The comment I originally replied to linked to a blog post arguing that in Haskell, the practice of wrapping type classes (which Rust calls traits) in an existen

The comment I originally replied to linked to a blog post arguing that in Haskell, the practice of wrapping type classes (which Rust calls traits) in an existential is an antipattern, because rather than using fancy type system features, you could just use a record with some functions in it, which is simpler and more direct. But dyn impl is a trait wrapped in an existential. There's no concrete type a -> b, only a trait for types that can be 'called.' So to pass an arbitrary function around in a record/struct you need to do the very thing the post is saying is an antipattern (in Haskell). So the argument doesn't really apply to Rust because there isn't a simpler more direct way to achieve the same thing.