---
title: We Already Have Go 2
date: 2022-05-25
tags:
- golang
- generics
- context
- modules
---
I've been using Go since Go 1.4. Since I started using Go so long ago, I’ve
seen the language evolve significantly. The Go I write today is roughly the same
Go as the Go I wrote back when I was still learning the language, but overall
it’s evolved and changed into something similar yet different feeling in
practice. Thinking back over the years, here are some of the biggest ticket
items that really changed how I use Go on a daily basis:
* The compiler rewrite in Go
* Go modules
* The context package
* Generics
This is a good thing. Go has had a lot of people use it. My career would not
exist in its current form without Go. My time in the Go community has been
_catalytic_ to my career goals and it’s made me into the professional I am
today. Without having met the people I did in the Go slack, I would probably not
have gotten as lucky as I have as consistently as I have.
Releasing a "Go 2" has become a philosophical and political challenge due to the
forces that be. "Go 2" has kind of gotten the feeling of “this is never going to
happen, is it?” with how the political forces within and without the Go team are
functioning. They seem to have been incrementally releasing new features and
using version gating in `go.mod` to make it easier on people instead of a big
semver-breaking release.
This is pretty great and I am well in favour of this approach, but with all of
the changes that have built up there really should be a Go 2 by this point. If
only to make no significant changes and tag what we have today as Go 2.
Take everything I say here with a grain
of salt the size of east Texas. I am not an expert in programming language
design and I do not pretend to be one on TV. I am also not a member of the Go
team nor do I pretend to be one or see myself becoming one in the
future.
If you are on the Go team and think that something I said here was observably
wrong, please [contact me](/contact) so I can correct it. I have tried to
contain my personal feelings or observations about things to these conversation
snippets.
This is a look back at the huge progress that has been made since Go 1 released
and what I'd consider to be the headline features of Go 2. Most of this is a
whirlwind tour of over a half-decade of improvments to the Go compiler, toolchain
and standard library. I highly encourage you read this fairly large post in chunks
because it will feel like _a lot_ if you read it all at once.
## The Compiler Rewrite in Go
When the Go compiler was first written, it was written in C because the core Go
team has a background in Plan 9 and C was its lingua franca. However as a result
of either it being written in C or the design around all the tools it was
shelling out to, it wasn’t easy to cross compile Go programs. If you were
building windows programs on a Mac you needed to do a separate install of Go
from source with other targets enabled. This worked, it wasn’t the default
though and eventually the Go compiler rewrite in Go changed this so that Go
could cross compile natively with no effort required.
This has been such an amazingly productive
part of the Go toolchain that I was shocked that Go didn’t have this out of the
gate at version 1. Most people that use Go today don’t know that there was a
point where Go didn’t have the easy to use cross-compiling superpower it
currently has, and I think that is a more sure marker of success than anything
else.
This one feature is probably at the heart of more CI flows, debian package
releases and other workflows than we can know. It's really hard to understate
how simple this kind of thing makes distributing software for other
architectures, especially given that macOS has just switched over to aarch64
CPUs.
Having the compiler be self-hosting does end up causing a minor amount of
grief for people wanting to bootstrap a Go compiler from absolute source code
on a new Linux distribtion (and slightly more after the minimum Go compiler
version to compile Go will be raised to Go 1.17 with the release of Go 1.19
in about 6 months from the time of this post being written). This isn't too
big of a practical issue given how fast the compiler builds, but it is a
nonzero amount of work. The bootstrapping can be made simpler with
[gccgo](https://gcc.gnu.org/onlinedocs/gccgo/), a GCC frontend that is mostly
compatible with the semantics and user experience of the Go compiler that
Google makes.
## Go Modules
In Go's dependency model, you have a folder that contains all your Go code
called the GOPATH. The GOPATH has a few top level folders that have a well-known
meaning in the Go ecosystem:
* bin: binary files made by `go install` or `go get` go here
* pkg: intermediate compiler state goes here
* src: Go packages go here
GOPATH has one major advantage: it is ruthlessly easy to understand the
correlation between the packages you import in your code to their locations on
disk.
If you need to see what `within.website/ln` is doing, you go to
GOPATH/src/within.website/ln. The files you are looking for are somewhere in
there. You don’t have to really understand how the package manager works (mostly
because there isn’t one). If you want to hack something up you just go to the
folder and add the changes you want to see.
