7 ms·
Be careful with Go struct embedding
- tymscar 1y agoThat’s actually crazy. Why is this even a feature?
- jrockway 1y agoBecause type Foo struct { sync.Mutex whatever string } var foo Foo foo.Lock() foo.whatever = 42 foo.Unlock() is convenient.
- echelon 1y agoIt's dangerous. This is awful. Any coding construct that can cause defects is an antipattern. Your language should discourage defects by design. Especially if the faults crop up at runtime. This struct field dereferencing is like NULLs and "goto". Language design that is anti-defect yet ergonomic include the modern Option<T> and Result<T, E> as seen in languages such as Swift and Rust, with first class destructuring that doesn't make it painful to use. They're almost impossible to misuse, yet feel convenient instead of frictionful. Rust's sum types and matching are another set of examples. Hopefully these patterns spread to more languages, because they're safe and convenient.
- eru 1y agoI mostly agree. > Language design that is anti-defect yet ergonomic include the modern Option<T> and Result<T, E> as seen in languages such as Swift and Rust, with first class destructuring that doesn't make it painful to use. Funny enough, this is only 'modern' in imperative languages. It's been a staple in the ML family since approximately forever. (But hey, I do appreciate progress when we get it!)
- thrill 1y agoHmm, never realized the convenience came this way. Seems the compiler could emit a warning if two equal depth names might cause confusion, which could be ignored if acceptable.
- sethammons 1y agoalmost always, the recommendation is to not embed your mutex; give it a name. foo.mu.Lock() This way you don't expose your primitives, preventing poor usage from causing a deadlock. Generally you don't want the user of your struct to have to know when or when to not lock.
- eru 1y agoAlas, locks don't compose, ie often your users will have to know about the internals when you are using locks. But it's good advice when it works.
- resonious 1y agoI thought Go was all about being "simple" at the cost of convenience. It is a bit ironic that this language that was was designed around "all of these features of other languages cause trouble, we will omit them" also has a bunch of features that cause trouble and get avoided. Just to make my own stance clear: I like language features. I think this struct embedding feature looks pretty cool. But I also like interfaces and polymorphism. I think it's OK for a programming language to be powerful, and to put the onus on developers to not go too crazy with that power. And for that reason, I've always gravitated away from Go, and always jump on an opportunity to make fun of it (as I have here).
- mikepurvis 1y agoAt risk of being excessively sassy this looks like a case of wanting the ergonomics of multiple inheritance without fully grappling with the complexities or implications of it.
- gdbsjjdn 1y agoIn most cases people just want any inheritance, this is the backwards way the Golang devs decided to implement it based on their 80s view of programming languages.
- metadat 1y agoIMHO it should be a compiler error. This is just so loose... a wheel fell off.
- ShroudedNight 1y agoA wheel is generous. This seems more like inviting the computing equivalent of spilling twenty thousand tons of crude into the sea, which then promptly catch fire.
- catlifeonmars 1y agoEh it’s about the same level of footgun you might see in C99. It’s not great but you’re being hyperbolic if you ask me.
- ShroudedNight 1y agoI agree that most scenarios are not going to be so perilous. However, GOTO FAIL[1] would fit your C99 foot gun description neatly, with repercussions that would approach the peril of my metaphor within at least an order of magnitude. https://en.wikipedia.org/wiki/Unreachable_code#goto_fail_bug https://en.wikipedia.org/wiki/Unreachable_code#goto_fail_bug
- dgl 1y agoSee how it's used in the standard library io types, it makes for quite nice composition: https://go.googlesource.com/go/+/refs/heads/master/src/io/io.go#130 https://go.googlesource.com/go/+/refs/heads/master/src/io/io...
- mananaysiempre 1y agoI’m sympathetic to parts of the Go design philosophy, but the only thing that comes to mind looking at this is “damn, that’s some awkward (nominal-looking) syntax for (structural) intersection types”. (It also feels to me that this sort of anonymous embedding is materially different for interfaces vs structs, though I admit that from a type-theoretic perspective it’s not.)
- bilbo-b-baggins 1y agoYou can’t have ambiguous methods so the problem illustrated here fails at compile time for interfaces.
- whatevertrevor 1y agoUnioning interfaces like this does seem convenient for composition/mixin patterns, I'm not sure if extending it to structs in general seems worth the cost of potential footguns though, especially external libraries and such where you probably don't want to think about the full potential tree of embedding conflicts.
- mananaysiempre 1y agoAt the very least, the Go authors have been convinced this should be a feature since the Plan 9 C dialect[1]. [1] http://doc.cat-v.org/plan_9/4th_edition/papers/comp http://doc.cat-v.org/plan_9/4th_edition/papers/comp, look for “anonymous structure or union” and note that a (different) part of that extension has since been standardized.
