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fakes

A Go library and CLI for generating fake data, built for internationalization. It exists because the existing Go fake libraries lacked the locale coverage and format control we needed.

Standards first — formats, names and structures follow international and local standards first and foremost.
Locale-aware — names, addresses, postal codes and phone numbers follow per-locale data and formats. The shipped data is organised by full locale tag (sv_SE), but the engine treats folders as plain namespaces — name yours anything.
Data lives in JSON — all source data is recursive JSON on disk, read when you create a faker and then served from memory. Add or change data without touching the library. Behavior belongs in data too: the engine grows a built-in function only for what data can't express (a checksum, a time-based id), never for what character classes and choices already do.
Composable — templates nest without limit: weighted choices, character classes and sub-templates combine to model any format.
Reproducible — seed a faker and it emits the same sequence every time, for a given version of the data: changing how a value is composed shifts the stream for that value and for everything drawn after it in the same faker. Every built-in draws only from that seed — no wall-clock, no crypto/rand — so determinism holds end to end.
Zero dependencies — standard library only.

CLI

Install the fakes command, then give it one or more -data-path directories and a path — it prints one value to stdout. Each dot segment descends one level: folders, then the category (a JSON file), then fields inside it.

go install github.com/Timewave-AB/fakes/cmd/fakes@latest

fakes -data-path ./data/sv_SE person               # Sara Eriksson
fakes -data-path ./data/sv_SE person.last          # Eriksson  (dotted path into a category)
fakes -data-path ./data sv_SE.person               # point at the tree; the folder is a segment
fakes -data-path ./data/sv_SE -data-path ./mydata word  # layer dirs; the last wins a name clash
fakes -seed 42 -data-path ./data/sv_SE address
fakes -repeat 3 -data-path ./data/sv_SE person             # three values, one per line
fakes -repeat 3 -separator ', ' -data-path ./data/sv_SE word  # nät, barn, sol
fakes -data-path ./data/sv_SE -list                        # every path this data offers

-data-path is repeatable (last wins a name clash) and the path comes last — all flags must precede it. -repeat N renders the path N times — each an independent draw — joined by -separator (default a newline, so values land one per line). Not sure what a data set offers? -list prints every path you can ask for; -version prints the build version.

Without installing, run it from a checkout with go run ./cmd/fakes …. Exit codes: 0 success (including -list, -version, -h), 1 runtime error (missing dir, unknown path), 2 misuse.

Generating a file from a custom template

A category is just a JSON file in a data directory, so you can drop in your own and render it — no code change. Save this as data/sv_SE/sql.json:

{
  "format": "INSERT INTO users V#ALUES({sql-username});",
  "sql-username": {
    "format": "'{username}'",
    "repeat": 3,
    "separator": "),(",
    "username": ["pixelfox", "snork", "turbohund", "blip", "zoom", "wahoo"]
  }
}

sql-username renders '{username}' repeat times and joins the results with the ),( separator; the outer V#ALUES(…) wraps that into one valid row list. (#A escapes the literal A, which a format string would otherwise read as a letter token — see Data format.)

fakes -seed 1 -data-path ./data/sv_SE sql
# INSERT INTO users VALUES('zoom'),('wahoo'),('blip');

Raise the template's repeat for more rows per statement; use the CLI's -repeat for more statements — together they build a whole seed file:

fakes -repeat 100 -data-path ./data/sv_SE sql > seed.sql

Library

go get github.com/Timewave-AB/fakes   # requires Go 1.22+ (for math/rand/v2)

Point New at one or more data directories, then generate values by path with Fake. Each dot segment descends one level: folders, then the category (a JSON file), then fields inside it.

package main

import (
	"fmt"
	"log"

	"github.com/Timewave-AB/fakes"
)

func main() {
	f, err := fakes.New([]string{"./data/sv_SE"})
	if err != nil {
		log.Fatal(err)
	}

	for _, path := range []string{"person", "address", "phone", "address.locality"} {
		v, err := f.Fake(path)
		if err != nil {
			log.Fatal(err)
		}
		fmt.Printf("%-18s %s\n", path, v)
	}
}

person             Sara Eriksson
address            Kungsvägen 68
                   379 17 Stockholm
phone              072-402 91 67
address.locality   Linköping

Seed a faker for reproducible output — same seed + locale yields an identical sequence, handy for stable tests:

a, _ := fakes.New([]string{"./data/sv_SE"}, fakes.WithSeed(42))
b, _ := fakes.New([]string{"./data/sv_SE"}, fakes.WithSeed(42))
av, _ := a.Fake("person")
bv, _ := b.Fake("person")
av == bv // true

f.List() returns the sorted paths the loaded data offers — the categories, their dotted fields and folder segments (what the CLI's -list prints). Every path it lists renders.

A *Fakes is not safe for concurrent use — create one per goroutine.