You can delete all of the intermediate compiler state easily in one fell swoop.
Just delete the `pkg` folder and poof, it’s all gone. This was great when you
needed to free up a bunch of disk space really quickly because over months the
small amount of incremental compiler state can really add up.
The Go compiler would fetch any missing packages from the internet at build time
so things Just Worked™️. This makes it utterly trivial to check out a project and
then build/run it. That combined with `go get` to automatically just figure
things out and install them made installing programs written in Go so easy that
it’s almost magic. This combined with Go's preference for making static binaries
as much as possible meant that even if the user didn't have Go installed you could
easily make a package to hand off to your users.
The GOPATH was conceptually simple to reason about. Go code goes in the GOPATH. The
best place for it was in the GOPATH. There's no reason to put it anywhere else.
Everything was organized into its place and it was lovely.
This wasn’t perfect though. There were notable flaws in this setup that were
easy to run into in practice:
* There wasn't a good way to make sure that everyone was using the _same copies_
of every library. People did add vendoring tools later to check that everyone
was using the same copies of every package, but this also introduced problems
when one project used one version of a dependency and another project used
another in ways that were mutually incompatible.
* The process to get the newest version of a dependency was to grab the latest
commit off of the default branch of that git repo. There was support for SVN,
mercurial and fossil, but in practice Git was the most used one so it’s almost
not worth mentioning the other version control systems. This also left you at
the mercy of other random people having good code security sense and required
you to audit your dependencies, but this is fairly standard across ecosystems.
* Dependency names were case sensitive on Linux but not on Windows or macOS.
Arguably this is a "Windows and macOS are broken for backwards compatibility
reasons" thing, but this did bite me at random times without warning.
* If the wrong random people deleted their GitHub repos, there's a chance your
builds could break unless your GOPATH had the packages in it already. Then you
could share that with your coworkers or the build machine somehow, maybe even
upload those packages to a git repository to soft-fork it.
* The default location for the GOPATH created a folder in your home directory.
Yeah, yeah, this default was added later
but still people complained about having to put the GOPATH somewhere at first.
Having to choose a place to put all the Go code they would use seemed like a big
choice that people really wanted solid guidance and defaults on. After a while
they changed this to default to `~/go` (with an easy to use command to influence
the defaults without having to set an environment variable). I don't personally
understand the arguments people have for wanting to keep their home directory
"clean", but the arguments are valid regardless.
Overall I think GOPATH was a net good thing for Go. It had its downsides, but as
far as these things go it was a very opinionated place to start from. This is
something typical to Go (much to people's arguments), but the main thing that it
focused on was making Go conceptually simple. There's not a lot going on there.
You have code in the folder and then that's where the Go compiler looks for
other code. It's a very lightweight approach to things that a lot of other
languages could learn a lot from. It's great for monorepos because it basically
treats all your Go code as one big monorepo. So many other languages don’t
really translate well to working in a monorepo context like Go does.
### Vendoring
That making sure everyone had the same versions of everything problem ended up
becoming a big problem in practice. I'm assuming that the original intent of the
GOPATH was to be similar to how Google's internal monorepo worked, where
everyone clones and deals with the entire GOPATH in source control. You'd then
have to do GOPATH juggling between monorepos, but the intent was to have
everything in one big monorepo anyways, so this wasn't thought of as much of a
big deal in practice. It turns out that people in fact did not want to treat Go
code this way, in practice this conflicted with the dependency model that Go
encouraged people to use with how people consume libraries from GitHub or other
such repository hosting sites.
The main disconnect between importing from a GOPATH monorepo and a Go library
off of GitHub is that when you import from a monorepo with a GOPATH in it, you
need to be sure to import the repository path and not the path used inside the
repository. This sounds weird but this is the difference between importing
`github.com/Xe/x/src/github.com/Xe/x/markov` and `github.com/Xe/x/markov`. This
means that things need to be extracted _out of_ monorepos and reformatted into
“flat” repos so that you can only grab the one package you need. This became
tedious in practice.
In Go 1.5 (the one where they rewrote the compiler in Go) they added support for
[vendoring code into your
repo](https://medium.com/@freeformz/go-1-5-s-vendor-experiment-fd3e830f52c3).
