CHANGELOG.md

@@ -26,10 +26,10 @@ and any release may change what a seed draws, so pin one exact version
   a record's columns, one record per row. The CLI exits `0` on success, `1` on a runtime
   error such as data that fails to load, and `2` on misuse.
 - The library, `github.com/larvit/fejkdata`, for Go 1.22 or later on a 64-bit target.
-  `New` loads the modules `WithDataFS` names and the folders `WithDataPath` names, and
-  fails, naming `WithDataFS`, where neither is given. `WithSeed` is required, and `New`
-  fails naming it where none is given; its draws repeat while the version and data stay
-  the same. `Fake` renders a path, and `FakeTemplate` and `NewTemplate` render a
+  `New` loads the modules `WithDataFS` names and the folders `WithDataPath` names, drawing
+  from the seed `WithSeed` gives, and fails naming each of `WithSeed` and `WithDataFS`
+  that is missing. The same seed repeats the draws while the version and data stay the
+  same. `Fake` renders a path, and `FakeTemplate` and `NewTemplate` render a
   template. `FakeRecord`, `FakeRecordTemplate`
   and `NewRecordTemplate` render a record, whose `JSON`, `CSVHeader`, `CSVLine` and
   `SQLInsert` methods write its columns as the CLI does. `FakeStruct` fills a struct's

README.md

@@ -372,8 +372,8 @@ links all of them. Two locales, with `enus` from `github.com/larvit/fejkdata/dat
 are `WithDataFS(append(svse.Modules(), enus.Modules()...)...)`. To stand in your
 own places, pass the locale's `FS` alone beside a tree providing `geo.SE.address`, a record
 with a `format` and the columns `sv_SE.address` reads, `street`, `street-number`,
-`postal-code` and `locality`: `WithDataFS(svse.FS, myPlaces)`. `New` fails where no `WithDataFS` or `WithDataPath` names the
-data to load.
+`postal-code` and `locality`: `WithDataFS(svse.FS, myPlaces)`. `New` fails where no `WithSeed` gives the seed, or no
+`WithDataFS` or `WithDataPath` names the data to load.
 
 `New` refuses data that breaks the grammar, could mean two things or cannot render a valid
 value, and `NewTemplate` refuses such a template. A category of an indexed source is
@@ -430,7 +430,7 @@ first call for a type compiles its tags and reports what they get wrong, with th
 error on every later call; a `datatype` in a tag other than the one its Go type sets is
 refused, naming that type.
 
-A `*Generator` is safe for concurrent use; a seeded sequence is reproducible only
+A `*Generator` is safe for concurrent use; a sequence is reproducible only
 when drawn from one goroutine. Changing how a value is composed shifts the seeded
 stream for that value and everything drawn after it.
 

cli/cli.go

@@ -320,7 +320,7 @@ func (in invocation) check(name string) error {
 	return nil
 }
 
-func (in invocation) options(modules []fs.FS) []fejkdata.Option {
+func (in invocation) options(modules []fs.FS, systemSeed uint64) []fejkdata.Option {
 	if in.noShipped {
 		modules = nil
 	}
@@ -330,7 +330,7 @@ func (in invocation) options(modules []fs.FS) []fejkdata.Option {
 	}
 	seed := in.seed
 	if !in.seeded {
-		seed = rand.Uint64()
+		seed = systemSeed
 	}
 	return append(opts, fejkdata.WithSeed(seed))
 }
@@ -572,7 +572,7 @@ func Main(modules ...fs.FS) int {
 	if fi, err := os.Stdin.Stat(); err == nil && fi.Mode()&os.ModeCharDevice != 0 {
 		stdin = nil
 	}
-	return run(commandName(os.Args[0]), modules, os.Args[1:], stdin, os.Stdout, os.Stderr)
+	return run(commandName(os.Args[0]), modules, os.Args[1:], rand.Uint64(), stdin, os.Stdout, os.Stderr)
 }
 
 // commandName is the name the command runs under: the file arg0 names, less a Windows
@@ -588,9 +588,9 @@ func commandName(arg0 string) string {
 	return name
 }
 
