pub trait Arbitrary: Clone + 'static {
// Required method
fn arbitrary(g: &mut Gen) -> Self;
// Provided method
fn shrink(&self) -> Box<dyn Iterator<Item = Self>> { ... }
}Expand description
Arbitrary describes types whose values can be randomly generated and
shrunk.
Aside from shrinking, Arbitrary is different from typical RNGs in that
it respects Gen::size() for controlling how much memory a particular
value uses, for practical purposes. For example, Vec::arbitrary()
respects Gen::size() to decide the maximum len() of the vector.
This behavior is necessary due to practical speed and size limitations.
Conversely, i32::arbitrary() ignores size() since all i32 values
require O(1) memory and operations between i32s require O(1) time
(with the exception of exponentiation).
Additionally, all types that implement Arbitrary must also implement
Clone.
Required Methods§
sourcefn arbitrary(g: &mut Gen) -> Self
fn arbitrary(g: &mut Gen) -> Self
Return an arbitrary value.
Implementations should respect Gen::size() when decisions about how
big a particular value should be. Implementations should generally
defer to other Arbitrary implementations to generate other random
values when necessary. The Gen type also offers a few RNG helper
routines.
Provided Methods§
sourcefn shrink(&self) -> Box<dyn Iterator<Item = Self>>
fn shrink(&self) -> Box<dyn Iterator<Item = Self>>
Return an iterator of values that are smaller than itself.
The way in which a value is “smaller” is implementation defined. In
some cases, the interpretation is obvious: shrinking an integer should
produce integers smaller than itself. Others are more complex, for
example, shrinking a Vec should both shrink its size and shrink its
component values.
The iterator returned should be bounded to some reasonable size.
It is always correct to return an empty iterator, and indeed, this is the default implementation. The downside of this approach is that witnesses to failures in properties will be more inscrutable.