The idea here was to make it easy to get closer to the model that the Go authors
envisioned for how people should use Go. Go code should all be in one big happy
repo and everything should have its place in your GOPATH. This combined with
other tools people made allowed you to vendor all of your dependencies into a
`vendor` folder and then you could do whatever you wanted from there.
One of the big advantages of the `vendor` folder was that you could clone your
git repo, create a new process namespace and then run tests without a network
stack. Everything would work offline and you wouldn't have to worry about
external state leaking in. Not to mention removing the angle of someone deleting
their GitHub repos causing a huge problem for your builds.
Save tests that require internet access or
a database engine!
This worked for a very long time. People were able to vendor their code into
their repos and everything was better for people using Go. However the most
critical oversight with the `vendor` folder approach was that the Go team didn't
create an official tool to manage that `vendor` folder. They wanted to let tools
like `godep` and `glide` handle that. This is kind of a reasonable take, Go
comes from a very Google culture where this kind of problem doesn't happen, so
as a result they probably won't be able to come up with something that meets the
needs of the outside world very easily.
I can't speak for how `godep` or `glide`
works, I never really used them enough to have a solid opinion. I do remember
using [`vendor`](https://github.com/bmizerany/vendor) in my own projects though.
That had no real dependency resolution algorithm to speak of because it assumed
that you had everything working locally when you vendored the code.
### `dep`
After a while the Go team worked with people in the community to come up with an
"official experiment" in tracking dependencies called `dep`. `dep` was a tool
that used some more fancy computer science maths to help developers declare
dependencies for projects in a way like you do in other ecosystems. When `dep`
was done thinking, it emitted a bunch of files in `vendor` and a lockfile in
your repository. This worked really well and when I was working at Heroku this
was basically our butter and bread for how to deal with Go code.
It probably helped that my manager was on
the team that wrote `dep`.
One of the biggest advantages of `dep` over other tools was the way that it
solved versioning. It worked by having each package declare
[constraints](https://golang.github.io/dep/docs/the-solver.html) in the ranges
of versions that everything requires. This allowed it to do some fancy
dependency resolution math similar to how the solvers in `npm` or `cargo` work.
This worked fantastically in the 99% case. There were some fairly easy to
accidentally get yourself in cases where you could make the solver loop
infinitely though, as well as ending up in a state where you have mutually
incompatible transient dependencies without any real way around it.
`npm` and `cargo` work around this by
letting you use multiple versions of a single dependency in a
project.
However these cases were really really rare, only appearing in much, much larger
repositories. I don't think I practically ran into this, but I'm sure someone
reading this right now found themselves in `dep` hell and probably has a hell of
a war story around it.
### vgo and Modules
This lead the Go team to come up with a middle path between the unrestricted
madness of GOPATH and something more maximal like `dep`. They eventually called
this Go modules and the core reasons for it are outlined in [this series of
technical posts](https://research.swtch.com/vgo).
These posts are a very good read and I'd
highly suggest reading them if you've never seem then before. It outlines the
problem space and the justification for the choices that Go modules ended up
using. I don't agree with all of what is said there, but overall it's well
worth reading at least once if you want to get an idea of the inspirations
that lead to Go modules.
Apparently the development of Go modules came out as a complete surprise,
even to the core developer team of `dep`. I'm fairly sure this lead my
manager to take up woodworking as his main non work side hobby, I can only
wonder about the kind of resentment this created for other parts of the
`dep` team. They were under the impression that `dep` was going to be the
future of the ecosystem (likely under the subcommand `go dep`) and then had
the rug pulled out from under their feet.
The `dep` team was as close as we've
gotten for having people in the _actual industry_ using Go _in production_
outside of Google having a real voice in how Go is used in the real world. I
fear that we will never have this kind of thing happen again.
Go modules does solve one very critical problem for the Go ecosystem though: it
allows you to have the equivalent of the GOPATH but with multiple versions of
dependencies in it. It allows you to have `within.website/ln@v0.7` and
`within.website/ln@0.9` as dependencies for _two different projects_ without
having to vendor source code or do advanced GOPATH manipulation between
projects. It also adds cryptographic checksumming for each Go module that you
download from the internet. This allows you to avoid having to shell out to
`git` every time you fetch a module that someone else has fetched before.
