Equality in Pony
Pony features two forms of equality: by structure and by identity.
Identity equality checks in Pony are done via the
is verifies that the two items are the same.
if None is None then // TRUE! // There is only 1 None so the identity is the same end let a = Foo("hi") let b = Foo("hi") if a is b then // NOPE. THIS IS FALSE end let c = a if a is c then // YUP! TRUE! end
Equality by structure check in Pony is done via
==. It verifies that two items have the same value. If the identity of the items being compared is the same then, by definition they have the same value. You can define how equality is defined on your object by implementing
fun eq(that: box->A): Bool
class Foo let _a: String new create(a: String) => _a = a fun eq(that: box->Foo): Bool => this._a == that._a actor Main new create(e: Env) => let a = Foo("hi") let b = Foo("bye") let c = Foo("hi") if a == b then // won't print e.out.print("1") end if a == c then // will print e.out.print("2") end if a is c then // won't print e.out.print("3") end
If you don't define your own
eq, you will inherit the default implementation that defines equal by value as being the same as by identity.
interface Equatable[A: Equatable[A] #read] fun eq(that: box->A): Bool => this is that fun ne(that: box->A): Bool => not eq(that)
Primitives and equality
As you might remember from Chapter 2, primitives are the same as classes except for two important differences:
- A primitive has no fields.
- There is only one instance of a user-defined primitive.
This means, that every primitive of a given type, is always structurally equal and equal based on identity. So, for example, None is always None.
if None is None then // this is always true end if None == None then // this is also always true end