Hitchhikers Guide to Go ⋆ Mark McDonnell

Introduction

A few years ago when I was learning the Go programming language I created a gist and updated it on a regular basis as a sort of cheat sheet. I stumbled across this gist recently and decided I’d try and port it over to some form of semi-coherent blog post.

Note: the code will not be updated in any way so YMMV

What this isn’t, is a walk-through of how to write Go code. I’d suggest trying the official Go Tour which is really good and covers a lot of ground. Instead I’m going to provide lots of example code in the vein of a resource like Go by Example.

What this is, is a barren wasteland of old code. Passers by are forewarned to tread carefully.

Note: there’s also not much in the way of code explanation, to the extent that some of the testing examples are long and very context specific - you’ve been warned

Most of this Go code is old, so you may find some packages or information possibly out of date (as in not the latest awesome thing) or maybe not that great quality either.

Take this as what it is, a sharing exercise. Take what you need and leave the rest. I wont be offended.

Private Repo Access

go get uses HTTPS so to be able to pull dependencies from a private repository, you’ll need to force it to use SSH so it can access your keys and authorise the connection:

git config --global url."git@github.com:".insteadOf "https://github.com/"

You can also restrict this to a single specific organisation if you prefer:

git config --global url."git@github.com:foo/".insteadOf "https://github.com/foo/"`

So when you want a private dependency like: git@github.com:foo/private.git:

go get github.com/foo/private

Build and Compilation

As of Go 1.5 you can use:

GOOS=darwin GOARCH=386 go build foo.go

Here’s a quick reference of the values you can specify:

$GOOS     $GOARCH
darwin    386      -- 32 bit MacOSX
darwin    amd64    -- 64 bit MacOSX
freebsd   386
freebsd   amd64
linux     386      -- 32 bit Linux
linux     amd64    -- 64 bit Linux
linux     arm      -- RISC Linux
netbsd    386
netbsd    amd64
openbsd   386
openbsd   amd64
plan9     386
windows   386      -- 32 bit Windows
windows   amd64    -- 64 bit Windows

You can get a full list with:

go tool dist list

Gox

Gox is an alternative build tool.

One time only commands for purpose of download/setup:

go get github.com/mitchellh/gox
gox -build-toolchain (only necessary for Go 1.4.x and lower)

Compilation example:

gox -osarch="linux/amd64" -osarch="darwin/amd64" -osarch="windows/amd64" -output="foobar.{{.OS}}"

This will generate three files:

foobar.darwin
foobar.linux
foobar.windows.exe

Other information

Use the -a flag when running go build.

In short, if you dont’ use go build -a -v . then Go won’t know if any packages are missing (you can find the gory details here)

Build Time Dynamic Variables

Imagine you have a global variable called version in your main package and you want to update that value at build time:

go build -ldflags "-X main.version=foobar"

A more realistic example would be to use some form of revision number of commit hash:

go build -ldflags "-X main.version=$(git rev-parse HEAD)"

Another approach would be to have separate files for different ’environments’ (all under the main package). You would then use a code comment to indicate what the environment was, and at build time you would tell the compiler which version of the file to compile.

// +build prod

package main

func init() {
    version = 123
}

You would compile the above version variable using:

go build -tags prod

Package Naming

In Go, the name of the package is used to refer to the exported item: fmt.Println, http.RegisterFunc etc. Because the package name is so visible, the package name should describe what the exported items are. Meaning, we shouldn’t have packages named util (as a common example of bad package naming) because util.JSONMarshal isn’t as efficient and effective as json.Marshal.

Another example of this that I found in my own code was utils.CreateUser. Later on during the project I had added utils.CreateLegacyUser. When I discovered what I had done I went back and made two separate packages legacy and aws so that I could have a consistent CreateUser function within both (e.g. aws.CreateUser and legacy.CreateUser).