Companies could run their own Go module proxy and then use that to provide
offline access to Go code fetched from the internet.
Wait, couldn't this allow Google to see the
source code of all of your Go dependencies? How would this intersect with
private repositories that shouldn't ever be on anything but work
machines?
Yeah, this was one of the big privacy
disadvantages out of the gate with Go modules. I think that in practice the
disadvantages are limited, but still the fact that it defaults to phoning home
to Google every time you run a Go build without all the dependencies present
locally is kind of questionable. They did make up for this with the checksum
verification database a little, but it's still kinda sus.
I'm not aware of any companies I've worked at running their own internal Go
module caching servers, but I ran my own for a very long time.
The earliest version of Go modules basically was a glorified `vendor` folder
manager named `vgo`. This worked out amazingly well and probably made
prototyping this a hell of a lot easier. This worked well enough that we used
this in production for many services at Heroku. We had no real issues with it
and most of the friction was with the fact that most of the existing ecosystem
had already been using `dep` or `glide`.
There was a bit of interoperability glue
that allowed `vgo` to parse the dependency definitions in `dep`, `godep` and
`glide`. This still exists today and helps `go mod init` tell what dependencies
to import into the Go module to aid migration.
If they had shipped this in prod, it probably would have been a huge success. It
would also let people continue to use `dep`, `glide` and `godep`, but just doing
that would also leave the ecosystem kinda fragmented. You’d need to have code
for all 4 version management systems to parse their configuration files and
implement algorithms that would be compatible with the semantics of all of them.
It would work and the Go team is definitely smart enough to do it, but in
practice it would be a huge mess.
This also solved the case-insensitive filesystem problem with
[bang-casing](https://go.dev/ref/mod#goproxy-protocol). This allows them to
encode the capital letters in a path in a way that works on macOS and Windows
without having to worry about horrifying hacks that are only really in place for
Photoshop to keep working.
### The Subtle Problem of `v2`
However one of the bigger downsides that came with Go modules is what I've been
calling the "v2 landmine" that Semantic Import Versioning gives you. One of the
very earliest bits of Go advice was to make the import paths for version 1 of a
project and version 2 of a project different so that people can mix the two to
allow more graceful upgrading across a larger project. Semantic Import
Versioning enforces this at the toolchain level, which means that it can be the
gate between compiling your code or not.
Many people have been telling me that
I’m kind of off base for thinking that this is a landmine for people, but I am
using the term “landmine” to talk about this because I feel like it reflects the
rough edges of unexpectedly encountering this in the wild. It kinda feels like
you stepped on a landmine.
The core of this is that when you create major version 2 of a Go project, you
need to adjust all your import paths everywhere in that project to import the
`v2` of that package or you will silently import the `v1` version of that
package. This can end up making large projects create circular dependencies on
themselves, which is quite confusing in practice. When consumers are aware of
this, then they can use that to more gradually upgrade larger codebases to the
next major version of a Go module, which will allow for smaller refactors.
This also applies to consumers. Given that this kind of thing is something that
you only do in Go it can come out of left field. The go router
[chi](https://github.com/go-chi/chi/issues/462) tried doing modules in the past
and found that it lead to confusing users. Conveniently they only really found
this out after the Go modules design was considered final and Semantic Import
Versioning has always been a part of Go modules and the Go team is now refusing
to budge on this.
My suggestion to people is to never
release a version `1.x.x` of a Go project to avoid the “v2 landmine”. The Go
team claims that the right bit of tooling can help ease the pain, but this
tooling never really made it out into the public. I bet it works great inside
google3 though!
When you were upgrading a Go project that already hit major version 2 or
higher to Go modules, adopting Go modules forced maintainers to make another
major version bump because it would break all of the import paths for every
package in the module. This caused some maintainers to meet Go modules with
resistance to avoid confusing their consumers. The workarounds for people that
still used GOPATH using upstream code with Semantic Import Versioning in it
were also kind of annoying at first until the Go team added "minimal module
awareness" to GOPATH mode. Then it was fine.
