Taken from karlsruhe's blog.
Many languages get this confused or wrong.
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Equality checks whether two things are equal, based on some library-defined definition of equality.
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Identity checks whether two things are identical, based on a built-in definition of identity."
Every language has a way to compare things. There are two types of comparison though: referential equality and value equality.
== and @eq - checks whether two things are equal, based on some library-defined definition of equality.
=== and @is - checks whether two things are identical, based on a built-in definition of identity.
equality = language definition.
identity = user / library definition.
So by default a type doesn't have identity defined i.e.
only available when the type is constrained appropriately
@fn same[E : Identity](a: E, b: E) = { a === b }