When calling methods that are exposed by embedding anonymous fields on a struct, the receiver is always the inner type that declares the method. it is making implementation decisions. Getter and setters are not behaviours, they are state. It's just about being aware of the tradeoffs. We use interface variable to store any type of value in go golang. It's left to your judgment whether it's reasonable to use public access to an attribute as part of your API in a given case, rather than discouraging any exposure because it could possibly complicate or prevent an implementation change later. What are the "disks" seen on the walls of some NASA space shuttles? You can use embedding to implement an interface whether it's defined in terms of getters and setters or a pointer to the data (or, a third option for readonly access to tiny structs, a copy of the struct). Without a real-world example Im having a tough time seeing the tradeoffs. is an important skill in creating software. That is: we already have those sorts of constraints, and to me it seems strictly positive to promote at the least the common ones from ad-hoc comments into the language proper. Sign in Yep. That may be fine or even inevitable; it's just a consequence to know about. I've personally had types in the past that needed to implement the following interface: and the associated structs would look a bit weird, like: This proposal would lead to interfaces forcing structs as types to implement them, whereas today it's possible to have the implementation be a completely different type, like a func or a string, for example, see https://play.golang.org/p/irTsnlldaea as an example for this. Find centralized, trusted content and collaborate around the technologies you use most. If I don't own the type, then situation is worse, I have to define a new type alias for each of the types I am passing to the function and then add new methods to them. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. But to my point, your are describing properties of the implementation; it Getter Trying to access Hello on the root struct A will not compile. Lets modify our example slightly to fix this issue in a simple way. One interface is having get_name method which will return names of student and teacher. > It's a straining towards generics. So: with getters/setters, you can change struct fields while maintaining a compatible API, and add logic around property get/sets since no one can just do p.Name = "bob" without going through your code. I ended up coming up with that exact same solution afterwards in the wee hours, but then realized in my case, the "you'd need to know the types beforehand" part is what I wanted to avoid. > You put a set of methods into an interface, but you are unable to specify any fields that would be required on anything that implements that interface. Using the example from Effective Go, for interfaces, embedding allows borrowing pieces of an implementation by embedding types. code's dependency on an uncertain implementation onto a well-defined If you have a PersonCollection, it might be internally backed by an sql.Rows, a []*Person, a []uint of database IDs, or whatever. Go interfaces are not like Java interfaces or C++ abstract classes. Dave Cheney Modelling an interface field as a getter and setter undermines the notion of interfaces describing abstract behaviour. Considering that I sometimes influence computer science students with my

Retracting it. Presumably you define these interfaces because you have some code that can utilize those methods and perform those behaviors without knowing (or caring) what the underlying types are. I am not a purist, I do what is convenient and readable. I am not a fan as it destroys the notion of an interface being an abstract type which doesn't imply a specific implementation.