Overall though, Go modules has been a net positive for the community and for
people wanting to create reliable software in Go. It’s just such a big semantic
break in how the toolchain works that I almost think it would have been easier
for the to accept if _that_ was Go 2. Especially since the semantic of how the
toolchain worked changed so much.
Wait, doesn’t the Go compiler have a
backwards compatibility promise that any code built with Go 1.x works on go
1.(x+1)?
Yes, but that only applies to _code you
write_, not _semantics of the toolchain_ itself. On one hand this makes a lot of
sense and on the other it feels like a cop-out. The changes in how `go get` now
refers to adding dependencies to a project and `go install` now installs a
binary to the system have made an entire half decade of tool installation
documentation obsolete. It’s understandable why they want to make that change,
but the way that it broke people’s muscle memory is [quite frustrating for
users](https://github.com/golang/go/issues/40276#issuecomment-1109797059) that
aren’t keeping on top of every single change in semantics of toolchains (this
bites me constantly when I need to quick and dirty grab something outside of a
Nix package). I understand _why_ this isn’t a breaking change as far as the
compatibility promise but this feels like a cop-out in my subjective
opinion.
## Contexts
One of Go’s major features is its co-operative threading system that it calls
goroutines. Goroutines are kinda like coroutines that are scheduled by the
scheduler. However there is no easy way to "kill" a goroutine. You have to add
something to the invocation of the goroutine that lets you signal it to stop and
then opt-in the goroutine to stop.
Without contexts you would need to do all of this legwork manually. Every
project from the time before contexts still shows signs of this. The best
practice was to make a "stop" channel like this:
```go
stop := make(chan struct{})
```
And then you'd send a cancellation signal like this:
```go
stop <- struct{}{}
```
The type `struct{}` is an anonymous
structure value that takes 0 bytes in ram. It was suggested to use this as your
stopping signal to avoid unneeded memory allocations.
This did work and was the heart of many event loops, but the main problem with
it is that the signal was only sent _once_. Many other people also followed up
the stop signal by closing the channel:
```go
close(stop)
```
However with naïve stopping logic the closed channel would successfully fire a
zero value of the event. So code like this would still work the way you wanted:
```go
select {
case <- stop:
haltAndCatchFire()
}
```
### Package `context`
However if your stop channel was a `chan bool` and you relied on the `bool`
value being `true`, this would fail because the value would be `false`. This
was a bit too brittle for comfortable widespread production use and we ended
up with the [context](https://pkg.go.dev/context) package in the standard
library. A Go context lets you more easily and uniformly handle timeouts and
giving up when there is no more work to be done.
This started as something that existed
inside the Google monorepo that escaped out into the world. They also claim to
have an internal tool that makes
[`context.TODO()`](https://pkg.go.dev/context#TODO) useful (probably by showing
you the callsities above that function?), but they never released that tool as
open source so it’s difficult to know where to use it without that added
context.
One of the most basic examples of using contexts comes when you are trying to
stop something from continuing. If you have something that constantly writes
data to clients such as a pub-sub queue, you probably want to stop writing data
to them when the client disconnects. If you have a large number of HTTP requests
to do and only so many workers can make outstanding requests at once, you
want to be able to set a timeout so that after a certain amount of time it gives
up.
Here's an example of using a context in an event processing loop (of course while
pretending that fetching the current time is anything else that isn't a contrived
example to show this concept off):
```go
t := time.NewTicker(30 * time.Second)
ctx, cancel := context.WithCancel(context.Background())
defer cancel()
for {
select {
case <- ctx.Done():
log.Printf("not doing anything more: %v", ctx.Err())
return
case data := <- t.C:
log.Printf("got data: %s", data)
}
}
```
This will have the Go runtime select between two channels, one of them will
emit the current time every 30 seconds and the other will fire when the
`cancel` function is called.
Don't worry, you can call the `cancel()`
function multiple times without any issues.
If you want to set a timeout on this (so that the function only tries to run
for 5 minutes), you'd want to change the second line of that example to this:
```go
ctx, cancel := context.WithTimeout(context.Background(), 5 * time.Minute)
```
The context will be automatically cancelled after 5 minutes. You can cancel it
sooner by calling the `cancel()` function should you need to. Anything else in
the stack that is context-aware will automatically cancel as well as the
cancellation signal percolates down the stack and across goroutines.
