FStar

hich are structurally typed can demonstrate functional extensionality and (https://github.com/FStarLang/FStar/wiki/SMT-Equality-and-Ext...) and check whether you've flipped the arguments, which would be a more valid criticism (but such checks are expensive and conflict with compile time requirements). Also Java has the same problem, if you define two interfaces with the same method signature and a single class implements both you can't disambiguate. Thirdly, the structural typing has a huge advantage, namely looser coupling and more tightly defined interfaces. If you follow the "accept interfaces return structs" go idiom, you'll see why. An open source library that does so leaves their returned structs open to be used by consumer code, that itself uses interfaces, without modification required. This means most go code has small, tightly defined interfaces, where every function on the interface is invoked in the relevant function. For example if you have a library with this definition: type Baz struct {} func (b Baz) Foo(){} func (b Baz) Bar(){} I can use Baz in my code like so: type Fooer interface { Foo() } func DoSomething(f Fooer) { } And use the underlying library, while being decoupled from it, without having to modify it. Fourthly: You can explicitly say a type implements an interface... * A good study: https://blog.boot.dev/golang/golang-interfaces/ * In Idris we would do: even : Nat -> Bool even Z = True even (S k) = odd k where odd Z = False odd (S k) = even k int -> even doubler a = 2 * a