README.md

@@ -22,6 +22,16 @@ Forked from [github.com/Timewave-AB/fakes](https://github.com/Timewave-AB/fakes)
 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.
 
+## Audience, scale and horizon
+
+fejkdata is for developers generating test and fixture data, from Go or the CLI —
+in-memory and single-process, with no persistence or networking, so it reads no
+configuration beyond what the caller passes. A single value is bounded at 1 048 576
+renders (`MaxRepeat`); how large each render is stays what the data asked for. The
+data format becomes the frozen public API at the first tagged release, which is the
+promotion trigger for a compatibility review; until then there is no compatibility
+promise.
+
 ## CLI
 
 ```sh
@@ -98,8 +108,9 @@ 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}")      // an inline template
-v, err = f.FakeTemplate(`{"format":"name: {x}","x":["bosse","lina"]}`)
+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 | |
@@ -121,8 +132,8 @@ 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`,
@@ -369,8 +380,9 @@ tokens add cost in proportion to the output.
   make `-d=./x` a directory named `=./x`.
 - **An argument is a template by its shape, not by a flag.** A JSON object or
   array, or a string carrying a `{` token, is an inline template; anything else is
-  a path. A path can never contain a brace — a name may not use one — so the two
-  never collide, and no `--template` flag is needed to disambiguate them.
+  a path. A name may not contain a brace or a bracket, so a path can never collide
+  with either spelling, and the `[` of a JSON array is gated on valid JSON so a
+  stray copied bracket never swallows an argument. No `--template` flag is needed.
 - **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.

cmd/fejkdata/main.go

@@ -227,10 +227,20 @@ func (in invocation) options() []fejkdata.Option {
 // 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 {
+	arg := in.paths[0]
+	var draw func() (string, error)
+	if isTemplate(arg) {
+		t, err := f.NewTemplate(arg)
+		if err != nil {
+			return err
+		}
+		draw = func() (string, error) { return t.Fake(), nil }
+	} else {
+		draw = func() (string, error) { return f.Fake(arg) }
+	}
 	out := bufio.NewWriter(w)
 	for i := 0; i < in.repeat; i++ {
-		arg := in.paths[0]
-		v, err := renderArg(f, arg)
+		v, err := draw()
 		if err != nil {
 			return err
 		}
@@ -244,9 +254,10 @@ func (in invocation) write(f *fejkdata.Generator, w io.Writer) error {
 }
 
 // isTemplate reports whether an argument is an inline template rather than a
-// path: a format string carrying a { token (a path can never contain a brace),
-// or a JSON object or array. A [ can begin a real category name, so a [
-// counts as a template only when the whole argument is valid JSON.
+// path: a format string carrying a { token, or a JSON object or array. A name may
+// not contain a brace or bracket, so both spellings collide with no path; the [
+// gate is valid-JSON so a [ alone never swallows an argument that merely began
+// with a copied bracket.
 func isTemplate(arg string) bool {
 	if strings.ContainsRune(arg, '{') {
 		return true
@@ -254,14 +265,6 @@ func isTemplate(arg string) bool {
 	return strings.HasPrefix(arg, "[") && json.Valid([]byte(arg))
 }
 
-// renderArg renders one positional argument: an inline template, or a path.
-func renderArg(f *fejkdata.Generator, arg string) (string, error) {
-	if isTemplate(arg) {
-		return f.FakeTemplate(arg)
-	}
-	return f.Fake(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.

node.go

@@ -335,23 +335,23 @@ 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 a bracket would be
+// misread by the command line as a JSON array. 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 CLI 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 CLI grammars reserve",
 			name, name[i:i+1], reservedList)
 	}
 	return nil

render.go

@@ -30,30 +30,52 @@ func (f *Generator) Fake(path string) (string, error) {
 	return render(f.rand, n), nil
 }
 
-// FakeTemplate renders an inline template — a format string or a JSON value — the
-// way a category's data is compiled and rendered, references reaching the loaded
-// tree with {/path}. It shares [Fake]'s lock: an inline node is compiled and
-// referenced per draw, so a seeded sequence is reproducible when drawn from one
-// goroutine.
-func (f *Generator) FakeTemplate(input string) (string, error) {
-	f.mu.Lock()
-	defer f.mu.Unlock()
+// Template is an inline template compiled, referenced and validated against a
+// generator's loaded data once, ready to render many times with [Template.Fake].
+type Template struct {
+	g *Generator
+	n node
+}
+
+// Fake renders the template with one draws.
+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 "", fmt.Errorf("fejkdata: %w", err)
+		return nil, fmt.Errorf("fejkdata: %w", err)
 	}
 	if err := linkNodeRefs(n, f.categories); err != nil {
-		return "", fmt.Errorf("fejkdata: %w", err)
+		return nil, fmt.Errorf("fejkdata: %w", err)
 	}
 	// No cycle is possible: a reference binds only into the loaded tree, which has
 	// no path into this node, so rendering it cannot reach itself.
 	if err := checkNodeRepeatReach(n); err != nil {
-		return "", fmt.Errorf("fejkdata: %w", err)
+		return nil, fmt.Errorf("fejkdata: %w", err)
 	}
 	if err := checkNodeBoundLevelsHeld(n); err != nil {
-		return "", fmt.Errorf("fejkdata: %w", err)
+		return nil, fmt.Errorf("fejkdata: %w", err)
 	}
-	return render(f.rand, n), nil
+	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
 }
 
 // descend walks named fields to the node a path names. It is the one render-side