This library provides property based testing with support
for internal integrated shrinking: integrated in the sense
of Hedgehog, meaning that there is no need to write a
separate shrinker and generator; and internal in the sense
of Hypothesis, meaning that this works well even across
monadic bind. However, the actual techniques that power
falsify are quite different from both of these two
libraries.
Most users will probably want to use the integration with
tasty,
and use Test.Tasty.Falsify from the tasty-falsify
package as their main entrypoint into the library.
The Test.Falsify.Interactive module can be used to
experiment with the library in ghci.
Changes
Revision history for falsify
0.4.0 – 2026-07-01
This release is a major cleanup release, and backwards incompatible with 0.3 in
various ways. Below we provide a list of changes as well as a migration guide.
New features
Test.Falsify.Interactive now offers a pure function sampleUsing (#65),
using ReplaySeed to initialize the PRNG. ReplaySeed is no longer opaque.
Property now offers getContext, which allows properties to vary their
behaviour across different test iterations; for example, tests might want to
start with generating values from small ranges and then slowly increase that
range. There is also a derived function sized for that specific application.
(#102, together with Peter Lebbing and Martijn Bastiaan from QBayLogic)
Package split: falsify vs tasty-falsify
The main falsify package no longer provides tasty integration (indeed,
it does not depend on tasty at all anymore); instead, tasty integration
is provided by a new tasty-falsify package (#80).
Module Test.Tasty.Falsify now lives in tasty-falsify, and it only
provides the functionality required for the use of falsify with tasty; it
no longer re-exports anything from falsify.
Test.Falsify.Driver now provides the “official” API for use in driver
integration.
Module hierarchy
The module hierarchy has been improved.
There is a new top-level module Test.Falsify, which is intended to provide
all definitions that are meant for unqualified import. A typical falsify
test suite will therefore start with
import Test.Falsify
import qualified Test.Falsify.Generator as Gen
import qualified Test.Falsify.Predicate as P
import qualified Test.Falsify.Range as Range
Test.Falsify.Generator now really only provides generators
(it was previously an awkward mix of generators and custom datatypes)
Data.Falsify.* is a new public module hierarchy providing some general
purpose data structures and utilities:
Data.Falsify.Concrete
Data.Falsify.Permutation
Data.Falsify.ProperFraction
Data.Falsify.Tree
Data.Falsify.WordN
Some testing specific data structures now have dedicated modules:
Test.Falsify.Fun
Test.Falsify.Marked
Test.Falsify.SampleTree
Test.Falsify.ShrinkTree
The internal (private) module hierarchy has been simplified as well; the
Reexported.* module hierarchy is no longer used (the distinction had lost its
purpose).
Newtypes
A number of type aliases have been replaced by newtypes.
In Test.Falsify.Predicate, VarName is now a newtype rather than a type
alias, and we now also have FnName alongside VarName. Both of these have
IsString instances, so code that uses OverloadedStrings should not be
affected
ShrinkTree is now a newtype rather than a type alias, and is moved to
Test.Falsify.ShrinkTree
Other changes
The signature of path has been simplified: it no longer uses IsValidShrink
(which is now internal API)
Simplified signature of bst, which now only accepts inclusive bounds (#91)
Remove deprecated functions integral and enum
Rename Test.Falsify.Range.between to uniform (#92).
This avoid confusion with Test.Falsify.Predicate.between (since the values
produced by the former do not necessarily satisfy the latter!). Also improved
documentation.
Predicates alwaysPass and alwaysFail have been renamed to pass and
fail respectively, and fail now takes an error messages. The new names
are a more natural fit when used together with lam to construct predicates
of arbitrary arity. To consider the old alwaysFail, use Fail "Fail".
Predicate documentation has been significantly improved.
Dropped support for GHC < 8.10.7
0.3.0 – 2026-03-05
Introduce new Range constructor called between, which can be used for
better uniform selection of large bit-size Integral types.
[#81, reported by Andrea Vezzosi]
Support generating functions from empty types [#84, Sjoerd Visscher]
Add minimalValue function [#86, Sjoerd Visscher]
Fix overflow in Fun Int8 [#89, reported by Jake McArthur]
The primitive Range constructor is now based on WordN rather than
ProperFraction. Most users will not notice this difference, but the
signature of the primitive eval function has changed.
Relax package bounds and test with ghc 9.14.1
0.2.0 – 2023-11-08
Avoid use of Expr in at (#48)
Add oneof (#54; Simon Kohlmeyer)
Generalize Range, so that it can be used for types like Char (#51).
As a consequence, Gen.integral and Gen.enum are now deprecated, and
superseded by Gen.inRange.
Add GenDefault class and DerivingVia helpers to derive generators
(Eric Conlon; #61, #64).
0.1.1 – 2023-04-07
Better verbose mode for test failures
New predicates: split and pairwise.
Shrink towards the second half of the range in withOrigin