Dependency Information

To see a list of dependencies for a given Go package you can utilise the go list command:

go list -json strconv

Which returns:

{
  "Dir": "/usr/local/Cellar/go/1.5.2/libexec/src/strconv",
  "ImportPath": "strconv",
  "Name": "strconv",
  "Doc": "Package strconv implements conversions to and from string representations of basic data types.",
  "Target": "/usr/local/Cellar/go/1.5.2/libexec/pkg/darwin_amd64/strconv.a",
  "Goroot": true,
  "Standard": true,
  "Root": "/usr/local/Cellar/go/1.5.2/libexec",
  "GoFiles": [
    "atob.go",
    "atof.go",
    "atoi.go",
    "decimal.go",
    "doc.go",
    "extfloat.go",
    "ftoa.go",
    "isprint.go",
    "itoa.go",
    "quote.go"
  ],
  "IgnoredGoFiles": [
    "makeisprint.go"
  ],
  "Imports": [
    "errors",
    "math",
    "unicode/utf8"
  ],
  "Deps": [
    "errors",
    "math",
    "runtime",
    "unicode/utf8",
    "unsafe"
  ],
  "TestGoFiles": [
    "internal_test.go"
  ],
  "XTestGoFiles": [
    "atob_test.go",
    "atof_test.go",
    "atoi_test.go",
    "decimal_test.go",
    "example_test.go",
    "fp_test.go",
    "ftoa_test.go",
    "itoa_test.go",
    "quote_test.go",
    "strconv_test.go"
  ],
  "XTestImports": [
    "bufio",
    "bytes",
    "errors",
    "fmt",
    "log",
    "math",
    "math/rand",
    "os",
    "reflect",
    "runtime",
    "strconv",
    "strings",
    "testing",
    "time",
    "unicode"
  ]
}

If you don’t specify the -json flag then the default behaviour is to filter out the ImportPath field from the above JSON output. For example:

go list strconv

Will return just the import path strconv.

Documentation: go help list | less

You can also utilise Go’s templating functionality on the returned JSON object by adding the -f flag:

go list -f '{{join .Deps " "}}' strconv

Which filters out the Deps field, joins up all items it contains using whitespace and subsequently returns:

errors math runtime unicode/utf8 unsafe

You can do more complex things such as:

go list -f '{{.ImportPath}} -> {{join .Imports " "}}' compress/...

Which will return something like:

compress/bzip2 -> bufio io sort
compress/flate -> bufio fmt io math sort strconv
compress/gzip -> bufio compress/flate errors fmt hash hash/crc32 io time
compress/lzw -> bufio errors fmt io
compress/zlib -> bufio compress/flate errors fmt hash hash/adler32 io

Dependency Management

Update (August 2017): there is an official tool now called dep.

There are many dependency management tools, these are the few that I’ve tried (in this order): godeps, gb, glide. Let’s review each of them to see how they work:

Godeps

When running go get <dependency> locally, Go will stick the dependency in the folder defined by your $GOPATH variable. So when you build your code into a binary using go build <script> it’ll bake the dependencies into the binary (i.e. the binary is statically linked).

But if someone pulls down your repo and tries to do a build from your code, then they’ll need to have a network connection to pull down the dependencies as their $GOPATH might not have those dependencies yet (unless the user manually executes go get for each dependency required). Also the dependencies they subsequently pull down could be a more recent (and untested version) of each dependency.

So to make this situation better we can use Godep to stick all your dependencies within a Godeps folder inside your project directory. You can then use godep save -r ./... to automatically update all your references to point to that local folder.

Note: you might need to remove the Godeps folder and run go get if you get strange conflicts. The ./... means to target all .go files

This way users who clone your repo don’t need an internet connection to pull the dependencies, as they already have them. But also they’ll have the correct versions of the dependencies. This acts like a Gemfile.lock as you would typically find in the Ruby world.

Gb

go get -u github.com/constabulary/gb/...
gb vendor fetch <pkg>
gb build all

You’ll need the following structure:

├── src
│   ├── foo
│   │   └── main.go
└── vendor
    ├── manifest
    └── src

The vendor directory is auto-generated by the gb vendor fetch <pkg> command.

Glide

This is now my preferred dependency management tool, as it works just like existing tools in other languages (e.g. Ruby’s Bundler or Node’s NPM) and so consistency is a plus.

It also provides the ability (like gb) to not commit dependencies but have specific versions vendored when running a simple command.

go get github.com/Masterminds/glide
export GO15VENDOREXPERIMENT=1       # or use 1.6
glide init                          # generates glide.yaml
glide install                       # installs from lock file (creates it if not found)
glide update                        # updates dependencies and updates lock file
glide list                          # shows vendored deps
go test $(glide novendor)           # test only your package (not vendored packages)

Note: to add a new dependency glide get <pkg_name>

Documentation

Godoc is the original implementation for viewing documentation. Previous to Godoc there was go doc, but that was removed and then added back with totally different functionality.

