[higherkinded]

It's "typeclass", not "interface". Though it works similar and is, in fact, just a way to define a constraint.

As of your described way of desugaring the contract, looks way too cluttered. It's exactly the reason why Reader and ReaderT exist in Haskell. Even though the proposal's contract syntax doesn't look very beautiful, it's still drastically better than keeping track of drilled structs of functions, the latter will become a mess really really fast, so having a way to conveniently constraint the accepted types while having a full-blown parametric polymorphism is essential for a modern language. Not sure exactly why anyone would be against that.

[lalaithion]

Sigh, I really need to read my posts before submitting. You're right, of course it's typeclass.

The whole point is to implement the bare bones of a parametric polymorphic system without constraining further development of the language. Additionally, it discourages complexity and requires explicitness, two goals of Go.

As far as my description of desugaring the contract, that is literally how Haskell implements typeclasses.

[higherkinded]

Well, yeah, Reader is that kind of thing.

I guess that the complexity people are concerned about is that once you implement a parametric polymorphism, HKTs are inevitable and that probably requires some brain flexing in order to get the meaning of a structure, which defeats the main purpose that is being explicit and simple. Pretty much a deadlock. I now understand why the community is so in odds with the parametric polymorphism as the concept but I'm also sure that simplicity of use and convenience of writing a robust solution is the two things that require it. You can't keep your head near the sand and avoid HKTs when you want flexibility and you can't allow that type of polymorphism whilst striving for what's Go trying to do.

While understanding the both standpoints, though, I still hope that Go will get parametric polymorphism as it's just so much more pleasant and convenient than carrying around a mess of structures to be explicit no matter the costs.