Is Go an Object-Oriented Language?

Is Go an Object-Oriented Language?(spf13.com)

73 points by spf13 11 years ago | 69 comments

optymizer 11 years ago |

The problem with the author's case for Go's is-a relationships is that it breaks down the moment you want to pass the object to a function expecting the original object.

For example, http://play.golang.org/p/EmodogIiQU

    type A struct { }
    type B struct { A }  //B is-a A

    func save(A) { //do something }
 
    b := &B{}

    save(b);  //OOOPS! b IS NOT A

If Go had is-a relationships, the code above would be valid. Instead, Go only implements has-a relationships, and simply provides shortcuts to calling B.A.foo() as B.foo().

One could create B.save(), which would call save(b.A), but the very reason you're now proxying the call to save() is because there is no is-a relationship in Go.

We all know about interfaces, but the problem is that is-a relationships do exist, and you can't always use interfaces, because often you want to share the data encapsulated by the objects, not only the behavior. One ends up creating methods to fetch each piece of data, but in code that is supposed to be performant, calling methods instead of accessing fields is suboptimal.

tolmasky 11 years ago | |

In my experience its always really telling that there are no incredibly simple is-a relationships to use in inheritance justifications (as opposed to tons of goto real world examples for just about every other feature under the sun). Its always either incredibly abstract (B is-a A), or incredibly contrived (Triangle is-a Shape). I've spent a lot of time in inheritance heavy code, and I've yet to find something that wouldn't be just as, if not more, elegant without inheritance. I've spent the most time in Cocoa (which I believe to be very well designed BTW), but the inheritance there is clearly not needed IMO. I usually find one of the following to be true:

1) Very shallow inheritance trees that could have very easily (and more logically) been replaced with interfaces. For example, NSResponder being everything's superclass even though just about all of its methods are empty implementations ("subclassers responsibility"), aka, it clearly should have been an interface.

2) Confused/strangely complex is-a relationships (mutable array is-a immutable array, what? so if I specifically specify NSArray, I may still get a mutable array and the type system will be happy???).

3) Strange rules around what methods are overridable, and more importantly, how specifically they can be overridden. Why can't I in a UIView subclass override -subviews to ensure that it always has the same subviews? Well, implementation detail, that's why. Normally this would be fine, but since anyone is allowed to muck around in a subclass, suddenly I need to know the way it was implemented. This of course conflicts other parts of the framework where you are definitely expected to override the method and not use a setter.

I have yet to be presented with one of these "killer" is-a relationships that must exist. If the sole excuse is "performance", then sure I'll conceded. I guess I don't work in environments where member access is the performance bottleneck so I guess I can't relate.

millstone 11 years ago | | |

Views in UI toolkits are the best example I know for an is-a relationship. A button is a view, a slider is a view. A container may have heterogenous children, but all of them are views. An interface doesn't work, because you would have to implement basic properties like containment, geometry, etc. separately for every widget.

To my knowledge, nobody has found any better design than this, though some have found worse ones (e.g. HTML and its bizarre input element).

To your specific points:

1. Yes, NSResponder should definitely have been an interface. See for example the weird way that documents and delegates are spliced into the responder chain.

2. The fact that NSMutable* is-a NS* is a lovely design. Mutability can be understood as adding setters to a class that doesn’t have them. The alternative seems to be weird duplicative splits like ArrayList/Array, or String/StringBuilder/CharSequence.

3. Agreed; it’s a failing of ObjC that it’s usually unspecified whether a method is designed to be called, to be overridden, or both. I wish this were enforced at the language level.

pcwalton 11 years ago | | |

> In my experience its always really telling that there are no incredibly simple is-a relationships to use in inheritance justifications (as opposed to tons of goto real world examples for just about every other feature under the sun). Its always either incredibly abstract (B is-a A), or incredibly contrived (Triangle is-a Shape). I've spent a lot of time in inheritance heavy code, and I've yet to find something that wouldn't be just as, if not more, elegant without inheritance.

The flow tree (render object tree) in Servo (or any other browser engine) must use inheritance: we have a heterogeneous tree of objects that all share a common set of fields (position, intrinsic widths, collapsible margins, some various bits that store state during reflow), but they all use virtual methods because they must lay out their contents differently.

We can't use composition because we wouldn't get virtual methods. We can't use an interface because then we would be forced into virtual dispatch for all of those fields that are shared between flows.

Rust doesn't have OO yet either, so we're forced to hack around it in weird ways (usually via a small amount of unsafe code to simulate inheritance).

> I have yet to be presented with one of these "killer" is-a relationships that must exist. If the sole excuse is "performance", then sure I'll conceded. I guess I don't work in environments where member access is the performance bottleneck so I guess I can't relate.

