DCI in Rust
Matthew Browne
I have been learning a bit about Rust, and I was able to get a
pretty good DCI example working based on this
existing example:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=a007eb593e3e12f5b965a3db5775f691
Rust is an interesting language from an OO and DCI standpoint...
It has objects but doesn't have classes or inheritance. It has
"traits," which are different from the kind of traits I was
referring to in my previous thread—in Rust, traits are more like
interfaces, although they can optionally provide default method
implementations. The interesting thing for DCI is that trait
methods are only available on an object if that trait has been
imported into the current file or module—not quite as good as the
context automatically enforcing the privacy of role methods like
in trygve, but it avoids the need to inject or un-inject role
methods. I suppose it's similar to the C++ implementation in some
ways, including how role-object contracts are enforced at
compile-time.
Here are the improvements I made on the original example:
1. I moved the data methods (available_balance(),
decrease_balance(), and increase_balance()) to the data module
(they were previously defined within the context).
2. I'm still learning Rust and don't really know what I'm doing,
but I managed to add a macro so that the above methods would be
available both on the Account type and on the role-player traits.
This causes compiler warnings about the Account.available_balance,
Account.increase_balance, and Account.decrease_balance being
unused...I wonder if there is a better way to do it that doesn't
require the use of a macro. But the code executes just fine with
my current solution, and the available_balance, increase_balance,
and decrease_balance methods can be called from anywhere.
3. The role methods in the context are now private and can't be
accessed outside the context.
In summary, I think Rust is a promising language for DCI
programming, and BTW it compiles to WebAssembly too which means it
can run in the browser. Rust also runs on every platform that LLVM
supports (AKA most common platforms).
P.S. This is a good article about Rust's classless design and
some of its advantages:
https://www.thecodedmessage.com/posts/oop-3-inheritance/