The syntax structure for go doc is as follows:

go doc <pkg>
go doc <sym>[.<method>]
go doc [<pkg>].<sym>[.<method>]

Here are some examples of using go doc:

go doc json # same as go doc encoding/json
go doc json.Number
go doc json.Number.Float64

Here is the same thing but using godoc (where the syntax structure is godoc <pkg> <symbol>):

godoc encoding/json # unlike "go doc json", "godoc json" doesn't work as it's not a fully qualified path
godoc encoding/json Number
godoc -src builtin make | less

Unlike with go doc, godoc doesn’t allow filtering by <method>
It only goes as far as <pkg> <symbol>

You can use <pkg> <symbol> <method>
and the method will be included in the results
but you’ll need to search for the method manually
godoc -src net/http Request ParseForm | less
here is a similar result using go doc
go doc http.Request.ParseForm | less

The purpose of go doc was to provide a simplistic cli documentation viewer, whereas Godoc has many more features available.

The go doc command also works not only with Go’s own library’s but your own custom packages as well.

There are some differences in what is returned though between godoc and go doc (mainly the latter is more succinct/compact so you can find the functions/types you’re after and then you can expand into those once you’ve found them; godoc is harder to sift through on the command line)…

`godoc encoding/json Encoder`

type Encoder struct {
    // contains filtered or unexported fields
}
    An Encoder writes JSON objects to an output stream.

func NewEncoder(w io.Writer) *Encoder
    NewEncoder returns a new encoder that writes to w.

func (enc *Encoder) Encode(v interface{}) error
    Encode writes the JSON encoding of v to the stream, followed by a
    newline character.

    See the documentation for Marshal for details about the conversion of Go
    values to JSON.

`go doc encoding/json Encoder`

type Encoder struct {
        // Has unexported fields.
}

    An Encoder writes JSON objects to an output stream.

func NewEncoder(w io.Writer) *Encoder
func (enc *Encoder) Encode(v interface{}) error

Notice the functions don’t have their documentation notes printed with go doc

One other thing godoc has over go doc is the ability to view the source code using the -src flag:

godoc -src builtin make | less

The godoc tool also has a full browser documentation suite available and allows you to generate HTML documentation for your project…

Full Browser Documentation

Start a local documentation server and allow indexing (which takes a few minutes; you have to just keep trying the search until it’s done)

godoc -http ':6060' -index

You can then open a new terminal pane and search via cli if you prefer (rather than open up a browser to http://localhost:6060/)

godoc -q tls | less

You can also have the playground available if you need it in the browser, but it does require an internet connection to compile:

godoc -http ':6060' -play

Testing

Note: see also examples here

Test files are placed in the same directory as the file/package being tested. The convention is to use the same file name but suffix it with _test. So foo.go would have another file next to it called foo_test.go.

Run the tests: go test -v ./...

You can also run a specific test like so: go test -v command/foo_test.go

Note: remember that your test file should have the same package name as your code being tested. This means the test file will have access to all the public functions and variables of that package (and so subsequently it’ll have access to the code being tested)

Here is a simple test example:

package foo

import "testing"

func TestBasics(t *testing.T) {
  expect := "abc"
  actual := "def"

  if actual != expect {
    t.Errorf("expected %s, actual %s", expect, actual)
  }
}

The output from running this test will be:

=== RUN   TestBasics
--- FAIL: TestBasics (0.00s)
        foo_test.go:10: expected abc, actual def
FAIL
exit status 1
FAIL    command-line-arguments  0.004s

The following example program demonstrates how to mock an io function so that we’re not reliant on reading from disk in our test suite.

Here’s the program:

package main

import (
  "fmt"
  "io/ioutil"
)

type fooIO func(string) ([]byte, error)

func getContents(fio fooIO, path string) (string, error) {
  f, err := fio(path)
  if err != nil {
    return "", err
  }
  return string(f), nil
}

func main() {
  contents, err := getContents(ioutil.ReadFile, "example.txt")
  if err != nil {
    fmt.Println(err)
  }
  fmt.Println(contents)
}

Here’s the test:

package main

import "testing"

var expectation = "pretend to read from disk"

func fakeReadFileSuccess(path string) ([]byte, error) {
  return []byte(expectation), nil
}

func TestIO(t *testing.T) {
  contents, _ := getContents(fakeReadFileSuccess, "dont-exist.txt")

  if contents != expectation {
    t.Errorf("Expected '%s' but got '%s'", expectation, contents)
  }
}

You could also probably use an interface there, but we’ve gone with a func type.

