Golang concepts from an OOP point of view

https://github.com/luciotato/golang-notes/blob/master/OOP.md

by markkit • 10 years ago

200 46 10 years ago

This is a discovery process, I'm writing this document to help myself understanding golang and maybe help others.

First: I took a brief look through this, and I didn't see anything that jumped out as wrong. It's a pretty comprehensive document, especially for someone who says they're just starting out, so kudos to James for writing it!

That said, from my experience in teaching Go classes and workshops for beginners, comparing embedding to inheritance is usually a bad idea. It's not that there aren't similarities, but in my experience it prompts new Go programmers to write unidiomatic Go code when they think of embedding in that way. They end up embedded structs the way they'd use inheritance in object-oriented languages, and since Go isn't really intended as an object-oriented language, it makes for unidiomatic code.

The main use cases of inheritance in OOP are accomplished by interfaces in Go, not struct embedding. But at the same time, the main benefits of embedding come from embedding interfaces - embedding structs is almost an afterthought in the language. In fact, I tend to advise people against embedding structs altogether. There are a few cases where it's worthwhile to embed structs, but the use case for embedding interfaces is much more powerful.

My advice for Go beginners: use interfaces more than you think you need to, and forget that struct embedding is even possible. This rule of thumb helps to write more idiomatic Go code when you're just getting started, before you have a feel for what that really means.

weberc24• 10 years ago

Interesting. I'm moving away from interfaces and toward plain structs (not consciously, but naturally). I tend to only use interfaces when I want polymorphism, not modularity. This is probably a consequence of me moving away from a mock-all-the-things approach to testing.

skybrian• 10 years ago

I recently heard a rule of thumb: "accept interfaces, return structs".

I haven't put it into practice, but it seems like this should give callers enough flexibility to pass in whatever they like.

solipsism• 10 years ago

Toward what kind of testing, mind sharing?

catnaroek4• 10 years ago

Go discourages classical inheritance, but otherwise it seems pretty object-oriented to me. Interfaces are object types. The only operation they support is calling methods. That's pretty much what objects are: self-contained entities with internal state (fields) and externally visible behavior (methods).

If anything, C++ is less object-oriented than Go, and more oriented towards abstract data types (which C++ programmers call “concepts”).

aikah3• 10 years ago

Go discourages classical inheritance,

Go makes classic inheritance impossible, it is a bit more than discouraging it.

oelmekki• 10 years ago

Is embedding interfaces still composition, in your opinion? And if not, does this means you recommend not using composition?

ratstew• 10 years ago

I just want to point out that embedding structs does not look like an afterthought at all, the Plan 9 C compilers had a feature akin to it for ages.

http://plan9.bell-labs.com/sys/doc/comp.html

lloeki3• 10 years ago

"Go is a great OO language" is quite a simple and interesting take on it.

https://www.youtube.com/watch?v=HqZQBmNVhBw

timtadh• 10 years ago

If you like these notes you might also like my article on the subject: http://hackthology.com/object-oriented-inheritance-in-go.htm...

In general, using struct embedding to simulate sharing code and data is pretty limited. I rarely use this feature in my own code. However, having structs which automatically implement interfaces is awesome because you can "say what you need." You can also compose the interfaces together by embedding them. For example of that, checkout the Map type in my data-structures repository: https://godoc.org/github.com/timtadh/data-structures/types#M...

dizzy3gg• 10 years ago

thanks, this is helpful for quick understanding of go

gtrubetskoy• 10 years ago

"receiver implict this parameter"

The parameter is actually quite explicit (unlike some other languages).

It would probably help your write up to talk about the difference between (using your example):

  func (r Rectangle) Area() float64 {

and

  func Area(r Rectangle) float64 {

As well as (this is a common rookie mistake):

  func (r Rectangle) SetWidth(w float64) {
    r.Width = w // does nothing

and

  func (r *Rectangle) SetWidth(w float64) {
    r.Width = w

SomeCallMeTim• 10 years ago

You might want to point to point that (if?) the second example does work? (And that the difference is the *)

Animats• 10 years ago

Go has most of the machinery of objects, but not the encapsulation. There are no private fields in structs.

barsonme2• 10 years ago

Well, yes kind of.

