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Comment by grumpyprole | Hacker News Reader

Go's lack of sum types mean that there is no static check for whether the error has actually been handled or not. Go's designers went to all the trouble of having a static type system, but then failed to properly reap the benefits. Sum types are the mathematical dual of product types. It makes sense for any modern language to include them.

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Ever since I learned of sum types, they have ruined my enjoyment of programming languages which don't have them. I sorely miss them in C++ for example (and std::variant is not a worthy alternative). I don't understand why any new language wouldn't have them.

Pedantic typechecking is like learning to spot improper kerning, you think it’s a good thing but you spend your entire life cringing at the world around you.

Just wait until you learn union types, type classes, type providers, and so on. It's even worse afterwards. :-)

std::variant is a good example of many things bad with c++ improvement process, as a language.

If you want to just pattern match on type of visitor there is “another convenience helper” that you need to bring, and result still looks not pleasant.

Introduced in like c++17, even in c++23 you still need to write a std::visit to process it. Committee members waste time on yak shaving that std::print

In theory the lack of sum types sounds like a drawback for Go error handling, in practice it does not matter at all IMO.

So far I have never worked a Go project without a strict linter enabled on the pipeline checking that you handled the case when err != nil. I don't care if it is the compiler or the linter doing it, the end result in practice is that there actually is no chance of forgetting to check the error, and works just as well as a stronger type system while also making the code stupidly obvious to read.

> no chance of forgetting to check the error, and works just as well as a stronger type system

A linter-based syntactic check is no substitute for a proper type system. A type system gives a machine checked proof. A heuristic catches some but not all failures to handle errors, it will also give false positives.

Error handling via sum types only enforces the rather weak constraint that you cannot access a non-error return value in the case where the function returns an error. It certainly doesn’t catch all failures to handle errors. For example, in Rust you are perfectly free to call a function which returns a Result and then ignore its return value (hence ignoring the case where an error occurred). Go’s error checking linters impose stricter constraints in some respects than the constraints on error handling imposed by Rust’s type system.

> only enforces the rather weak constraint that you cannot access a non-error return value in the case where the function returns an error.

This "rather weak constraint" as you put it, completely solves Tony Hoare's "billion dollar mistake": null pointer exceptions. Something Go also suffers from due to lack of Sum types. With regard to your Rust example, the compiler will give a warning that can be turned into an error to completely prevent this, if desired.

As the parent said, sum types are "foundational" and have many applications for writing safe statically checked code. Eradicating null pointers and enabling chainable result types are only the tip of the iceberg.

> This "rather weak constraint" as you put it, completely solves Tony Hoare's "billion dollar mistake": null pointer exceptions.

Yes. However, it doesn’t do what you seemed to be suggesting that it does, which is “catch all failures to handle errors”. You correctly note that Go’s linters can’t always do this, but also seem to erroneously suggest that Rust’s type system somehow can. This is backwards. Go’s error linters catch most instances of ignored error values, whereas Rust’s type system doesn’t do anything, in and of itself, to ensure that error values are not ignored. Of course there are compiler warnings to catch unused errors in Rust, but that’s fundamentally the same thing as the warnings you get from Go linters, and has nothing really to do with sum types. Whether or not an error is ‘ignored’ or ‘used’ in any interesting sense is not a formal property of a program that can be formally verified. (Yes, linear types, etc. etc., but you can formally use an error value in that sense while in practice ignoring the error.)

By the way, you don’t have to preach to me (or really anyone on HN) about the virtues of sum types. I’ve written a fair amount of Haskell and Rust code. My issue here is not with the utility of sum types, but with the erroneous claim that they somehow remove the need for linters or compiler warnings that flag unhandled errors.

> but also seem to erroneously suggest that Rust’s type system somehow can.

I didn't bring Rust into this discussion, it is hardly a model implementation of sum types and using them for proofs, but it is certainly a step in the right direction.

> My issue here is not with the utility of sum types, but with the erroneous claim that they somehow remove the need for linters or compiler warnings

You are misrepresenting my posts, I am responding to an erroneous claim that linters are a satisfactory substitute for sum types.

Thread A:

>> no chance of forgetting to check the error, and works just as well as a stronger type system

>A linter-based syntactic check is no substitute for a proper type system. A type system gives a machine checked proof. A heuristic catches some but not all failures to handle errors, it will also give false positives.

Thread B:

> Go's lack of sum types mean that there is no static check for whether the error has actually been handled or not.

