Crate contrafact[][src]

Expand description

A trait for highly composable constraints (“facts”) which can be used both to verify data and to generate arbitrary data satisfying those constraints.

This crate is mainly intended for use in writing tests, and in particular for generating meaningful fixture data. By defining composable, reusable constraints, they can be mixed and matched to handle the specific use cases of your tests. By defining what you need from a fixture rather than simply writing the fixture you need, the hope is that you save yourself duplicated effort over time.


The following example defines a simple struct S with two fields, and a simple Fact (constraint) about S which says that S::x must always equal 1. This Fact, like all Facts, can be used both to verify that an instance of S meets the constraint, or to generate new instances of S which meet the constraint.

use contrafact::{Fact, eq, lens};
use arbitrary::{Arbitrary, Unstructured};

#[derive(Debug, Clone, PartialEq, Arbitrary)]
struct S {
    x: u32,
    y: u32,

let mut fact = lens("S::x", |s: &mut S| &mut s.x, eq("must be 1", &1));

assert!(fact.check(&S {x: 1, y: 333}).is_ok());
assert!(fact.check(&S {x: 2, y: 333}).is_err());

let mut u = Unstructured::new(&[0; 9999]);  // NB: don't actually construct Unstructured this way!
let a = u);
assert_eq!(a.x, 1);

Things to know

The above example composes together existing Facts provided by this crate. You can also define your own facts by hand by implementing the Fact trait. TODO: example of this.

contrafact leans heavily on the arbitrary crate for generating arbitrary data. Get to know this library, because you will need to implement Arbitrary for any type you wish to write a Fact about.

Facts can be used to check if a constraint is matched via Fact::check() or check_seq, and also to build new values via Fact::build and build_seq. Building values requires the use of arbitrary::Unstructured.

Facts can also be stateful, such that the constraint changes while checking or building a sequence. TODO: example of stateful fact.

Facts can be easily “horizontally” composed together through the facts! macro, which boxes each Fact and lumps them together as trait objects, applying each fact one after the other.

Facts can be “vertically” composed together through the lens and prism combinators, which allow you to lift a Fact about one type into a Fact about another type.

See the Functions documentation for more examples and detailed instructions about each Fact defined by this crate.


pub use arbitrary;



Helper macro to run a check which may produce a Result, mapping any Err into a normal Check error string.


Convenience macro for creating a collection of Facts of different types. Each Fact will be boxed and added to a Vec as a trait object, with their types erased. The resulting value also implements Fact.



A brute-force fact. Use brute() to construct.


The result of a check operation, which contains an error message for every constraint which was not met.


A fact which uses a lens to apply another fact. Use lens() to construct.


A fact which is mapped from the data to be checked/mutated. Use mapped to construct.


A fact which uses a prism to apply another fact. Use prism() to construct.



A declarative representation of a constraint on some data, which can be used to both make an assertion (check) or to mold some arbitrary existing data into a shape which passes that same assertion (mutate)



A constraint which is always met


A constraint defined only by a predicate closure. Mutation occurs by brute force, randomly trying values until one matches the constraint.


A version of brute whose closure returns a Result


Build a sequence from scratch such that all Facts are satisfied. Each Fact will run Fact::advance after each item built, allowing stateful facts to change as the sequence advances.


Check that all of the constraints of all Facts are satisfied for this sequence. Each Fact will run Fact::advance after each item checked, allowing stateful facts to change as the sequence advances.


Specifies that a value should be increasing by 1 at every check/mutation


Specifies that a value should be increasing by 1 at every check/mutation, with no context given


Specifies an equality constraint


Specifies an equality constraint with no context


Specifies a membership constraint


Specifies a membership constraint


Lifts a Fact about a subset of some data into a Fact about the superset.


A fact which is defined based on the data to which it is applied. It maps the data into a fact to be applied.


A version of mapped whose closure returns a Result


Specifies an inequality constraint


Specifies an inequality constraint with no context


A constraint which is never met


Negates a fact


Negates a fact, with no context given


Combines two constraints so that either one may be satisfied


Lifts a Fact about some optional subset of data into a Fact about the superset.

Type Definitions


Type alias for a boxed Fact. Implements Fact itself.


Type alias for a Vec of boxed Facts. Implements Fact itself.


The Result type returnable when using check_fallible!