A browser engine is exactly that sort of environment. Forcing all member access to go through virtual dispatch would murder the performance of any browser.

Note that this was exactly the sort of thing that OO was designed for in Simula: heterogeneous trees of objects that all share some common fields but have different virtual methods. This generalizes to GUI libraries, game worlds etc—in short, simulations :)

noblethrasher 11 years ago | | |

Inheritance is usually a mess because people use it to model objects when they should instead be modeling types: a triangle is not a shape, but a Shape is a sum type of Circle, Square, Triangle, etc.

cheepin 11 years ago | | |

I see 'is a' and think algebraic data types.

lloeki 11 years ago | |

IOW, the Liskov substitution principle is unsatisfied. The language has chosen by design not to resolve "methods"† up the "hierarchy"†, therefore the only way to reinstate it is to implement func save(B) { }, which has either the benefit of making you think whether you need to persist more fields or make explicit that you don't. And as you said, implementing proxy accessors to satisfy an interface just to simulate inheritance is obviously not the right choice.

† neither of which exist, since there's no objects and no inheritance. In simplifying things, this brings a constraint. Given how method resolution is a pain point WRT implementation complexity and performance (see e.g Ruby), this is a reasonable tradeoff. I am glad we have such an interesting choice of languages.

jaekwon 11 years ago | |

Just call save(b.A) or save(&b.A).

No need for methods, is performant. I can't think of a reason--besides extra typing--why this wouldn't be sufficient.

http://play.golang.org/p/7vE_wN6EBv

AnimalMuppet 11 years ago | | |

It's sufficient, in that it does what you want - it calls save() on b's A.

But it's not sufficient if what you want is to treat a B as an A. There is no is-a, so you have to keep treating a B as something different from an A.

npat 11 years ago | |

Why not?

    type Saveable interface {
        ...
    }

    func save(a Saveable) { .... }

    save(a);  // ok, assuming A is Saveable
    save(b);  // necessarily ok, given than A is Saveable

adamlett 11 years ago |

I would say the defining property of OO, is polymorphism. It's the property that allows some code to call a Bar() method on object Foo and not be concerned with the exact type of object Foo. Without polymorphism, there is little differnce between Bar.Foo() and Foo(Bar).

Implementation inheritance is a property of _some_ OO languages and one that is hard to imagine separate from OO. Which is why perhaps so many insist upon it being a required property for some language to be called OO. I am firmly in the camp that thinks implementation inheritance is a bad idea and it is best to avoid it even in langauges that support it. Thus I don't agree with anyone who claims that it is an important characteristic of a language.

Whether object instances find their genesis in classes, factories or prototypes are IMO the least important aspect to consider when discussing whether or not some language is truly OO. It's the object instances that do the important work. Where they came from is not so interesting.

abrahamsen 11 years ago | |

> I would say the defining property of OO, is polymorphism.

Make it dynamic polymorphism, and I agree.

groovy2shoes 11 years ago | | |

Subtype polymorphism. Dynamic dispatch.

http://en.wikipedia.org/wiki/Subtype_polymorphism

http://en.wikipedia.org/wiki/Dynamic_dispatch

Polymorphism is a feature of the type system, and thus inherently static. Dynamic dispatch is something you often wind up with as a consequence of subtyping, but neither requires the other, strictly speaking.

panzi 11 years ago | | |

subtype polymorphism

asgard1024 11 years ago |

I have to say, I like Go model better than "standard" OOP (as in Java, C#..).

Class is a single construct used for three different abstractions, namely:

- modularity/hiding

- inheritance

- polymorphism

This eventually turned out to be a bad idea (as evidenced by all the mess with virtual methods, multiple inheritance and structural patterns), and interfaces (and namespaces) were added to partly remedy this.

In Go, you instead get three orthogonal constructs:

- modules

- embedded structures

- interfaces

These directly correspond to basic principles of OOP. Nice and clean.

dragonwriter 11 years ago | |

I like that Go and Rust reexamined some of the underlying traditions of the C++/Java/C# approach to OOP rather than reflexively repeating them -- and while I think Go and Rust both, on a high level, took good (but very different) approaches, I think the one thing that Rust did right that would be better even with the rest of Go's approach than the way Go did it is explicit and detached declaration of interface implementations for data types.

