[muraiki]

"To me this is one methodological weakness of type theory, the commitment of having types for all terms in the language. Why is that? Types are designed to facilitate composition, but the natural unit for program composition is not the term but the function, or even the module. It makes sense to assign types to modules — it is methodologically consistent. But how we calculate these types could conceivably be more flexible, allowing a global perspective within the unit of composition. Types, as properties at the interface could be calculated using the entire arsenal of (decidable!) logics and static analyses."

I admit to not being that familiar with type theory (I've done a little Scala but most of my FP experience is in Clojure) but when I read this my gut reaction was: "Isn't that kind of what Go is doing?"

Meaning that in Go, the focus of compositionality isn't on basic types and the functions that act on them (see Go's lack of generics) but rather the focus is on getting different modules to agree on "what should be done to some stuff" using interfaces. This is putting interaction first but also getting compositionality in a "generic" way given that interfaces in Go are implemented implicitly.

[Dewie3]

Yeah, Go has interfaces. Just like any sane, decently modern statically typed language. And it uses it to declare interfaces between things. A great concept, like the idea behind abstract data types -- defining data types by what operations can be performed on them.

But what is so special about Go's approach to interfaces? The only special thing I see is that interfaces are implemented implicitly, which I see more as a "implicit over explicit" decision rather than anything having to do with composition per se. Yet people keep raving about it.

[muraiki]

Well, I think that perhaps what makes Go different is not only interfaces, but interfaces as part of a larger package -- meaning the "not quite OO" approach it brings, in contrast to a language like Java's interfaces and general design philosophy.