- bilbo-b-baggins 1y agoNormally you wouldn’t contrive to use embedded struct fields in this way. And you can’t have the same kind of composition with methods - it’s a compiler error: https://go.dev/play/p/r04tPta1xZo https://go.dev/play/p/r04tPta1xZo So the whole article is basically about using the language in a way you normally would ever do.
- whatevertrevor 1y agoThis can be simplified, conflicting field names at the same level also don't compile: https://go.dev/play/p/D3eFi9_can8 https://go.dev/play/p/D3eFi9_can8 Conflicting functions at nested levels also compile: https://go.dev/play/p/xXXDZCjQJOh https://go.dev/play/p/xXXDZCjQJOh It's not about method vs field, it's about the nesting level of the conflicting identifier, if it's at the same level there's an error, if it's at different levels, the higher level hides the lower level identifier: https://go.dev/doc/effective_go#embedding https://go.dev/doc/effective_go#embedding
- amiga386 1y agoBecause it's useful. https://go.dev/doc/effective_go#embedding https://go.dev/doc/effective_go#embedding > Embedding types introduces the problem of name conflicts but the rules to resolve them are simple. First, a field or method X hides any other item X in a more deeply nested part of the type. If log.Logger contained a field or method called Command, the Command field of Job would dominate it. > Second, if the same name appears at the same nesting level, it is usually an error; it would be erroneous to embed log.Logger if the Job struct contained another field or method called Logger. However, if the duplicate name is never mentioned in the program outside the type definition, it is OK. This qualification provides some protection against changes made to types embedded from outside; there is no problem if a field is added that conflicts with another field in another subtype if neither field is ever used.
- typ 1y agoIt seems to come from a Plan 9 C idiom that GCC even has an extension for it. https://gcc.gnu.org/onlinedocs/gcc-5.3.0/gcc/Unnamed-Fields.html https://gcc.gnu.org/onlinedocs/gcc-5.3.0/gcc/Unnamed-Fields....
- divan 1y agoWhen you create a struct with another embedded struct (possibly from other package): type Foo struct { somepackage.Bar URL string } you don't want to depend on whether Bar already have URL. Your depth level has higher priority. Even more, imagine in the future authors of `somepackage` decided to add URL to their struct and it suddendly started to break your code from being compiled. Example in the OP article is a corner case where this behavior is creating ambiguity, indeed. Yet, it's documented and intentional.
- bilbo-b-baggins 1y agoHuh my IDE linter spits out warnings about this. Not sure which extension does it.
- digianarchist 1y agogolint possbily?
- Olreich 1y agoIf you need to grab a particular struct's version of the data, you can via `opts.BarService.URL` or `opts.FooService.URL`: https://go.dev/play/p/MUSYJhmoC2D https://go.dev/play/p/MUSYJhmoC2D Still worth being careful, but it can be useful when you have a set of common fields that everything of a certain group will have (such as a response object with basic status, debug info, etc. and then additional data based on the particular struct). I don't know why they let you embed multiple layers and multiple objects though. I've never gotten value out of anything but a "here's a single set of common fields struct embedding".
- tymscar 1y agoSo I got curious and I looked at the compiler source code, and it does a depth-first search. The fascinating bit to me is that there is a consolidateMultiples function in go/src/go/types/lookup.go (lines 286-304) that detects when multiple embedded types at the same depth provide the same field name. I wonder why they don’t do this for all levels. How deep could this even be in practice for it to matter? You could just have a hashmap with them all.
- dgl 1y ago> I wonder why they don’t do this for all levels. How deep could this even be in practice for it to matter? You could just have a hashmap with them all. While it may seem questionable for fields; it applies to methods too and is potentially more useful as a way to override them when doing struct embedding but wanting to preserve an interface.
- ankraft 1y agoIt performs a breadth-first search, not a depth-first search.
- divan 1y agoTo protect against changes in structs coming from external libraries. Or, better phrased, external structs should not dictate what names you're allowed to use in your own structs (so they have priority).
- linhan_dot_dev 1y agoI like the Go language because it's straightforward and clear, even if it looks a bit plain. I hope the feature mentioned in the article will cause a compiler error. However, I wouldn't use this approach when writing my own code.
- eru 1y ago> I hope the feature mentioned in the article will cause a compiler error. Read the article. It won't. At best you can perhaps find a linter that'll report it? > However, I wouldn't use this approach when writing my own code. You might use it by accident.
- linhan_dot_dev 1y agoI know, so this is a wish. Looking at other comment sections, there are actually linters that can warn about such behavior.
- linhan_dot_dev 1y agoThis comment unexpectedly received a few downvotes, which might be due to some misunderstanding. My point is as follows: 1. People choose Go for various reasons. 2. Beginners will encounter the issues mentioned in the article, so I hope there could be a feature that provides hints for newcomers. While the compilation errors might not be reasonable, at least the LSP/linter could provide some guidance.
