Go in particular is really unfortunate for a language so new. The language's weaknesses have dropped my estimation of its creators by several notches.
There, I fixed it for you.
Every language has product types. Every programmer understands how useful product types are. Why in the world would the same not apply to sum types?
Sum types can be useful. Other aspects of languages can be useful, too. Why fixate on that one? And if you're going to say that it's a minimum bar, well, some of the other features are missing in most languages that have sum types.
Pick the language that has the total set of features that makes it easiest to write whatever program you're trying to write. Don't get locked in to focusing on only one feature.
I haven't kept a tally of my frustrations with Java, but I can assure you that the most frequent is its lack of first-class sum types. Every time I need to encode (encode!) such a basic concept as "it could be this or that" using subclasses and some Design Pattern to Manage the Variants (usually Visitor, sometimes State), I metaphorically weep for my soul.
There are plenty of other minor gripes I have with the language -- there have been multiple instances where generic generics (HKTs, `class Foo<F<_>>`) would have made things much clearer -- but sum types are certainly the most prominent.
Our field has a terrible reputation for quality. In my estimation roughly 70-80% of that is directly or indirectly because of the lack of sum types.
Directly: people don't have the right tools to make invalid states in their business logic unrepresentable.
Indirectly: the lack of sum types results in broken null and error handling.
To me, sum types for error handling are isomorphic to checked exceptions. They both let you do dual-track programming - separating the normal path from the error path. Both have compiler support for enforcement. But checked exceptions are no longer considered to be the answer. What went wrong?
The problem turned out to be the programmers. They did at least two things that subverted checked exceptions.
First, they silently ate exceptions (that is, had an empty catch block just to make the exception go away. This is the equivalent of having a sum type that is either an integer or Nothing, and a function. Rather than return the sum type, the function returns an integer. In the Nothing case, the function just returns 0. That's about the same as the empty catch block to not have to declare the exception in the function's return type.
The second way checked exceptions went wrong was the opposite. When a function could throw Exception1, Exception2, Exception3, and Exception4, it was tempting to just declare it as throwing Exception (the base class). In the same way, a function that gets SumType1 back from one function, SumType2 back from another function, and SumType3 from a third function may return the sum of the sum types. It becomes an Everything type.
In both cases, the problem was that programmers were lazy. But here we are 20 years later, and programmers are still lazy. Until the programmers change, sum types won't fix things any more than checked exceptions did.
So, I think you're right about a result type being isomorphic to checked exceptions, for the reasons you laid out. If you compare it to Rust, what ends up happening is similar -- many libraries return an error type that's a union of all their dependencies' error types, and binaries end up using an "everything" anyhow::Error type in the end. However, I think where sum types in general end up working and checked exceptions don't are:
1. Sum types don't encourage a mix of checked and unchecked exceptions the way Java does. Translating from Rust, all exceptions that are meant to be handled by regular users are checked. The only exceptions that are unchecked are broken invariants (panics), which usually end up being handled either through aborting the program or through some sort of top-level restart logic. You could write Java in that style but it's not the ecosystem's convention.
2. Sum types are more general than checked exceptions: they can be used to express nulls and business logic as well. I suspect checked exceptions would have worked better if Java also had nullable and non-nullable types, because nullability is such a common source of errors.
3. You're right that you can drop an error on the floor with sum types as well, just like an empty catch block with exceptions. But that just doesn't happen nearly as often in practice, because result types form a closed set. With checked exceptions, in practice you often end up with a method throwing both checked and unchecked exceptions, and the same syntax is used to handle both. I think checked and unchecked exceptions are fundamentally very, very different and mixing the two is a mistake.
Once again, thank you for your response.
We can argue on the percentage. But the point on the direct and indirect damages makes a lot of sense.
Or, "I will not use any language which made software eat the world"
What more do you need?