> Reply to this email directly, view it on GitHub, or mute the thread. ***> wrote: That makes the interface a field of your struct, as in b := Boxed{Data: "foo", Boxable: somethingBoxable}. implementation, but it does not require it. It's just about being aware of the tradeoffs. The two workarounds are: use reflection is returning a behavior. They don't describe points in an inheritance tree, and it's probably wrong to think about them in that way. *Dave Cheney* But to my point, your are describing properties of the @Tom That's true. Accept interfaces and return structs, share by communicating, and rejoice. I hate which doesn't work since typically in go we do not have getter/setter methods for fields, they are expected to be accessed directly. Then you'd have. On those rare occasions I use Setters and Getters in my interface. Ask questions and post articles about the Go programming language and related tools, events etc. > I am philosophically opposed to this suggestion but if adopted, it will which drops the benefits of type checking at compile time. Here's the playground. Within interfaces, you can only implement interfaces! On 5 Jun 2018, at 08:42, David Skinner ***@***. Taking those things together, I would tend towards one extreme or the other for a given use case: either a) just make a public attribute (using embedding if applicable) and pass concrete types around or b) if exposing the data seems to complicate some implementation change you think is likely, expose it through methods. Ideally, since the types are equal, wed like to have the Color value be present regardless, right? Reply to this email directly, view it on GitHub Or you might want to be able to mark a database-backed object as "dirty" when it has unsaved changes; you can do that when data updates all go through SetFoo() methods. This comes up often when I have multiple types that I pass to a function. So now let's suppose you want to write a function that operates on several different protocol buffer types in the same way by accessing the field that exists in all of them. implementation is incomplete. Say you someday want to change Person to store not just a single "name" field but first/middle/last/prefix; if you have methods Name() string and SetName(string), you can keep existing users of the Person interface happy while adding new finer-grained methods. rev2022.7.21.42638. It is one with which I have In this case, we have the Hello field as part of two structs that are embedded by our main struct. I am not a purist, I do what is convenient and readable. and setters are not behaviours, they are state. How should I handle the maximum length for given names on the U.S. passport card? Before running the program again, well walk through the changes: The Kind field moved from the Animal struct to the AnimalBase struct, which Animal now embeds. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Coming up with useful abstract data types by properly defining interfaces be something I need not use. names of the properties with a lower case first letter. The concrete implementation; it has the field X, you can get it and set it thusly. Thanks for reading this post I would like to quickly plug a product I am working on to make teams move faster, happier. 5. Trending is based off of the highest score sort and falls back to it if no posts are trending. To learn more about golang, You can refer given below link: Your email address will not be published. By clicking Sign up for GitHub, you agree to our terms of service and Or you might want to be able to mark a database-backed object as "dirty" when it has unsaved changes; you can do that when data updates all go through SetFoo() methods. This can be explained by reading the documentation on Marshal, which in this case also applies to Unmarshal: Anonymous struct fields are usually marshaled as if their inner exported fields were fields in the outer struct, subject to the usual Go visibility rules amended as described in the next paragraph. I really do not want to get into an argument with Dave Cheney. However, exposing pointers still makes direct access to data available, so it only buys you limited additional flexibility for future changes. information hiding, since its values can only be manipulated by calling To enforce the level of behavioral abstraction you'd like today, you'd have to strike reflection from the language. Go collected all possible fields from our anonymous struct fields (embedded types), noticed that we had multiple fields of the same name at the same level, both of which were tagged, and ignored all without returning an error.

u/peterbourgon's comment definitely helps steer my brain into a right direction to utilizing the interfaces to ensure behavior, and not think of them as something that can be an "instance of" like in other languages. I'd like to point out that typescript supports this, and from what I've seen its used to allow passing literal objects to functions that accept such an interface. I When calling json.Unmarshal(, &animal) the next time, Go will invoke this custom function. Announcing the Stacks Editor Beta release! If the problem is accepting a restricted, common subset of fields, maybe a different venue to pursue this would be to allow passing of structs that get narrowed down to a subset of the fields. I'm pretty on the fence about this proposal. Dave Cheney Modelling an interface field as a getter and setter undermines the notion of interfaces describing abstract behaviour. Now we can use the interface and its implementations: Now, what you can't do is something like this: However, after playing around with interfaces and embedded structs, I've discovered a way to do this, after a fashion: Because of the embedded struct, Bob has everything Person has.

So, for example, the stdlib does things like let you initialize an http.Server with your config and promises that a zero bytes.Buffer is usable. By this, A receives all fields and methods from B, which receives all fields and methods from C, so Hello is available on A. Press J to jump to the feed. if the implementation changes, the clients do not have to change. You never access the concrete fields of the types that happen to implement the interface. What drives the appeal and nostalgia of Margaret Thatcher within UK Conservative Party? 465), Design patterns for asynchronous API communication. Asking for help, clarification, or responding to other answers. Hiding properties behind getters and setters gives you some extra flexibility to make backwards-compatible changes later. I ran into this in a very painful way when attempting to do experimental automated refactoring of the compiler that hid Node fields behind accessors. Your email address will not be published. // ReadWriter stores pointers to a Reader and a Writer. am open to corrections.. not behaviour, it is state. Trying to unmarshal JSON values into structs that embed other types can quickly lead to unexpected behaviour. There are two structures and one interface . One additional thing: the embedding approach is a bit more like inheritance, right? If you have a PersonCollection, it might be internally backed by an sql.Rows, a []*Person, a []uint of database IDs, or whatever.

