Crate contrafact

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.

Example

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 = fact.build(&mut 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.

Re-exports

pub use arbitrary;

Macros

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

Structs

BruteFact	A brute-force fact. Use brute() to construct.
Check	The result of a check operation, which contains an error message for every constraint which was not met.
LensFact	A fact which uses a lens to apply another fact. Use lens() to construct.
MappedFact	A fact which is mapped from the data to be checked/mutated. Use mapped to construct.
PrismFact	A fact which uses a prism to apply another fact. Use prism() to construct.

Traits

Fact	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)

Functions

always	A constraint which is always met
brute	A constraint defined only by a predicate closure. Mutation occurs by brute force, randomly trying values until one matches the constraint.
brute_fallible	A version of brute whose closure returns a Result
build_seq	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_seq	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.
consecutive_int	Specifies that a value should be increasing by 1 at every check/mutation
consecutive_int_	Specifies that a value should be increasing by 1 at every check/mutation, with no context given
eq	Specifies an equality constraint
eq_	Specifies an equality constraint with no context
in_iter	Specifies a membership constraint
in_iter_	Specifies a membership constraint
lens	Lifts a Fact about a subset of some data into a Fact about the superset.
mapped	A fact which is defined based on the data to which it is applied. It maps the data into a fact to be applied.
mapped_fallible	A version of mapped whose closure returns a Result
ne	Specifies an inequality constraint
ne_	Specifies an inequality constraint with no context
never	A constraint which is never met
not	Negates a fact
not_	Negates a fact, with no context given
or	Combines two constraints so that either one may be satisfied
prism	Lifts a Fact about some optional subset of data into a Fact about the superset.

Type Definitions

BoxFact	Type alias for a boxed Fact. Implements Fact itself.
Facts	Type alias for a Vec of boxed Facts. Implements Fact itself.
Result	The Result type returnable when using check_fallible!