Data

The library ships a ready-to-use set under data/: one folder per locale (en_US, sv_SE) plus a locale-neutral misc folder. Point either tool at the whole tree, a single folder, a copy, or your own directory — anywhere on disk. A category or folder name must not contain a dot, and dot-prefixed entries are skipped, so a data directory can also be a checkout.

A directory is just a namespace. Each JSON file is a category named after the file; each subdirectory is a dot-path segment — folders nest exactly like JSON objects do. So data/sv_SE/person.json is Fake("person") when you point at data/sv_SE, or Fake("sv_SE.person") when you point at data.

Pass several directories and they merge, left to right: matching folders combine by their children, and any other clash is won by the last directory loaded. That lets you layer your own data over the built-ins without copying them:

fakes.New([]string{"./data/sv_SE", "./mydata"}) // mydata overrides on a clash

Each shipped locale carries these categories, formatted per locale (e.g. date is MM/DD/YYYY in en_US, YYYY-MM-DD in sv_SE; ssn is a US SSN vs a Swedish personnummer): address, color, company, date, email, ip, person, phone, price, sentence, ssn, time, url, username, version, word.

data/misc carries locale-neutral categories — universal data that isn't tied to a language or region:

ids & networking: uuid (a proper random v4), mac, objectid (24 hex chars, MongoDB ObjectID-shaped — the leading bytes are random, not a real timestamp), creditcard (per-network numbers ending in a valid {luhn()} digit)
reference codes: currency (ISO 4217), country (ISO 3166), language (ISO 639), timezone (IANA)
web & systems: mimetype, httpstatus, useragent
misc: coordinate (a lat/long point), emoji, car

Many carry dotted sub-fields — currency.symbol, country.alpha2, mimetype.ext, httpstatus.code, car.maker. A time-ordered v7 UUID can't be expressed as data, so it's the {uuid()} builtin instead (see Functions).

Data format

Each JSON file in a data directory is a category named after the file (address.json → address), rendered by Fake("address"). Drop in a new file or folder — no code change, no recompile.

Every value is a node, one of three shapes, nestable without limit:

Node	JSON	Meaning
literal	`"Malmö"`	emitted verbatim — never formatted
choice	`["a", "b", …]`	one element, picked at random
template	`{"format": "…", …}`	a format string plus the named sub-nodes it references

Weight. A template node may carry a weight (default 1) to skew its odds within a choice:

[
  { "format": "#070-000 00 00", "weight": 10 },
  { "format": "#01-000 00 00" },
  { "format": "#010-000 00 00" }
]

Only template (object) nodes carry weight — a bare string or nested array in a choice always counts as 1. Weights are checked when you create the faker: a negative, non-numeric, or all-zero set is rejected at New, so a typo fails fast instead of silently skewing output.

Repeat. A template node may carry a repeat (default 1) to render its format that many times — each render an independent pick — joined by separator (default ""):

{ "format": "{word}", "repeat": 3, "separator": " ", "word": ["foo", "bar", "baz"] }

This yields e.g. bar foo baz. repeat must be a positive integer and separator a string, both checked at New.

format, weight, repeat and separator are the only options; any other key is a field. So write seperator and you get a field by that name while the 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 containing a dot: a category or folder no dot path could reach, or a field whose dot a token would read as a path (see Correlated fields).

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 in the current format (non-digits skipped but kept); unknown functions or wrong argument counts are rejected at New. This is what makes a generated Swedish personnummer valid — its last digit is a Luhn checksum over the nine before it:

{ "format": "00{mmdd}-000{luhn()}", "mmdd": [ … ] }

renders e.g. 811218-987, then {luhn()} appends 6 → 811218-9876. Place it after its payload (it reads what is to its left). The buffer it reads is per-expansion, so nesting keeps fixed parts out of the sum — e.g. a 12-digit form prefixes the century outside the checksummed core:

{ "format": "{century}{core}", "century": ["19", "20"],
  "core": { "format": "00{mmdd}-000{luhn()}", "mmdd": [ … ] } }

A function must be deterministic (no wall-clock), so a seeded faker stays reproducible. A time-based id (UUID v7, ULID) therefore draws its timestamp from the rng, not the clock — the result is a valid, reproducible value, not a real point in time.

There are four kinds. Derivations read the digits emitted so far, so put them after their payload; generators read only the rng, so they stand alone; one session counter (seq) advances state held on the faker; and one computation (calc) evaluates arithmetic over sibling fields. Arguments are validated at New (a bad count, range, country, or expression fails fast); a length, count or decimal place beyond a sane maximum is rejected there too, so a fat-fingered hex(2000000000) can't try to allocate gigabytes at render.