Here’s another program that does something similar where we mock a function that expects a particular interface:

package main

import "fmt"

type FooIO interface {
  Read() string
}

type Foo struct{}

func (f *Foo) Read() string {
  return "We READ something from disk"
}

func Stuff(f FooIO) string {
  return f.Read()
}

func main() {
  foo := &Foo{}
  contents := Stuff(foo)
  fmt.Println(contents)
}

Here’s our test:

package main

import (
  "testing"

  "github.com/stretchr/testify/assert"
)

type FakeFoo struct{}

func (s *FakeFoo) Read() string {
  return "We 'pretend' to READ something from disk"
}

func TestSomething(t *testing.T) {
  assert := assert.New(t)

  foo := &FakeFoo{}
  contents := Stuff(foo)

  assert.Equal(contents, "We 'pretend' to READ something from disk")
}

More Test Examples

Faking HTTP and WebServers can be a bit tricky:

package requester

import (
  "bytes"
  "encoding/json"
  "fmt"
  "io/ioutil"
  "net/http"
  "net/http/httptest"
  "os"
  "strconv"
  "testing"
  "time"

  "github.com/bbc/mozart-requester/src/aggregator"
  "github.com/julienschmidt/httprouter"
)

func TestSuccessResponse(t *testing.T) {
  upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
    w.Header().Set("Content-Type", "application/json")
    fmt.Fprintln(w, `{"head":[ "foo" ],"bodyInline":"bar","bodyLast":[ "baz" ]}`)
  }))
  defer upstream.Close()

  router := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
    Process(w, r, httprouter.Params{})
  }))
  defer router.Close()

  var config = []byte(fmt.Sprintf(`{
    "components":[
      {"id":"foo","endpoint":"%s","must_succeed":true},
      {"id":"bar","endpoint":"%s","must_succeed":true}
    ]
  }`, upstream.URL, upstream.URL))

  req, err := http.NewRequest("POST", router.URL, bytes.NewBuffer(config))

  client := &http.Client{}
  resp, err := client.Do(req)
  if err != nil {
    panic(err)
  }

  defer resp.Body.Close()
  body, _ := ioutil.ReadAll(resp.Body)

  var result aggregator.Result
  json.Unmarshal(body, &result)

  expectedStatus := "success"
  if result.Summary != expectedStatus {
    t.Errorf("The response:\n '%s'\ndidn't match the expectation:\n '%s'", result.Summary, expectedStatus)
  }

  expectedLength := 2
  if len(result.Components) != expectedLength {
    t.Errorf("The response:\n '%d'\ndidn't match the expectation:\n '%d'", len(result.Components), expectedLength)
  }
}

func TestFailureResponse(t *testing.T) {
  healthyUpstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
    w.Header().Set("Content-Type", "application/json")
    fmt.Fprintln(w, `{"head":[ "foo" ],"bodyInline":"bar","bodyLast":[ "baz" ]}`)
  }))
  defer healthyUpstream.Close()

  failingUpstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
    w.Header().Set("Content-Type", "text/plain; charset=utf-8")
    w.WriteHeader(http.StatusNotFound)
    fmt.Fprintln(w, "404 page not found")
  }))
  defer failingUpstream.Close()

  router := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
    Process(w, r, httprouter.Params{})
  }))
  defer router.Close()

  var config = []byte(fmt.Sprintf(`{
    "components":[
      {"id":"foo","endpoint":"%s","must_succeed":true},
      {"id":"bar","endpoint":"%s","must_succeed":true}
    ]
  }`, healthyUpstream.URL, failingUpstream.URL))

  req, err := http.NewRequest("POST", router.URL, bytes.NewBuffer(config))

  client := &http.Client{}
  resp, err := client.Do(req)
  if err != nil {
    panic(err)
  }

  defer resp.Body.Close()
  body, _ := ioutil.ReadAll(resp.Body)

  var result aggregator.Result
  json.Unmarshal(body, &result)

  expectedSummary := "failure"
  if result.Summary != expectedSummary {
    t.Errorf("The response:\n '%s'\ndidn't match the expectation:\n '%s'", result.Summary, expectedSummary)
  }

  expectedLength := 2
  if len(result.Components) != expectedLength {
    t.Errorf("The response length:\n '%d'\ndidn't match the expectation:\n '%d'", len(result.Components), expectedLength)
  }

