- No trait based dispatch (
Arbitrary). Generators and shrinkers are straightforward coroutines instead
- If two tests need a different shape of the tested data type, just combine generators differently. No newtype wrappers necessary.
- No macros. Test cases are composed of function calls, ifs and loops
- Linear, top to bottom test code
- Only implement shrinker when it actually provides value, i.e. a large enough falsifier is found and it needs to be shrunk
- Panics are caught and treated as test failures subject to shrinking. An
.unwrap() in tested code is also a violation of tested properties
- RNG seed value is printed on stderr. Use
cargo test -- ... --nocapture to grab it
- RNG seed value is taken from
FALSIFY_SEED environment variable if set. It's used for test failure reproduction
- Need a custom generator that produces only a subset of possible values of a given type? Just copy paste
the generic generator code and specialize for your requirements. There's no "You wanted a banana but what
you got was a gorilla holding the banana and the entire jungle."
Guidelines for implementing generators/shrinkers
arb_*() coroutines should generate random values indefinitely and never return. Even if some data type
is small enough to generate all values exhaustively. The coroutine may be used as a building block in
testing a more complex data type. Terminating, then, would result in terminating the "big" generator
prematurely.- In contract to
arb_*(), shrink_*() coroutines should finish once they can't shrink further. It allows
the caller to attempt a different shrinking strategy if the falsifier is still unmanageably large.