I'm building a new language and just a couple of days ago the concept of guard methods came up as I was trying to tighten up equality semantics to be more like Swift.
Things like Array.contains() only work if the element type implements the Equatable interface, so it would be a guard method. Maybe something like:
class Array<T> {
contains(value: T): boolean where T extends Equatable { ... }
}
Or possibly a constraint on the `this` type, TypeScript style:
Nice article, but I don’t fully understand why the author argues for such an aversion to static functions.
> Moreover, it breaks the systematic approach of sending messages to an instance (often presented as one of the key arguments in favor of object-oriented programming).
Is this just a matter of “the code will become spaghetti once too many classes/methods/implementations exist”? And, conversely, is a non-static method with a constrained, i.e. somehow different type (or also the approach of moving a method outside of the class proper) not more confusing?
> Although guarded methods seem necessary, unfortunately, I don’t know of any mainstream languages that allow their definition.
C++ has this feature. When you define a template class, you can use SFINAE (or, in modern C++, concepts) to make it so that certain methods only exist if the template parameter meets certain requirements.
(Though even this is often unnecessary - for something like your `flatten` example, you could just go ahead and define it unrestricted. Methods of template classes are typechecked lazily - in other words, they don't need to successfully typecheck unless they're called. For this specific use case, SFINAE/concepts would just make the error message nicer.)
Rust also has this feature, with conditional impls.
The general term for this is an indexed type. Specifically what is shown here is indexed codata. Indexed data is more commonly known as generalized algebraic data types (GADTs), which encode equality constraints at construction.
GADTs are well established in FP, but codata in general is not[^1]. As far as I can tell this is partly historical and partly cultural. (Codata is basically OO. Recent FP languages, like Idris, have explicit support for codata.)
Guarded Methods in OCaml (2025)
(xvw.lol)84 points by birdculture 3 August 2026 | 7 comments
Comments
I'm building a new language and just a couple of days ago the concept of guard methods came up as I was trying to tighten up equality semantics to be more like Swift.
Things like Array.contains() only work if the element type implements the Equatable interface, so it would be a guard method. Maybe something like:
Or possibly a constraint on the `this` type, TypeScript style: https://github.com/elematic/zena/blob/8d77f2b36001078f4d5054...> Moreover, it breaks the systematic approach of sending messages to an instance (often presented as one of the key arguments in favor of object-oriented programming).
Is this just a matter of “the code will become spaghetti once too many classes/methods/implementations exist”? And, conversely, is a non-static method with a constrained, i.e. somehow different type (or also the approach of moving a method outside of the class proper) not more confusing?
C++ has this feature. When you define a template class, you can use SFINAE (or, in modern C++, concepts) to make it so that certain methods only exist if the template parameter meets certain requirements.
(Though even this is often unnecessary - for something like your `flatten` example, you could just go ahead and define it unrestricted. Methods of template classes are typechecked lazily - in other words, they don't need to successfully typecheck unless they're called. For this specific use case, SFINAE/concepts would just make the error message nicer.)
Rust also has this feature, with conditional impls.
GADTs are well established in FP, but codata in general is not[^1]. As far as I can tell this is partly historical and partly cultural. (Codata is basically OO. Recent FP languages, like Idris, have explicit support for codata.)
More in my book: https://functionalprogrammingstrategies.com/
^[1]: The degenerate form of codata is the function, which FP languages do tend to support very well.