Hashable trait (importable from std::hash) is used to transform an object instance into a u64. The interest is to easily compare objects, for example in std::collection::map::HashMap, or std::collection::set::HashSet. Hashable classes can be used in these collections as key. A default behavior is defined in this trait, but the method hash can be redefined, it takes a immutable object instance as parameter, and return a u64 value.

import std::io;
import std::collection::map;
import std::hash;

class Foo {
    let _v : [c8];

    pub self (v : [c8]) with _v = v {}
    
    impl Hashable, Streamable;
}


def main () {
    let dmut coll = HashMap!{&Foo, i32}::new ();
    coll:.insert (Foo::new ("X"s8), 12);
    coll:.insert (Foo::new ("Y"s8), 34);
    
    println (coll);	
    println (Foo::new ("X"s8) in coll);
}

Results:

{main::Foo(X)=>12, main::Foo(Y)=>34}
true