My problem with ad-polymorphism is primarily that it encourages more generic-ness in authoring code, which I am aware is often lauded as a positive characteristic of a given program. Taken to the extreme, there are whole communities of developers that pride themselves of writing the most generic code possible. I hold the opinion that all code should be clearly specified and particularized to processing the actual data required by the specification of the problem domain. I am certainly on the extreme end of the spectrum of generic to highly specified, but I genuinely feel that better code and the resulting programs come from a low level and data specific design ethos.

Additionally, and more relevant to Programming Language Theory, the entanglement between ad-hoc polymorphism implementations and the detention of a programming language are a huge mistake from my perspective. This is where Haskell becomes relevant, because although there are some differences (implicit self, orphan issues, etc) between Rust traits and Haskell’s typeclasses, they are an extremely similar mechanism for achieving ad-hoc polymorphism. At this point in its development, and due to the pervasiveness of usage throughout the stdlib, I see it as almost certain that Rust will attempt to kludge traits into a formal definition of its type theory and language definition making any attempts at coexisting with other language in a single application more difficult. Comparably, in Haskell, typeclasses are merely syntactic sugar allowing for a better developer experience using ad-hoc polymorphism. Specifically, Haskell, the language as defined, achieves ad-hoc polymorphism by passing a dictionary parameter to functions using overloaded function names. This is done using the standard terms in Haskell and has a clear denotaional semantics for such terms. Rust usage of traits is talked about and reasoned about as a more primitive part of the language, not just a pretty piece of sugar with a more basic existence in the formally defined core of a programming language.

If ad-hoc polymorphism is something a language designer wants to incorporate into the usage of a language, my position is that defining a typeclass style sugar over a clearly defined set of terms in the core, formally defined language would be at least a method of prohibiting issues with the resolution, definition, and various methods of implementation of ad-hoc polymorphism polluting the syntax and semantics of the language itself. But the above requires a firm definitional boundary between what is the language and what is syntactic sugar to be built into the definition of the language and built into the compiler infrastructure.

A more out of band way to achieve ad-hoc polymorphism would be to have a pre-processor that is a part of the language distribution and maintained by the language designer/org that does the resolution/solving of the ad-hoc polymorphism and then presents the compiler with source text with no overloading.

There are also type theoretic solutions to ad-hoc polymorphism l, but that’s a little outside the scope (even if it my personal solution to having and limiting ad-hoc polymorphism in a language).