You can attach this to an HTTP request by using
[`http.NewRequestWithContext`](https://pkg.go.dev/net/http#NewRequestWithContext):
```go
req, err := http.NewRequestWithContext(ctx, http.MethodGet, "https://christine.website/.within/health", nil)
```
And then when you execute the request (such as with `http.DefaultClient.Do(req)`)
the context will automatically be cancelled if it takes too long to fetch the
response.
You can also wire this up to the `Control-c` signal using a bit of code
[like this](https://medium.com/@matryer/make-ctrl-c-cancel-the-context-context-bd006a8ad6ff).
Context cancellation propagates upwards, so you can use this to ensure that things
get stopped properly.
Be sure to avoid creating a "god context"
across your entire app. This is a known anti-pattern and this pattern should only
be used for small command line tools that have an expected run time in the minutes
at worst, not hours like production bearing services.
This is a huge benefit to the language because of how disjointed the process of
doing this before contexts was. Because this wasn’t in the core of the language,
every single implementation was different and required learning what the library
did. Not to mention adapting between libraries could be brittle at best and
confusing at worst.
I understand why they put data into the context type, but in practice I really
wish they didn’t do that. This feature has been abused a lot in my experience.
At Heroku a few of our production load bearing services used contexts as a
dependency injection framework. This did work, but it turned a lot of things
that would normally be compile time errors into runtime errors.
I say this as someone who maintains a
library that uses contexts to store [contextually relevant log
fields](https://pkg.go.dev/within.website/ln) as a way to make logs easier to
correlate between.
Arguably you could make the case that people are misusing the
tool and of course this is what will happen when you do that but I don't know if
this is really the right thing to tell people.
I wish contexts were in the core of the language from the beginning. I know that
it is difficult to do this in practice (especially on all the targets that Go
supports), but having cancellable syscalls would be so cool. It would also be
really neat if contexts could be goroutine-level globals so you didn’t have to
"pollute" the callsites of every function with them.
At the time contexts were introduced,
one of the major arguments I remember hearing against them was that contexts
"polluted" their function definitions and callsites. I can't disagree with this
sentiment, at some level it really does look like contexts propagate "virally"
throughout a codebase.
I think that the net improvements to reliability and understandability of how
things get stopped do make up for this though. Instead of a bunch of separate
ways to cancel work in each individual library you have the best practice in
the standard library. Having contexts around makes it a lot harder to "leak"
goroutines on accident.
## Generics
One of the biggest ticket items that Go has added is "generic types", or being
able to accept types as parameters for other types. This is really a huge ticket
item and I feel that in order to understand _why_ this is a huge change I need
to cover the context behind what you had before generics were added to the
language.
One of the major standout features of Go is interface types. They are like Rust
Traits, Java Interfaces, or Haskell Typeclasses; but the main difference is that
interface types are _implicit_ rather than explicit. When you want to meet the
signature of an interface, all you need to do is implement the contract that the
interface spells out. So if you have an interface like this:
```go
type Quacker interface {
Quack()
}
```
You can make a type like `Duck` a `Quacker` by defining the `Duck` type and a
`Quack` method like this:
```go
type Duck struct{}
func (Duck) Quack() { fmt.Println("Quack!") }
```
But this is not limited to just `Ducks`, you could easily make a `Sheep` a
`Quacker` fairly easily:
```go
type Sheep struct{}
func (Sheep) Quack() { fmt.Println("*confused sheep noises*") }
```
This allows you to deal with expected _behaviors_ of types rather than having to
have versions of functions for every concrete implementation of them. If you
want to read from a file, network socket, `tar` archive, `zip` archive, the
decrypted form of an encrypted stream, a TLS socket, or a HTTP/2 stream they're
all [`io.Reader`](https://pkg.go.dev/io#Reader) instances. With the example
above we can make a function that takes a `Quacker` and then does something with
it:
```go
func main() {
duck := Duck{}
sheep := Sheep{}
doSomething(duck)
doSomething(sheep)
}
func doSomething(q Quacker) {
q.Quack()
}
```
If you want to play with this example,
check it out on the Go playground [here](https://go.dev/play/p/INK8O2O-D01). Try
to make a slice of Quackers and pass it to `doSomething`!