-// run is Main over the given streams, run as the command name. A nil stdin is a terminal,
-// which holds no template.
-func run(name string, modules []fs.FS, args []string, stdin io.Reader, stdout, stderr io.Writer) int {
+// run is Main over the given streams, run as the command name, seeded by systemSeed where
+// no --seed is given. A nil stdin is a terminal, which holds no template.
+func run(name string, modules []fs.FS, args []string, systemSeed uint64, stdin io.Reader, stdout, stderr io.Writer) int {
 	in, err := parseArgs(args)
 	if err != nil {
 		return misuse(stderr, name, err)
@@ -613,7 +613,7 @@ func run(name string, modules []fs.FS, args []string, stdin io.Reader, stdout, s
 			return fail(stderr, name, err)
 		}
 	}
-	f, err := fejkdata.New(in.options(modules)...)
+	f, err := fejkdata.New(in.options(modules, systemSeed)...)
 	if err != nil {
 		return fail(stderr, name, err)
 	}

fejkdata.go

@@ -20,7 +20,7 @@ const MaxRepeat = 1 << 20
 var _ [^uint(0)>>63 - 1]struct{}
 
 // Generator generates fake data from a namespace tree. Create one with [New].
-// It is safe for concurrent use; a seeded sequence is reproducible only when drawn
+// It is safe for concurrent use; a sequence is reproducible only when drawn
 // from one goroutine.
 type Generator struct {
 	// mu guards drawState, structs and root, which gains an indexed category on the
@@ -36,8 +36,8 @@ type config struct {
 	seeded     bool
 	sources    []datafiles.Source
 	nilModules error
-	// given is whether an option named the data to load, though it may name none.
-	given bool
+	// dataGiven is whether an option named the data to load, though it may name none.
+	dataGiven bool
 }
 
 // Option configures a [Generator].
@@ -54,7 +54,7 @@ func WithSeed(seed uint64) Option {
 // layer several; the last wins a name clash.
 func WithDataPath(dir string) Option {
 	return func(c *config) {
-		c.sources, c.given = append(c.sources, datafiles.Dir(dir)), true
+		c.sources, c.dataGiven = append(c.sources, datafiles.Dir(dir)), true
 	}
 }
 
@@ -71,7 +71,7 @@ func WithDataFS(modules ...fs.FS) Option {
 			}
 			c.sources = append(c.sources, datafiles.FS(fsys))
 		}
-		c.given = true
+		c.dataGiven = true
 	}
 }
 
@@ -91,24 +91,21 @@ func (loadError) Is(target error) bool { return target == ErrLoad }
 
 // New builds a generator from the options' sources, merged in order with the last
 // winning a name clash; it fails where no [WithSeed] gives the seed, or no [WithDataFS]
-// or [WithDataPath] names the data to load. Each JSON file becomes a category named after the file (address.json ->
-// "address") and each subdirectory a namespace segment. It errors on a missing
-// directory, invalid JSON or invalid data. Given only WithDataFS() with no modules, New
-// loads no data, and the generator renders templates that read none. A source whose .fejkdata.json carries an
-// index, as each shipped module's does, loads each category on the first call reaching
-// it. Data failing to load, in New or on that first call, gives an error matching
+// or [WithDataPath] names the data to load. Each JSON file becomes a category named
+// after the file (address.json -> "address") and each subdirectory a namespace segment.
+// It errors on a missing directory, invalid JSON or invalid data. Given only
+// WithDataFS() with no modules, New loads no data, and the generator renders templates
+// that read none. A source whose .fejkdata.json carries an index, as each shipped
+// module's does, loads each category on the first call reaching it. Data failing to load, in New or on that first call, gives an error matching
 // [ErrLoad].
 func New(opts ...Option) (*Generator, error) {
 	var c config
 	for _, opt := range opts {
 		opt(&c)
 	}
-	if err := c.missing(); err != nil {
+	if err := c.misuse(); err != nil {
 		return nil, err
 	}
-	if c.nilModules != nil {
-		return nil, c.nilModules
-	}
 	root, err := loadSources(c.sources)
 	if err != nil {
 		return nil, fmt.Errorf("fejkdata: %w", loadError{err})
@@ -128,14 +125,14 @@ func (f *Generator) List() []string {
 	return out
 }
 
-// missing names each option New requires that no option gave.
-func (c config) missing() error {
+// misuse names every mistake in the options, before any data loads.
+func (c config) misuse() error {
 	var err error
-	if !c.given {
+	if !c.dataGiven {
 		err = errNoData
 	}
 	if !c.seeded {
 		err = errors.Join(err, errNoSeed)
 	}
-	return err
+	return errors.Join(err, c.nilModules)
 }

inline.go

@@ -11,7 +11,7 @@ import (
 // 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
+// sequence is reproducible only when a generator — and its templates — are
 // drawn from one goroutine.
 type Template struct {
 	g *Generator