -47,3 +47,13 @@ func TestInertObjectIsRejected(t *testing.T) {
 		}
 	}
 }
+
+func TestInlineFolderSigilsAreRejected(t *testing.T) {
+	f := shipped(t)
+	for _, input := range []string{"{.sv_SE.person.last}", "{..sv_SE.person.last}"} {
+		_, err := f.NewTemplate(input)
+		if err == nil || !strings.Contains(err.Error(), "write {/sv_SE.person.last}") {
+			t.Errorf("NewTemplate(%q) = %v, want an error naming the root spelling", input, err)
+		}
+	}
+}

reference.go

@@ -72,6 +72,14 @@ func refSegments(name string, folder []string) ([]string, error) {
 // error, never a random render-time one.
 func linkRefs(root map[string]node) error {
 	return eachTemplate(root, func(folder []string, path string, t *template) error {
+		return linkTemplateRefs(folder, path, t, root)
+	})
+}
+
+// linkTemplateRefs binds one template's references against root. A template with
+// none is left untouched, so an inline format that references nothing costs only
+// the refTokens scan.
+func linkTemplateRefs(folder []string, path string, t *template, root map[string]node) error {
 	names := refTokens(t.format)
 	if len(names) == 0 {
 		return nil
@@ -100,7 +108,6 @@ func linkRefs(root map[string]node) error {
 		return fmt.Errorf("%s: %w", path, err)
 	}
 	return nil
-	})
 }
 
 // eachTemplate calls fn once per template, with the folder its category sits in

shipped_data_test.go

@@ -108,6 +108,12 @@ func TestFakeIsSafeForConcurrentUse(t *testing.T) {
 					return
 				}
 				f.List()
+				tmpl, err := f.NewTemplate("{/sv_SE.person.last}")
+				if err != nil {
+					t.Error(err)
+					return
+				}
+				tmpl.Fake()
 			}
 		}()
 	}

template.go

@@ -140,7 +140,7 @@ func checkTokens(format string, fields map[string]node) error {
 				}
 				continue // its target is checked at New (see linkRefs)
 			}
-			if err := checkArm(name, fields); err != nil {
+			if err := checkArm(name, fields, len(names) == 1); err != nil {
 				return fmt.Errorf("token {%s}: %w", t.body, err)
 			}
 		}
@@ -151,7 +151,9 @@ func checkTokens(format string, fields map[string]node) error {
 }
 
 // checkArm validates one sibling name or path against a template's fields.
-func checkArm(name string, fields map[string]node) error {
+// wholeToken says the name is the token's entire body, so {/name} would render
+// the same value and can be offered as the reference spelling.
+func checkArm(name string, fields map[string]node, wholeToken bool) error {
 	a := splitArm(name, nil)
 	if err := checkSegments(a); err != nil {
 		return err
@@ -164,6 +166,13 @@ func checkArm(name string, fields map[string]node) error {
 		if isOption(a.key) {
 			return fmt.Errorf("%q is an option and can never be a field", a.key)
 		}
+		if len(fields) == 0 {
+			hint := ""
+			if wholeToken && hintableRef(name) {
+				hint = fmt.Sprintf(" — write {/%s} to reference the data", name)
+			}
+			return fmt.Errorf("no field %q; a token names a sibling field, and this template has none%s", a.key, hint)
+		}
 		return fmt.Errorf("no field %q", a.key)
 	}
 	if err := checkPath(head, a.tail, a.key); err != nil {
@@ -172,6 +181,17 @@ func checkArm(name string, fields map[string]node) error {
 	return nil
 }
 
+// hintableRef reports whether {/name} is a reference the grammar accepts, so the
+// hint never names a spelling that fails too.
+func hintableRef(name string) bool {
+	for _, seg := range strings.Split(name, ".") {
+		if checkName(seg) != nil {
+			return false
+		}
+	}
+	return true
+}
+
 // tokenOperands lists the fields one {token} body reads as operands, empty for a
 // field token or a builtin that reads none.
 func tokenOperands(body string) []string {