>>I dunno, my IntelliJ calls out unhandled errors. I imagine go-vet does as well.

>>>A simple syntactic check will only ever work as a heuristic. Heuristics don't work for all cases and can be noisy. The point is, no modern language should need such hacks. This problem was completely solved in the 70s with sum types. [emphasis mine]

What you said in these two threads seemed to suggest that the functionality of Go error linters can be subsumed by a 'proper type system' that includes sum types, and that such a type system would catch all failures to handle errors. But sum types by themselves do nothing to force handling of errors. You could add sum types to Go and you'd still need a linter to perform the exact same error handling checks that Go linters currently perform. Whatever kind of value a fallible function returns, you can always ignore that value as far as the type checker is concerned (unless you add something like linear types to your type system, which are orthogonal to sum types).

Apologies for the confusion re Rust. For Rust just read 'a language that handles errors using sum types'.

> Sum types by themselves do nothing to force handling of errors,

If you have Haskell experience, then have you ever wondered how it is considered "null safe" and does not throw null pointer exceptions? Perhaps it is because optional "Maybe" types (the simplest form of error) must be explicitly unpacked? Yes, Haskell, being an old language without a sound type system, permits "fromJust" and its exceptions (a side effect) are not tracked like other effects. But despite this, are you seriously claiming that sum types "do nothing" to achieve this null safety?

If you want to understand the full proving power of sum types, I do not suggest Rust or Haskell as a model example. Coq, Agda, Idris or ATS will be better examples.

>But despite this, are you seriously claiming that sum types "do nothing" to achieve this null safety?

No. I said nothing about null safety. What I said is that “sum types by themselves do nothing to force handling of errors”. In fact I imagine that’s one of the reasons that Haskell uses exceptions for error handling in the IO monad. If Haskell had a non-raising function like

    openFile :: FilePath -> IOMode -> IO (Either IOError Handle)

then you could of course attempt to open a file without checking for an error:

    main :: IO ()
    main = do
        openFile “/foo/bar” ReadMode
        putStrLn “Did the file open successfully? No idea”

Sum types (by themselves) cannot be used to prove that all errors have been handled. In fact, formal proofs of this property using other type system features (such as linear types) are of fairly limited practical value, given that merely 'using' an error value in some type-theoretic sense doesn't necessarily entail actually taking appropriate action to handle it.

> No. I said nothing about null safety. What I said is that “sum types by themselves do nothing to force handling of errors”.

Maybe (null) types are the simplest form of error type, with null pointer exceptions being the simplest from of unhandled error. They are therefore the easiest example to illustrate my point. You cannot simply choose to ignore them and remain credible. Haskell's broken old IO APIs are hardly a model example. Your Haskell code will at least give a compiler warning for ignoring the output. I would configure the compiler to turn this into an error.

>with null pointer exceptions being the simplest from of unhandled error.

I don't know of any practical language that forces you to handle the Nothing condition of a Maybe. Haskell has fromJust (as you note), Rust has unwrap. I suppose Idris could become practical one day, but it's not there yet. More fundamentally, without something like linear types, nothing in the type system can force you to check that a value of a particular sum type instantiates a particular variant. You're always free to ignore values, which means that you're free to ignore error conditions.

> Haskell's broken old IO APIs are hardly a model example of anything

I don't quite see what you're getting at here. My example function isn't part of Haskell's IO API. It's an example of what Haskell's IO API might look like if it used sum types for error handling rather than throwing exceptions. I fail to see how there can be anything inherently 'broken' about a hypothetical function that opens a file and returns either a file handle or an error.

>Your Haskell code will at least give a compiler warning for ignoring the output. I would configure the compiler to turn this into an error.

So you're saying that you'd configure the compiler to do exactly the same checks that Go error linters do...none of which have anything to do with sum types.

> It's an example of what Haskell's IO API might look like if it used sum types for error handling rather than throwing exceptions.

Apologies I missed that bit, it is indeed a perfectly reasonable API.

> So you're saying that you'd configure the compiler to do exactly the same checks that Go error linters do...none of which have anything to do with sum types.

We are arguing semantics as to what constitutes a "handled error". If a user chooses to explicitly throw away the error and not use the value, then you are arguing it is not handled. I am arguing that it has been handled (and checked as such). Either way sum types are a step in the right direction, despite all the shortcomings and unsound type systems of "practical" languages.

Rust compiler warns about ignoring a result of functions annotated with #[must_use]. This is optional because if a function has no side effects, then ignoring its return value is not a problem and shouldn't be warned about.