NateDad 11 years ago | | |

So you're saying you'd prefer it if you had to explicitly tell Go that your type implements an interface? Something like this?

  type Shape interface {
      Area() int
  }

  type Square struct {
     sideLen int
  }

  // Somehow denote that this function is implementing
  // Shape's Area function
  func (s Square) Shape.Area() int {
      return sideLen * sideLen
  }

Because, one of the things I like best about Go's interfaces is that you don't have to do that.

stcredzero 11 years ago | |

One could say that Go errs on the side of caution, making sure it doesn't have too many of the trappings of OOP.

jnks 11 years ago |

The author of this blog post is a little confused about embedded structs. His examples of has-a and is-a are both has-a's, and the syntax change involved (leaving off a name for the embedded type) doesn't actually do anything.

  type Person struct {
     Name string
     Address Address
  }

is equivalent to

  type Person struct {
     Name string
     Address
  }

And in fact, these are equivalent too:

  p.Address.Zip = "01313"

  p.Zip = "01313"

http://play.golang.org/p/aKH3YxT5Mb

Go doesn't really support is-a for structs, as pointed out elsewhere in the comments here. Interface implementation is the only way to get the sort of "this type can be substituted for this other type" idea that is-a inheritance provides in other languages.

ChuckMcM 11 years ago |

Reminds me of the (possibly) apocryphal story of Nickolas Wirth telling his audience at Apple that Modula-2 was OO and having one of the audience members object. To which Dr. Wirth replied, "Who are we to say what object oriented means exactly?" and the objector, who turns out to be Alan Kay, says, "Well I invented the term so I get to define it, this isn't object oriented."

I would say that similar objections would be made about Go calling it self 'object oriented' however I also don't know what is being asserted.

Go has many constructs that make abstraction easier, and that is what many programmers want out of the OO idea, so its fine. I'm sure there specific things that some people require before they will label something as OO. Is it a functional question or a religious question as to whether or not Go is Object Oriented?

jbert 11 years ago |

Something I've done in some golang code recently is to fake some OO features. I'd appreciate some commentary on the approach.

So I want a few different, but similar things. These are actually stages in a processing pipeline, each stage doing different processing steps.

What I'm currently doing, which mostly works well, is to have a struct type ('Stage') which does all the generic work (equivalent to an abstract base class in C++). The Stage contains a function ptr ('Each') to actually do the processing step.

I can then have various 'derived' types which embed 'Stage'. Each one is assembled via a ctor which sets up the 'Each' function ptr. Effectively this provides inheritance with method overloading for the Each function.

I also have an interface ('Stager') which is satisfied by the Stage type, and so consequently by all the derived types (since they embed Stage).

So, I seem to have most of the benefits of C++ abstract base class and 'inheritance' (of data and methods), including overriding of methods by subclasses (using explicit assignment to function ptr).

It feels pretty nice to work with. The main concern I have is the slight klunkiness of the Stage/Stager duality. I also don't think I'd like this if I had many overridden methods (I just have one atm).

Anyone care to comment on a better way to this or other critique?

chimeracoder 11 years ago |

> Since a standard definition doesn’t exist, for the purpose of our discussion we will provide one.

Who said a standard definition doesn't exist? From Alan Kay, who 'invented' object-orientation and coined the term:

> OOP to me means only messaging, local retention and protection and hiding of state-process, and extreme late-binding of all things. It can be done in Smalltalk and in LISP. There are possibly other systems in which this is possible, but I’m not aware of them[0].

Yes, you can make the argument that the term has evolved in common parlance beyond what Kay originally conceived of, but it's silly to propose a "modern" definition of object-orientation and not at least mention the original definition.

[0] From a 2003 email: http://userpage.fu-berlin.de/~ram/pub/pub_jf47ht81Ht/doc_kay...

[1] Note that he does not mention Java, even though he write this during the height of Java's popularity: http://www.tiobe.com/index.php/paperinfo/tpci/Java.html

vidarh 11 years ago | |

Kay's definition is by no means "standard". "Standard" usage of the term started deviating from Kay's definition almost as soon as it was conceived. By time the term was widespread, it already meant something different to what he envisioned.

It may be "silly" not to mention the original, but in this context the original a distraction: the original definition would exclude pretty much every language we today tend to consider object-oriented.

kd0amg 11 years ago | | |

The blog post gives us, "To me this feels very much like an object. I am able to create a structured data type and then define methods that interact with that specific data." Defining OOP as functions that consume structured data fails to exclude a lot of languages we don't consider object-oriented.

AUmrysh 11 years ago |

You can do this same thing in C to get Objects.

Also I believe there may be a typo in your last paragraph

>while staying clear of the brittle mess than is inheritance

It should be "that" instead of "than", I think.

jerf 11 years ago | |

C is not an object-oriented language because it has no language features intended to make objects easier to use or work with. You can't actually attach "methods" to structs in C.

You can use objects in C, and there's numerous in-the-wild big examples. But you have to implement it yourself or use some library, and there isn't one "standard" for the language.

Go clearly does have some features designed to make objects a first-class element. Equally clearly there are some other languages that have "more" such features.

This post I'm making is merely descriptive; there's no positive or negative attached to these statements.

