Rust is a systems programming language following fairly standard imperative approaches and a C-style syntax. Haskell is a purely functional programming language, innovating in areas such as type theory and effect management. Viewed that way, these languages are polar opposites.
And yet, these two languages attract many of the same people, including the engineering team at FP Complete. Putting on a different set of lenses, both languages provide powerful abstractions, enforce different kinds of correctness via static analysis in the compiler, and favor powerful features over quick adoption.
In this post, I want to look at some of the philosophical underpinnings that explain some of the similarities and differences in the languages. Some of these are inherent. Rust’s status as a systems programming language essentially requires some different approaches to Haskell’s purely functional nature. But some of these are not. It wasn’t strictly necessary for both languages to converge on similar systems for Algebraic Data Types (ADTs) and ad hoc polymorphism (via traits/type classes).
Keep in mind that in writing this post, I’m viewing it as a consumer of the languages, not a designer. The designers themselves may have different motivations than those I describe. It would certainly be interesting to see if others have different takes on this topic.
This is so obvious that I almost forgot to include it. If there’s one thing that defines Rust versus any other language, it’s ownership and the borrow checker. This speaks to two core pieces of Rust:
The concept of ownership achieves both of these. Many additions have been made to the language to make it easier to work with ownership overall. This hints at the concept of ergonomics, which is fundamental to Rust philosophy. But ownership and borrow checking are also known as the harder parts of the language. Putting it together, we see a philosophy of striving to meet our goals safely, while making the usage of the features as easy as possible. However, if there’s a conflict between the goals and ease of use, the goals win out.
All of this stands in stark contrast to Haskell, which is explicitly not a systems language, and does not attempt in any way to address those cases. Instead, it leverages garbage collection quite happily, with the trade-offs between performance and ease-of-use inherent in that choice.
The underlying goal of Haskell is ultimately to create a purely functional programming language. Many of the most notable and unusual features of Haskell directly derive from this goal, such as using monads to explicitly track effects.
Other parts of the language follow from this less directly. For example, Haskell strongly embraces Higher Order Functions, currying, and partial function application. This combination turns many common structures in other languages (like loops) into normal functions. But in order to make this feel natural, Haskell uses slightly odd (compared to other languages) syntax for function application.
And this gets into a more fundamental piece of philosophy. Haskell is willing to be quite dramatically different from other programming languages in its pursuit of its goals. In my opinion, Rust has been less willing to diverge from mainstream approaches, veering away only out of absolute necessity.
This results in a world where Haskell feels quite a bit more foreign to others, but has more freedom to innovate. Rust, on the other hand, has stuck to existing solutions when possible, such as eschewing monadic futures in favor of async/.await syntax.
I undervalued how important this feature was for a while, but recently I’ve realized that it’s one of the most important features in both languages for me.
I used to think that the reason I loved both Haskell and Rust so much was their shared strong typing, ADTs, and pattern matching combination.
After a recent discussion, I think it may be more about being expression-oriented languages.
— Michael Snoyman (@snoyberg) January 11, 2021
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