tl;dr - No change in language, one minor consistency oversight fixed. Big changes on implementation, including compiler being bootstrapped.
- Generics in current form won't work across the board with all parts of Go.
- Generics of form that will work for Go across the board are not technically trivial.
Honestly, I appreciate Go's philosophy of not implementing until fully understood and accepted. Besides, Go's primary design principle is that of composability. So you're using reflection in one form or another. Achieving generic behaviour with interface and reflection is much more consistent than generics as we know from other languages.
From the outside, the two features (swift's protocol with extensions and go interfaces) seem a bit close, so that made me wonder. I didn't have the time to think too much in detail about it, so i'm just wondering if anybody already had.
Just off the top of my head - boxed types, for example lead to Java-style inheritance patterns. This sits awfully with composability and readability of Go code. You read Go code like a tree. You read Java code as multiply linked list.
There are technical drawbacks in the implementation of parametric polymorphism that don't suit Go's design goals: http://research.swtch.com/generic.
"We just can't be bothered" or "we don't think they're much good, and we don't have a need for them" would be a much more realistic answer.
Go isn't interested in either being slow. Any generics solution has to have fast runtime, because they need to replace the builtin parametric slices and maps. And the compiler is already too slow, in fact there's a lot of work slated to try and make it faster.
Lots of effort went into studying the problem, so it most definitely isn't an issue of "can't be bothered".
I think the error in that reasoning is attributing this issue to generics specifically when this is actually a problem with code reuse in general.
Say I have a linked list and want to use it to hold both integers or strings. My two options, in any language—whether that language has generics or not—are: (1) write specialized code for every type I want to use it with (go generate), potentially bloating the code and resulting in extra compilation time; (2) use some sort of existential type (interface{} in Go) and share the code but pay a performance cost at runtime. The dilemma arises because of the problem itself, not because of generics.
It is of course right that there is a tradeoff here (although there are many other potential solutions that aren't at one extreme or the other, .NET-style JIT compilation or intensional type analysis for example). But not having generics doesn't eliminate the tradeoff. Generics are just a way for the compiler to automate the work that a programmer would otherwise have to do. If you don't have generics, you still have that dilemma, except that you have to write the code yourself instead of the compiler doing it for you.
It seems (and this is purely an analysis of my own experience and the large amount of Go I read inside Google) that the types of data structures and algorithms I need parametrized follow a power law distribution. Mostly I need a list, or a hash map, and the builtin slice and map types almost always meet my needs. For the long tail of parameterized types in that power distribution, it appears that either it is not a performance hot spot, in which case I can borrow from the dynamic dispatch tools in the language (that is, some variant of an interface{}), or performance is so critical that even in C++ there is no obvious generic data type.
The latter is an interesting case I ran into in my prior life as a C++ programmer, and in OCaml. Someone had created a generic version of a data structure (usually more than one someone), but what I needed in my performance hot spot was some variant that wasn't captured by the parameterization.
An old example that comes to mind: I inherited a struct whose memory layout described a wire protocol. It had an 8-byte piece of padding in the middle of it. That was the perfect place to hide the pointer for a linked list I wanted to create in an intermediate step. Of course, none of the many template-based linked list data structures I had in C++ could do that, so I rolled my own.
So as unexpected as this may be, I find a combination of map/slice, interface{}, and rolling my own meets my generics needs in Go well enough that I wouldn't want to trade off slower compilation for it, or bad compile-time error messages. I still miss generics, and it may be that the particular kinds of programming I do mean I miss them less than others. A co-worker with nearly as much Go experience as I have says he misses them more often. The ones that come up most often for me are sorting and some kind of ordered map, but even those bug me less than once a month.
People that didn't find it to be a problem (e.g. due to the kind of stuff they are working on) are not the ones concerned about its lack though.
It's like someone saying "I just do web development with Python, so I see no reason for NumPy to exist".
I'm not trying to shift the argument, I'm to explain the importance of Go in practice. There is a generics dilemma that programmers pay today and it should be possible to turn that into a language feature with the same tradeoffs. In practice, I don't think a generics language feature that captures how I program in Go is possible. (Or at least, I haven't thought of one nor seen a proposal that does.)
Like many others, I'd like to see a prototype that proves me wrong.
It would not slow down compilation. Slices and maps are just built-in generics. Generics would codegen exactly the same way as slices and maps do now. Slices and maps compile down into calls to builtin runtime functions (using intensional type analysis IIRC); a generic version of them would, if implemented properly, call down into those exact same functions.
> There is a generics dilemma that programmers pay today and it should be possible to turn that into a language feature with the same tradeoffs.