What's inside the SPIKE Essential small angular motor? Sign up for a free GitHub account to open an issue and contact its maintainers and the community. One could say: why not use embedding of BaseStruct in the Struct1 and Struct2. One structure is for students details and another structure is for teachers details. This change is required by the next step: We have implemented the Unmarshaler interface by declaring an UnmarshalJSON function on the Animal struct. That flexibility is more relevant when the type is complicated (and the codebase is big). @jimmyfrasche Sounds like MOVE CORRESPONDING in COBOL (I have in fact written actual COBOL programs). With the Kind value available, we can switch over the supported animal kinds, and Unmarshal into the specific structs in case of a match. In retrospect, and after 5 years of using Go, it's clear to me that the above is not idiomatic Go. If your interface returns a *Person, instead of just strings or whatever, all PersonProviders have to import the package where Person is defined. Hmm to chime on to your first example.

somewhat antiquated ideas, golang They arise, for example, when using many different protocol buffer types in proto3. - We can have a transparent data type by declaring a structure with the Id try to use this for additional type expressiveness by composition via embedding. state. Getter and setters are not behaviours, they are state. - I define an interface for pen with the functions all pens have. There are times when I wish to define implementation. If your interface returns a *Person, instead of just strings or whatever, all PersonProviders have to import the package where Person is defined. Connect and share knowledge within a single location that is structured and easy to search. those rare occasions I use Setters and Getters in my interface. ***> wrote: add getter/setter methods to the types How can I drop the voltage of a 5V DC power supply from 5.5V to 5.1V? You are receiving this because you commented. Rather, they have names like Reader, describing behaviors. How to determine if a type implements an interface with C# reflection.

You can easily do it by the next approach. It's fine to do your own stuff like that, and, indeed, I don't think you should abstract things away preemptively if the more concrete, data-exposing version seems likely to work. Hey there I would like to quickly plug a product I am working on to make teams move faster, happier.

It's fine to do your own stuff like that, and, indeed, I don't think you should abstract things away preemptively if the more concrete, data-exposing version seems likely to work. This will be made the included struct field be available in Bob structure directly with some syntactic sugar. Reply to this email directly, view it on GitHub, or mute the thread. Here, We do not want to access structures variable from outside. (You could do it other ways, too, like stashing the original data somewhere and comparing when a Save() method is called.). I have to say, this is giving me flashbacks to 1999 and learning to write reams of boilerplate getters and setters in Java. What happens if I accidentally ground the output of an LDO regulator? I'm in the middle of trying to mock out some calls to os.Process for unit tests. There are times when I wish to define implementation. I found a similar approach by setting a struct that implements an interface as the property of another struct. This proposal would lead to interfaces forcing structs as types to implement them, whereas today it's possible to have the implementation be a completely different type, like a func or a string, for example, That is already possible today: if a particular type is inspected using reflection, it may be required to be a pointer-to-struct or a channel or obey any number of other invariants. golang encoding messagepack Thanks! Brain is stuck on a lot of past OOP languages, your comment helps shift that thinking for Go :) thinking of things as behaviors is the right route. relativity If you need this field to be present in all cases, leave it out of the interface and make something like: But it would only work if Package.Boxes contains pointer elements. My apologies. Obviously, embedding would be preferable but in this case please assume that Struct1 and Struct2 are coming from two different place which the author does not full control over. undermines the notion of interfaces describing abstract behaviour. Creating a ReadWriter interface that embeds other interfaces combines the functionality of both the Reader and Writer. Scientifically plausible way to sink a landmass. structure of his choice. - I then define 5 types of pens To learn more, see our tips on writing great answers. Also, Go conventions do not require you to always put an abstraction in front of your data attributes. Was there a Russian safe haven city for politicians and scientists? ***> wrote: They are supposed to have a particular field, but I cannot express that through the interface that the function accepts. It would compile and fail at runtime.). I am not a It's too bad Go's own standard library doesn't always do this. One specific thing: interfaces in Go have the peculiar property that you can implement one without importing the package that defines it; that can help you avoid cyclic imports. privacy statement. I'm familiar with the fact that, in Go, interfaces define functionality, rather than data. Our color field was completely ignored. So, for example, the stdlib does things like let you initialize an http.Server with your config and promises that a zero bytes.Buffer is usable. I think I should express myself more precisely, as I would to a student. Also, Go conventions do not require you to always put an abstraction in front of your data attributes. An example of an earlier discussion: https://groups.google.com/d/msg/golang-nuts/ZJ5DEv_36S8/opZ__-l6XxAJ .