- tsimionescu 1y agoThis may be a language issue, but saying "I hope this causes a compilation failure" implies that you don't know if it does - this is probably why you got down voted, since the article very explicitly says that this doesn't cause compilation failure. You may have meant that you hoped it wouldn't do this - in that case you should have used a construction like "I'd hoped this would have caused a compilation failure".
- 1y ago
- digianarchist 1y agoI'm surprised this wasn't in the recent post submitted here: https://blog.habets.se/2025/07/Go-is-still-not-good.html https://blog.habets.se/2025/07/Go-is-still-not-good.html It's a one of a few rough edges in Go.
- eru 1y agoYou could add it as a comment there, or on the HN discussion?
- nemo1618 1y agoOver the course of ~10 years of writing Go, my ratio of "embedding a struct" to "regretting embedding a struct" is nearly 1:1. I do not embed structs anymore. It is almost always a mistake. I would confidently place it in the "you should be required to import 'unsafe' to use this feature" bin.
- oefrha 1y agoUsing struct embedding for pure data to implement discriminated unions is fine, better than MarshalJSON() that is lost on a type definition. Using it to save typing, or going crazy with it (I consider embedding two things going crazy) is bad.
- nine_k 1y agoI think that using embedding for discriminating unions if a good idea. It would work, but it does not force the user to do the discrimination. I would say that explicit typecasting at the point of discrimination is safer. Without it, nothing prevents you from using one field from one variant of the union, and another from a different variant. Introduction of proper discriminated unions would be great.
- oefrha 1y agoI'm not sure I understand you, or you understand me. I'm saying this is okay: type Order struct { Type OrderType CommonAttr1 int CommonAttr2 string } type OrderTypeA struct { Order TypeAAttr1 int TypeAAttr2 string } type OrderTypeB struct { Order TypeBAttr1 int TypeBAttr2 string } And yes you should convert to OrderTypeA or OrderTypeB at the first opportunity in domain code, and only convert from them at the latest opportunity. You seem to be under the impression that I'm advocating for something like type OrderUnion struct { CommonAttr1 int CommonAttr2 string TypeAAttrs TypeBAttrs } That's what I consider going crazy.
- debugnik 1y ago> And yes you should convert to OrderTypeA or OrderTypeB at the first opportunity in domain code, Go can only downcast through interfaces so there's something missing to your approach to unions, isn't there?
- noisy_boy 1y agoAm I the only one who found the described behavior to be intuitively correct? I did expect it to print "abc.com".
- _2d30 1y agoI don’t write Go at all but given the first example, also expected this. I was very surprised that either example compiled, though.
- adastra22 1y agoWhat is the intuition for that?
- b_e_n_t_o_n 1y agoYeah it makes sense to me as well.
- preommr 1y agoMy personal conspiracy is that Golang is an epic prank. Make a language that's really good in some ways and just horrible in other ways for no reason whatsoever. So that when it's critics point out contradictory features like embedding, it's defenders can be the ultimate troll and say things like "but, actually, it's a simple language because it doesn't have while loops". It's the best explanation I have for some of the cognitive dissonance surrounding the language design.
- shadowgovt 1y agoAs with most small-team languages, it was built mostly to solve the problems that its initial author had in front of them.
- b_e_n_t_o_n 1y agoIs it possible that it's like every other language, with flaws and tradeoffs that don't always make sense to everyone? Why make it more complicated than that?
- hakunin 1y agoThis might sound bad, but having worked in a few different languages, I find it kind of cute how some parts of Go seem to promote simplicity while others are simply what the authors were used to growing up. Sometimes it frustrates me when I write Go, when I see a tradeoff due to the latter. But yeah. Languages tend to have a lot of subjectivity of their author in them. The more "strict" the language is, the more confined you are to the author's opinions.
- nine_k 1y ago> just horrible in other ways for no reason whatsoever I bet the reasons were very mundane: initial project scope, deadlines, performance review cycle. "This simplest thing that could possibly work", etc.
- jerf 1y agoI've been programming in Go for ten years. This problem has happened to me once. Would you care to make a list of all the problems your favorite language has served up to you at a rate of once in ten years, so I can also write a post making your language sound horrible as a result?
- MH15 1y agoHello from another Matt Hall! Interesting post, although I don't do much Golang.
- alpb 1y agoIt's my common code review comment to the beginners to not embed structs. There's rarely anything to be gained by doing so. The only use case I found to be useful is to embed something like: type MockHandlers struct { UnimplementedHandlers } func (m MockServer) LoginHandler{ /* ... */ } where I get to override only a part of a bigger interface at a time and have the embedding take care of the rest by saying panic("unimplemented") to satisfy the interface.