Function	Kind	Emits
`{luhn()}`	derivation	Luhn check digit (mod-10) over preceding digits
`{mod11()}`	derivation	weighted mod-11 check char (weights 2–7 from the right); `X` when it would be 10
`{ean()}`	derivation	EAN-13 / UPC-A / ISBN-13 / GTIN check digit
`{uuid()}`	generator	UUID v7 (v4 ships as data — see Data)
`{ulid()}`	generator	ULID, 26-char Crockford base32
`{nanoid(n)}`	generator	URL-safe Nano ID, `n` chars
`{hex(n)}`	generator	`n` lowercase hex digits
`{base64(n)}`	generator	`n` random bytes, base64
`{int(min,max)}`	generator	uniform integer in `[min, max]`
`{float(min,max,dp)}`	generator	number in `[min, max]` with `dp` decimals
`{iban(CC)}`	generator	a length- and mod-97-valid IBAN for country `CC` (BE, DE, DK, ES, FI, NO, SE)
`{seq()}`, `{seq(name)}`	session counter	next integer (from 1) in this faker's sequence; `name` selects an independent counter
`{calc(expr)}`, `{calc(expr,dp)}`	computation	value of an arithmetic expression over number literals and sibling fields; `dp` rounds

{ean()} is also the ISBN-13 check (an ISBN-13 is an EAN-13 — build the 978/979 prefix in data and call {ean()}). {iban()} is a generator, not a derivation: an IBAN's check digits sit before the account number, which a left-to-right reader can't reach, so it emits the whole value (a generic numeric BBAN — valid length and checksum, not real bank routing).

{seq()}'s counter lives on the faker, so it spans Fake calls (and repeat) and resets when you build a new faker — seq is reproducible by being ordered, not random. It's the natural fit for a primary-key column in the SQL example above.

Computation. {calc(expr)} evaluates an arithmetic expression — + - * /, parentheses and unary minus, the usual precedence — and emits the result. Operands are number literals and sibling field names, each rendered then read as a number; an optional second arg rounds to that many decimals ({calc(net * qty, 2)}), otherwise the value prints in minimal form. A hyphen is always subtraction, so a hyphenated field name can't be an operand. The expression is checked at New (parse, and that every name is a real field):

{ "format": "{net} x {qty} = {calc(net * qty, 2)}", "net": ["19.99"], "qty": ["3"] }

renders 19.99 x 3 = 59.97. Each name is rendered where it appears, so a field shown and used in a calc is drawn twice — keep a shared operand in a single-value field if the two must agree. A field that doesn't render to a number yields NaN, and a division by zero yields Inf; both print rather than failing the render.

References. A {..path} token renders a node from the data root instead of a sibling field — the dot path is the one Fake takes, resolved across every loaded directory. One category can borrow another, even across folders or layered data dirs:

{ "format": "Hej, {..en_US.person}!" }

renders e.g. Hej, Pat Smith!. References are bound when you create the faker, so a path that is unknown, names a folder, or steps through a multi-variant choice fails at New. A reference that leads back to its own value (directly, mutually, or through a chain) is a cycle that would never finish rendering, so it too is rejected at New.

Correlated fields. A {name.field} token addresses a path into a sibling, and a sibling addressed that way is drawn once per expansion — so several tokens read one row. That is how two facts that belong together, such as a locality and the postal code that really covers it, stay together:

{ "format": "{street} {number}\n{place.postal-code} {place.locality}",
  "place": [
    { "format": "{locality}", "locality": "Stockholm", "postal-code": { "format": "#100 00" }, "weight": 975 },
    { "format": "{locality}", "locality": "Tranås",    "postal-code": { "format": "#5#7#3 00" }, "weight": 18 }
  ] }

renders e.g. Kungsgatan 35 / 176 99 Stockholm, never a Stockholm postal code beside Tranås. Each row carries its own weight, so how often a place appears is data too. The rule holds both ways: two tails of one head come from the same row, and one path read twice reads one value ({p.first} … {p.first}@… gives one name).

One draw, one spelling. A format may not both render a level and read a path into it — {p} beside {p.first}, or {place} beside {place.locality}. Rendering a level expands it afresh while a path reads the level's held draw, so the two would disagree; naming the fields you want is the one spelling that always agrees, and the other is a load error. This covers every way a level can be rendered: a token, a {calc()} operand, and a {..path} reference — wherever the reference sits, including in a field the format renders.

token {p} renders a level that {p.first} reads a path into; name the fields you want instead

For the same reason a path may not read into a level carrying a repeat: it reads one draw, so the repeat could never apply.

A path is held at every level it passes through, not just the first, so the facts can nest as deeply as they belong:

{ "format": "{p.geo.town.name} {p.geo.town.zip}, {p.geo.region}", "p": [ … ] }

renders Kiruna 98100, Norrbotten — the town, the zip that covers it and the region it sits in all come from one draw. Two paths part company exactly where they diverge: {p.a.v} and {p.b.v} share the row and nothing below it.