AUmrysh 11 years ago | | |

Thank you for clarification. I've not really used Go, so I don't know how it differs from C for objects. I'm a little familiar with using structs in C for OOP, I apologize if my comment came across as to lessen the article, because I found it pretty interesting, and I love to see things like this where a new programmer might have their eyes opened to something those of us who've been around a while might already know from somewhere else.

seanmcdirmid 11 years ago | | |

C has nice infrastructure for building v-tables that I found very useful back when I was writing in C. Heck, I wasn't even aware that I was merely reinventing objects at the time.

Go is definitely an object-oriented language. It lacks classes, but those aren't required. We might argue that structural subtyping isn't really very OO. But given Go's lack of generics, this is a moot point.

slm_HN 11 years ago | | |

"You can't actually attach "methods" to structs in C."

You can have structure members that are function pointers which is basically attaching a "method".

ben336 11 years ago |

I think the "object"-less OO tag is a bit overblown. Structs are objects. Just because Go creators didn't choose to name them that doesn't mean there's a fundamental difference between Go structs and other languages' objects.

grey-area 11 years ago | |

The fundamental difference (and there is one), is that Go omits the concept of inheritance, preferring composition instead. So in contrast to many mainstream languages today - Java, Obj-C, C#, C++, Ruby, Python, PHP , etc. there is no concept of inheritance, only composition of objects and conforming to interfaces.

There's a good quote in the article about this under 'Inheritance Is Best Left Out' - James Gosling responds to someone asking what he'd change in Java in retrospect - “I’d leave out classes,” he replied.

voidlogic 11 years ago | | |

>>The fundamental difference (and there is one), is that Go omits the concept of inheritance, preferring composition instead

Right, but that doesn't mean Go is not object oriented, it simply eschews inheritance for composition. Go also offers something else n lieu of inheritance that many other OO languages don't have, embedding.

mseepgood 11 years ago | | |

"O-O is important because it provides uniformity of interface. Subtype inheritance encourages non-uniform interfaces." - Rob Pike http://www.infoq.com/presentations/Go-Google 43:00

afternooner 11 years ago | | |

Yea, I liked the idea of what go was waning to do. They wanted classes and objects without the classes and inheritance, but all they've done is replace that headache with an interface headache.

Jare 11 years ago |

Surprised there is no mention of hiding or message passing, which are two more common aspects of Object Orientation. Without them, I think the idea that methods are "attached" to objects is rather incomplete.

jdmichal 11 years ago | |

More common, but certainly not required. For instance, JavaScript does not provide hiding at all as part of its OO mechanisms. (Implementations that allow hiding use closures to provide it.)

Jare 11 years ago | | |

Exactly, that would fit with the tone of the article, which does away with the idea of OO defined in terms of axioms like inheritance, etc. and instead explores how these pieces of the OO picture work and relate to each other.

kd0amg 11 years ago |

What have’t we done.

Attached that behavior to the struct itself so that invoking a struct's "area" function could give any of several different behaviors depending on exactly what "rect" struct you have.

jerf 11 years ago | |

To do that in Go, you just add:

    type Area interface {
        area() int
    }

though I'd note I'm deliberately just copying the article as written, as an "area" that's confined to an int is awfully weird.

Note you can indeed just add that, and you're able to take "Area"s anywhere you like, and you can even do it in a different module entirely; nothing has to "declare" than a rect implements Area.

To forstall the usual next question, no, Go has no further overloading based on type or anything else. My personal advice if you want to use that a lot is to use a different language. I like Go, but I look on the people trying to use it for machine learning or matrix math or other intensely mathematical computational loads with a bit of mystification. It isn't what it's good for, and I see no sign the core devs even consider it a marginal use case (to say nothing of a core use case) and have no intentions of changing the language to make this easier. If you really, really need overloading for your core use case, pick something else. Go is built for the environments where overloading is generally dangerous and used by people to do excessively "clever" things on the server, not for the environments where it is necessary. (Or perhaps "environment", singular, since "intensely mathematical code" is the only such thing I know of; everywhere else I've ever seen it it's asking for trouble.)

rakoo 11 years ago | | |

> Go has no further overloading based on type or anything else

You can overload in Go: see for example how a zlib compressor is implemented [0].

The gist is that you take a standard io.Writer, embed it in your struct, and override the Write() method. This way you have a new io.Writer you can use wherever a io.Writer is needed. This pattern is actually standard in Go (and I guess in other languages where interfaces are more important than implementations)

Or maybe I didn't understand ?

[0] http://golang.org/src/pkg/compress/zlib/writer.go?s=4340:439...

NateDad 11 years ago |

This is honestly kind of a dumb question, because the answer depends on who is asking the question. Object oriented has become such an overused and little understood term, that you can't just give an answer without trying to discern what the person asking is looking for. There's almost always a better question to ask than "Is x language object oriented?".