Finding the spirit of L-World

[John Rose]

On Feb 22, 2019, at 6:57 PM, John Rose <john.r.rose at oracle.com> wrote:
> 
> On Feb 22, 2019, at 11:42 AM, Kevin Bourrillion <kevinb at google.com> wrote:
>> 
>> I think we should make users write `equals` to test value types. If they write `==`, they are indicating a special situation where they need identity semantics, which don't make sense for value types, and that should be an error.
> 
> This sounds like a proposal for the future, but as Brian points
> out it is also a constraint on large amounts of generic code
> that has already been written.
> 
> Let's make the best of op==; it's in our past and the future
> of comparison logic in Java is too tightly coupled with the past.
> 
> — John

P.S. Also, in exactly-typed code, the substitutability test
will *often* be exactly what the user wants.  The default
implementation of ValObject.equals will use that test,
and users will surely opt to inherit that rather than write
a local equals method that does the same.

(Not always, of course; there are value types and object
types where the default equals method doesn't DTRT.
But often, for cases like small numerics and tuples.)

In that value class, requiring the user to call x.equals(y) instead
of writing the known exact equivalent x==y feels like somebody's
code style policy, rather than a useful discipline.