You can define functions that explicitly interact with that embedded data and are unaware of the nature of the outer struct.

Thanks for contributing an answer to Stack Overflow! functions that have access to the missing information. If you are building a software product and want an easy way to deploy and manage your cloud resources, set up preview environments, and understand what you're running, make sure to check it out! > My apologies. - An interface which defines a required property, is not an interface, "I can make an interface that defines data rather than behavior". (The only way you could mess up, that I can see, would be embedding the interface PersonProvider in Bob, rather than a concrete Person. That is not behaviour,

difficulty. With regard to the topic under discussion. If the duplicate name is never used, theres no error. I do think getters/setters add more flexibility than exposing a pointer, but I. Bijan is correct, why can you only make assignments to the above using dot notation? We wouldn't need to compile to a function call. You're going to be weighing this on a per-attribute basis. The thread mentions http.ResponseWriters Header method which seems like a clear win in code simplicity with this. DebugPen does nothing but create log entries to debug my main program 1. This enforces By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. How to help player quickly make a decision when they have no way of knowing which option is best. You interact with it exclusively through its methods. The advantage of this proposal is that when several different struct types have fields with the same names and types, and interface could be used to access those fields directly, rather than requiring each type to define boilerplate getter/setter methods. Using this method, I can make an interface that defines data rather than behavior, and which can be implemented by any struct just by embedding that data. @pciet Lack of a real-world example is a fair complaint. That makes much more sense to me now. maintain data validity, data integrity, collateral effects, with the data What purpose are these openings on the roof? I'm familiar with the fact that, in Go, interfaces define functionality, rather than data. Do you want to seal them up? getters in the interface means that the underlying implementation can be Are there any ad hoc examples when this is needed for structs from vastly different sources (packages, types generated by different means, etc.)? has the field X, you can get it and set it thusly. It's very unlikely you actually wanted that. For example, maybe this would be a way to solve passing data around: https://play.golang.org/p/57BBaoUlMhu. For example: I haven't thought through all of the implications of this change. Its important to be aware of the underlying rules that determine how your program will run because it might diverge from your expected outcome. I can't think of any better answer than: if data is needed, accept a common struct, if functionality is needed, accept an interface. What's the reverse of DateValue[, "YearExact"]? Modelling an interface field as a getter and setter undermines the notion of interfaces describing abstract behaviour. More concretely, interfaces in Go don't have names like Boxable, describing concrete attributes. SharpiePens can be any color. In the given below example, We will learn how to access interface fields in go golang with code. Making statements based on opinion; back them up with references or personal experience. to your account. I do not like exposing Do I need to define a bunch of setters? An interface type that lists a field may only be implemented by a struct type that has a field with the same name and type. This is particularly clear in the fact that an interface that defines a field can only be implemented by a struct type. ChromePens cannot change colors.

Thank you. The Go visibility rules for struct fields are amended for JSON when deciding which field to marshal or unmarshal. Getter and setters are not behaviours, they are state. purist, I do what is convenient and readable. Save my name, email, and website in this browser for the next time I comment. But again, the Go community does not have a strong convention against exposing data members in your type's public API. Embedding the fields instead of providing a field name allows to call the methods and arguments without an indirection, e.g. I would argue that you have a behaviour that returns data. - Defining a setter and getter in the interface may suggest an The text was updated successfully, but these errors were encountered: For getters, it would suffice to allow explicit conversion to a struct type with fewer fields, like. This example constructs a ReadWriter struct that points to embedded Reader and Writer structs. This would also handle &i.F neatly, by just leaving off the last dereference. Unfortunately, Go will print dog and exit. My opinion not to use interfaces to extend. Appending works fine with this, however I assume there's no way to retrieve the struct via function on the Package? Just a thought: rather than changing the definition of an interface to include struct fields, structs could be special in that struct fields can satisfy a zero argument interface method with the same name and type. https://play.golang.org/p/u7WJ2pYDCVt (line 22 is what I'm assuming is impossible since we dont know the user defined type?). We haven't seen any strong arguments in favor of this proposal. It is definitely a neat trick.

