In practice this doesn't seem to work. In theory, Go should have a Result interface and two types Data and Error that implement it, and then functions that could error would return a Data or an Error and then... you'd have to call a function like `my_result.Do(callback)`? The fact that idiomatic Go code actually returns (data, err) and checks if err == nil -- and that it's not an error to forget this check -- suggests that the interface paradigm just doesn't work that well.

I have programmed Go for around 8 years now. This is not a problem in practice.

I am familiar with Haskell. I know the gospel of sum types. But I don't think it's good engineering to force an inferior solution in to solve a problem I don't have.

There are times when it solves problems. It is sort of ironic that as this conversation was occurring here on HN, I was programming with a sum type at work and doing a lot of type switches today. But it was solving a problem for me. Either/Result doesn't solve a problem I have in Go.

The vast majority of time a person bashes a sum type into Go, they should be using polymorphism instead of switches.

It is a well-known error to try to use a functional programming language as an OO language. It is the exact same error to use an OO language as if it's a functional language. One may be more in line with the zeitgeist, but that just makes it more popular, not a better idea. It's just as silly and just as gauche as the guy who runs to a Haskell community to complain about how they can't figure out how to implement inheritance in a nice way.