Fake

A pure, composable Haskell faker library for generating fake data in tests. Built with the State monad for deterministic, reproducible test data generation.

Features

• Pure Functions: No global state or I/O effects in generators
• Deterministic: Seeded generation for reproducible tests
• Composable: Monadic combinators for building complex generators
• Type Safe: Concrete return types, no stringly-typed APIs
• Minimal Dependencies: Only standard Haskell libraries
• Easy to Use: Simple, intuitive API

Installation

cabal build
cabal test

Usage

Basic Generation

importFakeimportqualifiedFake.PersonasPersonimportqualifiedFake.InternetasInternet-- Deterministic generationlet name = runFakerSeed 42Person.fullName
let email = runFakerSeed 42Internet.email
-- Non-deterministic generation
randomName <- runFaker Person.fullName

Complex Objects

dataUser=User{name::String
, email::String
, age::Int}deriving (Show)
generateUser::FakeUser
generateUser =do
n <-Person.fullName
e <-Internet.email
a <- integerRange 1880return$User n e a
user <- runFaker generateUser

Using Combinators

-- Generate 5-10 emails
emails <- runFaker $ vectorOf (5, 10) Internet.email
-- Optional value (50% chance)
username <- runFaker $ maybeGen Internet.username
-- Pick one from list
email <- runFaker $ oneof [Internet.safeEmail, Internet.freeEmail]

Architecture

The library uses a State monad wrapping StdGen for pure, composable random generation:

newtypeFakea=Fake (StateStdGena)
deriving (Functor, Applicative, Monad)

Three execution modes:

• runFaker :: Fake a -> IO a - System random
• runFakerSeed :: Int -> Fake a -> a - Deterministic
• runFakerWith :: StdGen -> Fake a -> a - Custom RNG

Available Generators

Primitives

• Numbers: integer, integerRange, natural, float, double, bool
• Characters: char, alpha, numeric, alphanumeric
• Strings: string, stringBounded
• Selection: elements, shuffle

Combinators

• Repetition: vector, vectorOf, sequence
• Optionality: maybeGen, oneof
• Functional: filterGen

Person

• Names: firstName, lastName, fullName, name
• Titles: title, jobTitle, prefix, suffix
• Bio: bio

Internet

• Email: email, safeEmail, freeEmail
• Usernames: username, domain, domainSuffix
• URLs: url, slug

Address

• Geography: city, country, countryCode
• Postal: postalCode, zipCode
• Coordinates: latitude, longitude

Testing

importTest.HspecimportFakeimportqualifiedFake.PersonasPersonspec::Spec
spec = describe "User generation"$do
it "generates consistent names with same seed"$dolet name1 = runFakerSeed 123Person.fullName
let name2 = runFakerSeed 123Person.fullName
name1 `shouldBe` name2

Run tests:

cabal test