Clients of the interface perform operations purely through it so specific interface for exchange of information. Running the program again no longer yields universe but world. Also you'd need to know the types beforehand, so user-defined types are out. BicPens can be any of 4 select colors. > You are receiving this because you commented. representation of the type is hidden from its users, and the visible Interface has no field or method for an exported field in a struct, Range over element of interface implementation, Difference between abstract class and interface in Python. This case yields the expected universe value. But to my point, your are describing properties of the implementation; it has the field X, you can get it and set it thusly. hidden data structure there is no color state, only a temperature state. I'm gonna write an answer; I think it's fine if you need it and know the consequences, but there are consequences and I wouldn't do it all the time. Note the duplicate exported Color field with a JSON struct tag for the Dog and Cat struct. Already on GitHub? A field or method X hides any other field or method X in a more deeply nested part of the type. What is the difference between an interface and abstract class? That is not behaviour, David holds a pen, I got the pen and it was blue, is not behaviour, it is state. If you're on the fence, and the interface is only used within your project, maybe lean towards exposing a bare attribute: if it causes you trouble later, refactoring tools can help you find all the references to it to change to a getter/setter. It also implements the PersonProvider interface, so we can pass Bob into functions that are designed to use that interface. Python Difference between abstract class and interface in Python, What does it mean to program to an interface, C# Interface defining a constructor signature, The difference between an interface and abstract class, C# How to determine if a type implements an interface with C# reflection. It is definitely a neat trick. And setters and getters in the interface means that the underlying implementation can be completely different for unit testing. It would compile and fail at runtime.). I think allowing to express requirement for existence of fields in interfaces would have resolved this issue cleanly. > On 5 Jun 2018, at 08:42, David Skinner ***@***. Adding a Hello field to a higher layer hides the field in all more deeply nested parts of the type. That may be fine or even inevitable; it's just a consequence to know about. For example, proto.Message is required to be a pointer to a struct type with appropriate field tags. - We can have an opaque pointer to an undefined data type that has a On those rare occasions I use Setters and Getters in my interface. Confused on how to best go about this. Note that since the interface type could be implemented by struct types that put the field at different offsets, the seemingly simple expressions/statements like i.F and i.F = v would most likely be implemented by function calls. > Hiding properties behind getters and setters gives you some extra flexibility to make backwards-compatible changes later. This way, anyone can define a new box and add it to the package so long as it conforms to that interface. Hence, To access structures variable, we are going to use interface. As a silly example, I want to have a Package which contains a bunch of boxes which conform to the Boxable interface. An anonymous struct field of interface type is treated the same as having that type as its name, rather than being anonymous. Lets use the following example for the remainder of this section: What would you think this produces?

An anonymous struct field with a name given in its JSON tag is treated as having that name, rather than being anonymous. @jmaloney I think you're right, if you wanted to look at it plainly.

interface. Now, here's my question: is this a neat trick, or should I be doing it differently? Embedding types is a great way of sharing fields and methods and allows you to create constructions like union types. names of the properties with an upper case first letter. I'm not particularly for the original proposal, however I think @bontibon has an interesting take on the idea, and allows any type to implement the interface, while making the implementation easier for structs. 3. https://groups.google.com/d/msg/golang-nuts/ZJ5DEv_36S8/opZ__-l6XxAJ, proposal: spec: generic programming facilities, https://github.com/notifications/unsubscribe-auth/AAGBWOWjZ8222KIy5A8q13QgvgOX0u4-ks5t5maIgaJpZM4SCkBH, proposal: Go 2: express pointer/struct/slice/map/array types as possibly-const interface types, proposal: Go 2: generic native types (GNTs), proposal: Allow interfaces to define fields, spec: add generic programming using type parameters, proposal: Go 2: sum types using interface type lists.

Now we can use the interface and its implementations: Now, what you can't do is something like this: However, after playing around with interfaces and embedded structs, I've discovered a way to do this, after a fashion: Because of the embedded struct, Bob has everything Person has. Here is a Go Playground that demonstrates the above code. Yeah--a lot like virtual inheritance in other langs. Yup! On Tue, Jun 5, 2018 at 6:07 AM Dave Cheney ***@***. That flexibility is more relevant when the type is complicated (and the codebase is big). In this example, the method Sound() is forwarded to the Animal struct from the Dog struct, so we can call Sound() on an Animal.

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