You can also embed interfaces into other interfaces, which will let you create
composite interfaces that assert multiple behaviours at once. For example,
consider [`io.ReadWriteCloser`](https://pkg.go.dev/io#ReadWriteCloser). Any
value that matches an `io.Reader`, `io.Writer` and an `io.Closer` will be able
to be treated as an `io.ReadWriteCloser`. This allows you to assert a lot of
behaviour about types even though the actual underlying types are opaque to you.
This means it’s easy to split up a [`net.Conn`](https://pkg.go.dev/net#Conn)
into its reader half and its writer half without really thinking about
it:
```go
conn, _ := net.Dial("tcp", "127.0.0.1:42069")
var reader io.Reader = conn
var writer io.Writer = conn
```
And then you can pass the writer side off to one function and the reader side
off to another.
There’s also a bunch of room for "type-level middleware" like
[`io.LimitReader`](https://pkg.go.dev/io#LimitReader). This allows you to set
constraints or details around an interface type while still meeting the contract
for that interface, such as an `io.Reader` that doesn’t let you read too much,
an `io.Writer` that automatically encrypts everything you feed It with TLS, or
even something like sending data over a Unix socket instead of a TCP one. If it
fits the shape of the interface, it Just Works.
However, this falls apart when you want to deal with a collection of _only one_
type that meets an interface at once. When you create a slice of `Quacker`s and
pass it to a function, you can put both `Duck`s and `Sheep` into that slice:
```go
quackers := []Quacker{
Duck{},
Sheep{},
}
doSomething(quackers)
```
If you want to assert that every `Quacker` is the same type, you have to do some
fairly brittle things that step around Go's type safety like this:
```go
func doSomething(qs []Quacker) error {
// Store the name of the type of first Quacker.
// We have to use the name `typ` because `type` is
// a reserved keyword.
typ := fmt.Sprintf("%T", qs[0])
for i, q := range qs {
if qType := fmt.Sprintf("%T", q); qType != typ {
return fmt.Errorf("slice value %d was type %s, wanted: %s", qType, typ)
}
q.Quack()
}
return nil
}
```
This would explode at runtime. This same kind of weakness is basically the main
reason why the Go standard library package [`container`](https://pkg.go.dev/container)
is mostly unused. Everything in the `container` package deals with
`interface{}`/`any` values, which is Go for "literally anything". This means
that without careful wrapper code you need to either make interfaces around
everything in your lists (and then pay the cost of boxing everything in an
interface, which adds up a lot in practice in more ways than you'd think) or
have to type-assert anything going into or coming out of the list, combined
with having to pay super close attention to anything touching that code
during reviews.
Don't get me wrong, interface types
are an _amazing_ standout feature of Go. They are one of the main reasons that
Go code is so easy to reason about and work with. You don't have to worry
about the entire tree of stuff that a value is made out of, you can just
assert that values have behaviors and then you're off to the races. I end up
missing the brutal simplicity of Go interfaces in other languages like Rust.
### Introducing Go Generics
In Go 1.18, support for adding types as parameters to other types was added.
This allows you to define constraints on what types are accepted by a function,
so that you can reuse the same logic for multiple different kinds of underlying
types or write collections that deal with values of a given type that meets an
interface without also having to make sure that everything else in that
collection is of the same type at runtime.
That `doSomething` function from above could be rewritten like this with
generics:
```go
func doSomething[T Quacker](qs []T) {
for i, q := range qs {
q.Quack()
}
}
```
We can totally refactor out the error return and any of that runtime fallible
code. This allows us to express constraints at _compile time_ so that
attempting to mix `Duck`s and `Sheep` in the same argument to `doSomething`
will fail to build.
- [ ] Overview of some of the types of collections it lets you make
- [ ] This is a huge improvement to the language
---
We already have Go 2. It’s just called Go 1.18 for some reason. It’s got so many
improvements and fundamental changes that I believe that this is already Go 2 in
spirit. I, as some random person on the internet that is not associated with the
Go team, think that if there was sufficient political will that they could
probably label what we have as Go 2, but I don’t think that is going to happen
any time soon. Until then, we still have a very great set of building blocks
that allow you to make easy to maintain production quality services, and I don’t
see that changing any time soon.