- Fire-Dragon-DoL 1y agoMaybe I see it differently, but it made sense: embeding works only at 0 depth, it's like a macro to access rapidly the fields of the embedded struct, it doesn't go beyond that,there is no inheritance. When embedding BarService, the field being embedded is BarConnectionOptions
- b_e_n_t_o_n 1y agoStruct embedding is sugar for type Foo struct { MyType MyType } myFoo.MyType.url So it would resolve myFoo.MyType1.url over myFoo.MyType2.NestedType.url
- joahnn_s 1y agoWho's actually expecting `xyz.com` here? Spec: https://go.dev/ref/spec#Selectors https://go.dev/ref/spec#Selectors > x.f resolves to the field/method at the shallowest depth in T. If there isn’t exactly one at that depth, it’s illegal. Embedding promotes fields; on name collisions the shallowest wins. So `opts.URL` is `FooService.URL` (depth 1), not `BarConnectionOptions.URL` (depth 2).
- Someone 1y agoThat something is clearly specced doesn’t imply all developers actively know it. Even given that it compiles, I wouldn’t exclude it being a runtime error. But the big problem isn’t that it behaves as advertised, it’s that it is way too easy to write opts.URL where you mean opts.Bar.URL. Auto-complete will happily compete the wrong thing for you.
- lowmagnet 1y agoI have code that wouldn't work without embedding. Basically, I have a type that annotates any AWS paginate and is capable of streaming all the results as a sequence. It embeds the original client so you get all the functionality of the client, but it also wraps the functions that support a pagination. I can't think of an easier or clearer way to do it.
- okibry 1y agoThe same issue with embbedding of field had reflect tag like json or bson ...
- okibry 1y agoAfter that time, I did not use json or bson Unmarshal to cast anymore. Any transform/converter function of me is manual directly. And thanks for LLM, with the their help, I only need to review the function, so no more typing.
- flowerthoughts 1y agoMaking the compiler fail that would indeed be nice. A clear example of a language feature that makes sense when there's a single developer, but in a code base with hundreds of developers, this could easily break something without anyone noticing.
- HelloNurse 1y agoThis is a "you had one job" level language defect. If there are excuses that allow Go to pretend that structs with duplicate member names are well formed, good programming language designers would reject them in horror and specify how the compiler should actively look for this type of mistake, without wasting effort to look for such excuses.
- divan 1y agoSome people in comments jump to the conclusion that Go allows conflicting names in embedded structs. It doesn't not – for the embedded structs of the same depth. These won't compile: type Bad struct { Name string Name string } type A struct{ Name string } type B struct{ Name string } type C struct { A B } bad.Name() // compile error: other declaration of Name c.Name() // compile error: ambiguous selector c.Name The case in article is about field names of the different depth. Spec is very clear about this behavior, and it was intentional. One of the reasons why handling same field names is different at different nesting levels is to protect against changes in structs coming from external libraries. Or, better phrased, external structs should not dictate what names you're allowed to use in your own structs (so they have priority). I.e. when you create a struct with another embedded struct (possibly from other package): type Foo struct { somepackage.Bar URL string } you don't want to depend on whether Bar already has URL. Your depth level has higher priority. Even more, imagine in the future authors of `somepackage` decided to add URL to their struct and it suddendly started to break your code from being compiled. I agree that behavior in the OP article example is confusing (and so is the code - do you want URL of Foo service or Bar service?). Yet, this behavior is intentional and documented. As usual, it's a subtle tradeoff here. If this feature would be implemented differently (say, compile time error for all depth levels), we would see an article with rant on how external structure changes breaks compilation.
- ActionHank 1y agoI don't really use golang all that much and even I was confused about OP's point because of the different depths. I feel like sometimes people just want to complain.
- divan 1y agoI think author just used to having consistency in Go. If you learned behaviour of one aspect of the language (i.e. compile-time error for conflicting field names), you would expect to have it in all cases. And that's not what's happened in the given example.
- NooneAtAll3 1y ago
- desumeku 1y agoWhy would you ever do opts.URL instead of opts.FooService.URL or opts.BarService.URL? What does ambiguity and imprecision gain you here when you can just write out from which struct you want it from? I don't even know why opts.URL would compile, it's completely unstated that you're not grabbing it from opts but grabbing it from some other structure contained within opts. Shouldn't even compile IMO, but at least I found something I disagree with Go's designers on.
- fishywang 1y agoI don't understand how this got so many upvotes. Embedded struct fields are never "promoted", you always need to access them via the embedded type's name, so there's nothing to conflict. The only thing "promoted" are the functions associated with the embedded types, and when those actually conflicts, the compiler will tell you, as expected.
- jlokier 1y ago> you always need to access them via the embedded type's name, so there's nothing to conflict. The article talks about "opts.URL" in its example being accepted by the compiler, which accesses "opts.FooService.URL" without using the embedded type's name.