The binding lasts for one expansion, so each repeat iteration draws again and a nested template keeps its own. A field no dotted token addresses is unaffected: {word} {word} still draws twice.

New checks a path the way Fake resolves one: every variant of a multi-variant choice must carry the whole path, so a row missing a field is named at load:

token {place.postal-code}: field "place": not every variant of this 2-way choice
carries "postal-code"; all carry [locality]

The sub-fields stay addressable on their own — Fake("address.place.locality") renders, and List advertises it.

Format string. Every character is literal except:

Token	Expands to
`0`	digit 0–9
`1`	digit 1–9
`A`	letter A–Z
`a`	letter a–z
`#`	escape — the next char is literal (`#0` → `0`, `##` → `#`)
`{name}`	render the sibling field `name`
`{name.field}`	render `field` of one draw of the sibling `name` (see Correlated fields)
`{name()}`	call a built-in function (see Functions)
`{..path}`	render the node at a dot path from the data root (see References)

{a|b} renders one of the sibling fields a or b, chosen at random; an arm may be a {..path} reference too ({name|..en_US.person}).

Inside a format, 0 1 A a are always character classes — so a fixed 0, 1, A or a must be escaped (#1, #A) or it becomes random. A format of 100 Main St renders e.g. 506 Mdin St — the 1, 0, 0 and a were random, the M, in and St were not. A half-fixed string is the trap: 555-0000 keeps 555- and randomises the last four digits. For a value with no tokens at all, use a bare string node ("100 Main St"), emitted verbatim.

Putting it together (person.json):

[
  {
    "format": "{prefix}{femalefirst|malefirst} {last}",
    "femalefirst": ["Anna", "Astrid", "Elin"],
    "malefirst": ["Anders", "Erik", "Gustav"],
    "last": [
      { "format": "{first}sson", "first": ["Ander", "Erik", "Karl"] },
      ["Berg", "von Flemming"]
    ],
    "prefix": [
      "",
      { "format": "{string} ", "string": ["dr", "prof"], "weight": 0.05 }
    ]
  }
]

This yields e.g. Anna Eriksson, Erik Berg, or rarely dr Astrid von Flemming. Any field is reachable by dotted path — Fake("person.last") renders just a surname; choices along the path are resolved at random. A path may continue through a choice only where every variant carries the rest of it — every currency variant carries symbol, so currency.symbol resolves — which keeps a path from rendering on one call and failing on the next.

Performance

Each file is parsed, validated and weight-indexed once, in New. After that a Fake call costs about what its output costs — it scans the chosen format and renders nested tokens, independent of how large your lists are:

Picking from a list is O(1) whatever its length — a 10-name list and a 100 000-name list cost the same.
Giving entries a weight makes that list's pick O(log n) instead (a search over cumulative weights). Still tiny, but an unweighted list is the cheapest — only add weight where you actually want skew.
Long format strings, deep nesting and many {tokens} add cost in proportion to the output produced.

Development

Everything runs in Docker — no local tooling beyond Docker is needed. Source is bind-mounted; build caches persist in the gocache volume.

docker compose run --rm test    # run tests
docker compose run --rm ci      # vet + format check + tests
docker compose run --rm cover   # tests with coverage
docker compose run --rm bench   # benchmarks
docker compose run --rm build   # compile the library
docker compose run --rm vet     # go vet
docker compose run --rm dev     # interactive shell

Commands that rewrite source keep your file ownership when run with --user:

docker compose run --rm --user "$(id -u):$(id -g)" fmt   # gofmt -w .
docker compose run --rm --user "$(id -u):$(id -g)" tidy  # go mod tidy

Every pull request runs docker compose run --rm ci against both the latest and the lowest supported Go, and must pass before it can be merged. docker build . runs the same vet and tests locally.

Tests run against the latest Go by default. Set GO_VERSION to check the lowest supported version too:

GO_VERSION=1.22.12 docker compose run --rm test   # lowest supported
docker compose run --rm test                      # latest

Layout

fakes.go        Fakes, New, List, options, seeding
node.go         the node model and JSON -> node compilation
render.go       Fake and the recursive renderer (choices, format strings, paths, bound draws)
template.go     the {token} grammar: scanning, function and path tokens, validation
reference.go    {..path} binding across the tree, and cycle detection
builtins.go     the {name()} function registry and its implementations
calc.go         the {calc()} arithmetic evaluator: parser, eval, validation
data.go         data loading: folders/files -> namespace tree, multi-path merge
cmd/fakes/      the `fakes` CLI (New + Fake/List over stdout)
data/           shipped data (JSON): locale folders + a misc folder

To add a category, drop a JSON file into a data directory; to add a locale, add a subdirectory of JSON files.

License

MIT — see LICENSE.

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