Yes, but that’s just the sort of linting you can also get vía Go tools. It’s not something that’s possible because of sum types.

> if a function has no side effects

A property which of course is not tracked by Rust’s type system. My only point here is that neither sum types in general nor Rust’s specific implementation of them provide any means of ensuring that errors are handled. They do other nice things, just not that.

> no chance of forgetting to check the error

I remember a similar discussion here a couple months ago and some guy linking numerous open bug reports in Docker's repo with this very issue.

No language has a 'static check for whether the error has actually been handled or not'. In Rust, for example, you can just 'unwrap' an error. In Haskell you can use 'fromJust'. And in Go you can just ignore 'err' and assume it is 'nil'.

Sum types might be the 'mathematical dual of product types' but programming languages are not mathematics. The possibly implementations of sum types are quite varied. It makes sense in low-level languages for the programmer to use what makes sense in the particular situation.

Unwrap and fromJust can be disallowed if need be, they are "unsafe" convenience functions whose use can and should be tracked. Not all languages with sum types will permit them. Rust also has "unsafe" code blocks, should we also claim it is therefore not memory safe? Some would try to do so, but at least this unsafe code is tracked and not idiomatic.

> programming languages are not mathematics

This may be how you choose to view them. But many of us seeking to build safer and more correct software aim to make programming more like mathematics. Mathematics tells us how to compose and tells us how to prove. Both things the software industry is currently failing at.

>Unwrap and fromJust can be disallowed if need be, they are "unsafe" convenience functions whose use can and should be tracked.

And with the same sort of third-party tools you use to 'track' those and ensure they're not used, you can track unused error returns in Go or C.

> Not all languages with sum types will permit them.

All do.

>Rust also has "unsafe" code blocks, should we also claim it is therefore not memory safe? Some would try to do so, but at least this unsafe code is tracked and not idiomatic.

Absolutely we should! Rust fanatics try to claim it is a memory-safe language when it isn't. Real memory-safe languages like Java have existed for far longer.

>This may be how you choose to view them. But many of us seeking to build safer and more correct software aim to make programming more like mathematics. Mathematics tells us how to compose and tells us how to prove. Both things the software industry is currently failing at.

You missed the entire point of what I said.

> All do

No, this is not true. A total functional programming language can disallow partial functions that circumvent the type checker.

Again, linters only get you so far. For example, sum types eradicate null pointer exceptions, linters do not.

I don't give a shit what languages CAN do in theory.

I care what languages DO do. And all languages (that aren't research projects like agda) provide escape hatches here. Even Haskell.

>Again, linters only get you so far

You brought up linters. Not me.

He still has a point. In theory you might need a language like Idris/Agda, but in praxis it still makes a big difference.

It is true that you will see that a function can return an error and that you choose to ignore it. It's also true that you can do the same in many other languages that use sumtypes.

But it is still different. Because while ignoring an error in go is as easy as putting an underscore next to the happy-case, in languages with sumtypes that doesn't work.

The equivalent in other languages would be to return a struct and then just access one value and ignore the other one. In that case, the practical implications would be the same.

But when using a sumtype, a few things change.

First, you can not just access the happy-case value, you are (or at least can be) forced to also "access" the unhappy-case value. Either in a pattern match, in a fold-function and so on.

You know have to return something, even if it is an empty value our "escaping" by throwing an error.

On top of that, what happens if a function can partially succeed? Take a graphql request as a practical example where this is quite common.

With Go's style of error handling, how do you model that? I.e. say you need to redactor a function that previously either succeeded or failed into one that now can partially succeed and fail.

In a language with sumtypes I would now switch from a sumtype Success|Error to more complex type Success|Error|PartialSuccess which makes it a breeze to refactor my code because the compiler will tell me all the places where I have to consider a new case and what it is.

I'm genuinely curious, how would you model that in Go and what implication would such a refactoring have on existing code?

I imagine it would be quite different in praxis.

You can always implement tagged unions in any language with untagged unions, so in a broad sense you can emulate sum types in situations where they make sense but use simpler code elsewhere. I might do that in C. Depends on the situation. It also obviously isn't a proper answer, I am sure you will agree, to just emulate the feature that I am saying is unnecessary. That works in Lisp where you can elegantly add language features with proper macros. In C, you cannot.