Yes, it is possible and many languages have done it. All Golang needs to do is what Swift (just to name probably the closest analogue) does, with interfaces for runtime dispatch and generics for compile-time monomorphization. Or, if you want to continue using intensional type analysis to reduce compile time, implement that as certain variants of OCaml did (though you'll pay a performance cost to do so and I don't think it's worth it).
That said, your second point is good, my argument is weak because the compiler could stick to just specializing maps and slices and otherwise boxing. I strongly suspect any passable generics implementation for Go will need to do this (which due to necessary stdlib changes is off the table until Go 2, so understand I comment on this topic purely for the sake on conversation, I don't think anything can be done any time soon). The degree to which it needs to do it, I'm not sure.
There is still an orthogonality issue with interfaces. There's a lot of overlap between a dynamic dispatch mechanism and parametric polymorphism. As an API designer I'm a bit worried about it. I suspect if a good generics implementation came along though that argument would get pushed aside.
On this topic: one of the better prototypes I saw had a lot of trouble producing good error messages. I suspect this is a solvable problem, but how is not clear to me yet.
Also, we call it Go, not Golang.
Or do you think that just maybe using speed as a reason not to do something is a bit of a cop out. Especially without any benchmarks to back it up.
The Go 1.5 compiler is not slower because it is written in Go, it is slower because it was mechanically translated from another language. Human eyes will improve it over time.
Garbage collection is a throughput hit the language designers (and I) are willing to accept for improved safety and simpler APIs. I'd rather not pay it, but I choose it over spending a large chunk of my API documentation describing various ownership scenarios like I did in C++. It's a tradeoff I find acceptable for most programs. You won't find me using a GC on a clock slower than 100 MHz, or in a sub-millisecond realtime system (but I probably won't be using linux either there).
I'm also willing to pay the performance hit on non-critical generic code, and I use interface{} for that where I can. If benchmarks show it's a problem, I'll do something differently. That might be hand-rolling an algorithm, which is unfortunate for the programmer who follows me and has to read it. But it doesn't come up much.
The performance price for generating large amounts of extra code in the compiler is reasonably well understood, and not something that would be amenable to simply trying and benchmarking. One would have to implement generics, then spend several programmer-years tuning the compiler over various programs, compilers are complex machines.
Again, I haven't met a compiler expert who doesn't think that generating code for widely used generics would be expensive. And they would be widely used, any good generics solution would have to replace maps and slices, and would permeate the standard library.
Citation needed.
I've seen absolutely zero sample implementations, testing of implementation options, etc work going in with regard to adding Generics in Go.
And, aside from a couple of blog posts and shooting down people asking for it in comments, I've seen no real organized discussion, e.g. like what would go on for a Python PEP to be accepted.
They've been saying that from the begining. There are no real technical constraints, it's been done in all kinds of languages and it has been a well understood feature for 2+ decades. They just don't want to do the compromises needed, while letting the developers continue to deal with all of them...
See also pcwalton's (of Rust fame) comment below.
The lessons learned don't necessarily translate well between languages, because different problems need to be solved. Each language is a bag of features that need to work well with all the other features included in the language.
I have yet to see a full technical proposal that integrates well with the existing language features and has no nasty corner cases. If such a thing existed, I'd be among the first to stand up and applaud it.
The best solution currently (IMHO), is code generation with things like gen [1]. Though I have yet to use that in a real project, so I can't say for sure it is actually the best solution.
Apparently they don't want to support only boxed types (like Java), and they also don't want to generate multiple implementations of each generic function for each size, resulting in code bloat (like C++), or to generate code at runtime (like, say, Julia).
You could argue that they should just make a choice and go with it because generics are so important, and some language designers would do that, but then these differences in design goals are why we have different languages in the first place.
This is based on an early article by Russ Cox [1]; I don't know how the teams' position has evolved since then.
type V interface{}
Don't hold your breath when it comes to generics. There is no way they can retrofit them without breaking the language, it's too late. Even features like covariance or unions are just out of question.
An option would be to write a super-set of Go with generics that would compile to Go code. what it would do basically is use interface{} everywhere the variable type T is required and insert type assertions for the user. Been thinking about that.
I think that's pretty much how you'd want to do it, it's type erasure just like generics in Java - sure, some people complain about not having reified generics, but I'm more concerned about type-safety, Go already has some runtime reflection capabilities anyway.
You could call it go++!
(But seriously, that's pretty much what C++ did to C.)
We have this discussion after every minor release. 1.6 will be released in December. See you then.