    type foo struct {
        a int // "private"
        B int // "public"
    }

You cannot specify default values and Go refers to them as "exported" not private, but technically they are "private" in the sense that they can only be accessed via methods.

kuschku2• 10 years ago

is a field named "ß" private or public? Or ĸ?

gtremper• 10 years ago

Technically, exported/unexported is different than public/private since its at the package level. You can access unexported fields of a struct from anywhere within the same package (not just from that struct's "methods"), which isn't true of a 'private' field.

david-given• 10 years ago

There are, actually --- any field which begins with a lower case letter is private, any field which begins with an upper case letter is public. (Don't ask me what happens if the field begins with a letter which has a single form and does not have an explicit upper or lower case.)

Privacy is only enforced between packages, so anything in the same package as the struct can see the private fields.

lloeki3• 10 years ago

Don't ask me what happens if the field begins with a letter which has a single form and does not have an explicit upper or lower case.

So, on to the spec: https://golang.org/ref/spec#unicode_letter

In The Unicode Standard 8.0, Section 4.5 "General Category" defines a set of character categories. Go treats all characters in any of the Letter categories Lu, Ll, Lt, Lm, or Lo as Unicode letters, and those in the Number category Nd as Unicode digits.

Which reads:

    Lu = Letter, uppercase
    Ll = Letter, lowercase
    Lt = Letter, titlecase
    Lm = Letter, modifier
    Lo = Letter, other

So I suppose (but am not sure) the case you describe falls into Lo.

lucio• 10 years ago

Author here, any pull request fixing typos, spelling and grammar, is welcomed.

pjmlp• 10 years ago

I would use the real name of the language, Go.

Other than that, nice article.

morecoffee• 10 years ago

Struct embedding has been mostly unpleasant in my experience. When a type is embedded, only the methods that are explicitly on the interface/struct actually get propagated, rather than all of public methods. This means that wrapper types break optimizations around type punning. For example: https://play.golang.org/p/vIo1HtAeb4

This happens within the standard library in ways that hurt performance. The 'image' package allows people to register their own decoders but limits the API to an io.Reader. It can be an optimization if you could cast the inputted reader to another more efficient type (like a ReaderAt), but you can't because the image package wraps the Reader in its own "peeking" type. There's no way to tell the type was embedded, or what other actual methods might have worked.

gabesullice• 10 years ago

While I appreciate that this is a work in progress and meant for learning, i just want to caution readers and those new to Go that this is a somewhat dangerous way to approach Go.

Simply mapping familiar vocabulary from an traditional OO language into Go implies that the differences are superficial and the underlying concepts are largely the same. The truth is that a lot of the traditional vocabulary was very intentionally left out of Go.

For example, mapping embedded stucts to "multiple inheritance" masks the fact that Go is trying to guide you into understanding the power of composition over inheritance. It's not that it's impossible to understand that, but by carrying over the terminology, you burden yourself with some preconceptions.

Like I said, I know this is a work in progress, but I encourage you to perhaps map the terms but then write a paragraph or two about how they're _different_, not the same.

This was a really nice resource for me when I first came to Go: http://spf13.com/post/is-go-object-oriented/

coldtea• 10 years ago

Simply mapping familiar vocabulary from an traditional OO language into Go implies that the differences are superficial and the underlying concepts are largely the same. The truth is that a lot of the traditional vocabulary was very intentionally left out of Go.

I find that what has been "intentionally left out of Go" is not necessarily a good choice.

I also don't get the obsession with "idiomatic" programming.

Idiomatic Java/J2EE circa 1998-2008 was total crap (the programming style, not the language). Even its then adherents admit it. Same holds for many other languages/times.

Programmers should be consistent with a style, and teams should work with an agreed upon style. But some languages designers to impose their ideas upon all users of a language is wrong.

logicchains• 10 years ago

Idiomatic Java/J2EE circa 1998-2008 was total crap (the programming style, not the language)

I think part of the aim of Go was to make this style impossible, by e.g. removing inheritance, which plays a key role in most crappy Java code.

matteuan• 10 years ago

I think it also highlights how a lot of programmers have learned everything only from one point of view. Concepts like interfaces, structures and types are general and abstract, they should not be explained in terms of OOP, instead they should be used as tools to explain OOP.

Golang concepts from an OOP point of view

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