I would probably simply do in C the same thing as usual:

    int
    function1(int arg1, int arg2, int *out1, struct foo *out2)
    {
        if (part1(arg1, out1))
            return 1;
        if (part2(arg2, *out1, out2))
            return 1;
        return 0;
    }

    // Oh hmm, some callers can do something useful with a partial result.
    // Assume the internals are more complex, because obviously in this simplified example you
    // could just make them call part1 directly.

    enum { SUCCESS, PARTIAL_SUCCESS, FAILURE }
    function2(int arg1, int arg2, int *out1, struct foo *out2)
    {
        if (part1(arg1, out1))
            return FAILURE;
        if (part2(arg2, *out1, out2))
            return PARTIAL_SUCCESS;
        return SUCCESS;
    }

This is compatible with old callers, even, who treat any nonzero result as failure and any zero result as complete success (the normal pattern in C).

Yes, the caller needs to check the result and avoid looking at out2 if you dont get SUCCESS and avoid looking at out1 if you get FAILURE. But this sort of thing is de rigeur in C. Your compiler (or a linter, and optional warning flags are essentially linters anyway) will warn you if you ignore the result and if you switch on the result will warn you if you ignore a case.

But obviously it is left up to you to avoid the "dont touch X if Y" stuff. Eh, that is in my experience not the hard bit of writing C. The hard bit is anything involving dynamic lifetimes or shared mutable state. The nice thing is that you can avoid this in C! Most people don't. The easy path is calling malloc everywhere and getting yourself into a muddle. The simple path, which is better in the long run, is to use values and sequential, imperative code. And if you do that, you realise that C's design makes way more sense. That is how it was designed to be used. Dynamic lifetimes of objects? It is like trying to use Rust to represent linked lists. People that say 'Rust sucks because double linked lists lol' are morally equivalent to people that say 'C sucks because malloc and free lol', it is like.... Yeah you aren't meant to do that!

> You can always implement tagged unions in any language with untagged unions, so in a broad sense you can emulate sum types in situations where they make sense but use simpler code elsewhere

I'm a bit confused now, since I don't see how this is related to the point I was making. You are right - with the exception that sumtypes are still more powerful since you cannot e.g. emulate GADTs with tagged unions, but for most cases in practice, I agree. Still, what's the point?

I also think we have a general misunderstanding, since you are saying:

> That works in Lisp where you can elegantly add language features with proper macros

But Lisp is dynamically typed, so talking about union types is meaningless in a dynamically typed language. That doesn't make any sense to me in this context.

And about C (which is statically typed): C does not have union types (and hence also no tagged union types). What C does have are (untagged) enums, but that's not the same thing. The crucial difference is that union types are ad-hoc whereas enums are statically defined. I think it is a bit confusing since C calls them unions - but in the context of this discussion it's important that they are very different things.

E.g., with union types you can do:

    type union1 = string | int
    type union2 = string | boolean
    type union3 = union1 | union2 | float
    // same as type union3 = string | int | boolean | float

The compiler must be able to resolve those things automatically. I'm hope I'm not completely mistaken here, But I believe there is no way to combine unions like this in C at the type-level. You would have to write those out by hand or generate the code. But if there is, please correct me.

> This is compatible with old callers, even, who treat any nonzero result as failure and any zero result as complete success (the normal pattern in C).

The idea or moviation was though that in a language with sumtypes (or tagged unions) the old callers would not be compatible. Trying to compile code against `function2` should fail. But it should not fail in an arbitrary way - it should fail by the compiler saying "hey, look, you handled the error case and the success case, but you also have to handle the partial-success case; and here is how the data you need to handle looks if it is partial-success: ...". That is what sumtypes give you, and I find that this enormously useful in practice. In a language without sumtypes you will not get this level of support by the compiler - that is the point I was trying to make.

> I don't give a shit what languages CAN do in theory.

Then I'm sure you'll be very satisfied with languages like Go and Zig. I am not satisfied with them, because I know what can be done.

> You brought up linters. Not me.

I did not. They were brought up by others as apparently one way that Go programmers work around lack of sum types.

A good point. Newer languages influenced by Go, like Vlang[1], have sum types partially for such reasons.

[1]: https://github.com/vlang/v/blob/master/doc/docs.md#sum-types

> Go's lack of sum types mean that there is no static check for whether the error has actually been handled or not.

I dunno, my IntelliJ calls out unhandled errors. I imagine go-vet does as well.

A simple syntactic check will only ever work as a heuristic. Heuristics don't work for all cases and can be noisy. The point is, no modern language should need such hacks. This problem was completely solved in the 70s with sum types.

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