  expectedStatus := []int{}
  for _, value := range result.Components {
    if value.Status == 404 {
      expectedStatus = append(expectedStatus, value.Status)
    }
  }
  if len(expectedStatus) < 1 || len(expectedStatus) > 1 {
    t.Errorf("The response length:\n '%d'\ndidn't match the expectation:\n '%d'", len(expectedStatus), 1)
  }
}

func TestSlowResponse(t *testing.T) {
  healthyUpstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
    w.Header().Set("Content-Type", "application/json")
    fmt.Fprintln(w, `{"head":[ "foo" ],"bodyInline":"bar","bodyLast":[ "baz" ]}`)
  }))
  defer healthyUpstream.Close()

  slowUpstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
    timeout, err := strconv.Atoi(os.Getenv("COMPONENT_TIMEOUT"))
    if err != nil {
      t.Errorf("COMPONENT_TIMEOUT: %s", err.Error())
    }
    time.Sleep(time.Duration(timeout) * time.Millisecond)
    w.Header().Set("Content-Type", "application/json")
    fmt.Fprintln(w, `{"head":[ "foo" ],"bodyInline":"bar","bodyLast":[ "baz" ]}`)
  }))
  defer slowUpstream.Close()

  router := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
    Process(w, r, httprouter.Params{})
  }))
  defer router.Close()

  var config = []byte(fmt.Sprintf(`{
    "components":[
      {"id":"foo","endpoint":"%s","must_succeed":true},
      {"id":"bar","endpoint":"%s","must_succeed":true}
    ]
  }`, healthyUpstream.URL, slowUpstream.URL))

  req, err := http.NewRequest("POST", router.URL, bytes.NewBuffer(config))

  client := &http.Client{}
  resp, err := client.Do(req)
  if err != nil {
    panic(err)
  }

  defer resp.Body.Close()
  body, _ := ioutil.ReadAll(resp.Body)

  var result aggregator.Result
  json.Unmarshal(body, &result)

  expectedStatus := 408
  for _, value := range result.Components {
    if value.ID == "bar" && value.Status != expectedStatus {
      t.Errorf("The response:\n '%d'\ndidn't match the expectation:\n '%d'", value.Status, expectedStatus)
    }
  }

  expectedSummary := "failure"
  if result.Summary != expectedSummary {
    t.Errorf("The response:\n '%s'\ndidn't match the expectation:\n '%s'", result.Summary, expectedSummary)
  }
}

I typically run my tests using Make, but it ultimately looks like this:

pushd src && APP_ENV=test COMPONENT_TIMEOUT=100 go test -v $(glide novendor) && popd

Here’s another example of a test needing to fake things:

package retriever

import (
  "bytes"
  "io/ioutil"
  "net/http"
  "strings"
  "testing"

  "github.com/PuerkitoBio/goquery"
)

const href = "http://bar.com/"
const url = "http://foo.com/"

var body string

func fakeNewDocument(url string) (*goquery.Document, error) {
  body = strings.Replace(body, "{}", href, 1)

  resp := &http.Response{
    Status:        "200 OK",
    StatusCode:    200,
    Proto:         "HTTP/1.0",
    ProtoMajor:    1,
    ProtoMinor:    0,
    Body:          ioutil.NopCloser(bytes.NewBufferString(body)),
    ContentLength: int64(len(body)),
    Request:       &http.Request{},
  }

  return goquery.NewDocumentFromResponse(resp)
}

func TestRetrieveReturnValue(t *testing.T) {
  // {} interpolated with constant's value
  body = `
    <html>
      <body>
        <div class="productInfo">
          <a href="{}">Bar</a>
        </div>
      </body>
    <html>
  `
  coll, _ := Retrieve(url, fakeNewDocument)

  if response := coll[0]; response != href {
    t.Errorf("The response:\n '%s'\ndidn't match the expectation:\n '%s'", response, href)
  }
}

func TestRetrieveMissingAttributeReturnsEmptySlice(t *testing.T) {
  // href attribute is missing from anchor element
  body = `
    <html>
      <body>
        <div class="productInfo">
          <a>Bar</a>
        </div>
      </body>
    <html>
  `
  coll, _ := Retrieve(url, fakeNewDocument)

  if response := coll; len(response) > 0 {
    t.Errorf("The response:\n '%s'\ndidn't match the expectation:\n '%s'", response, "[http://bar.com/]")
  }
}

And…

package scraper

import "testing"

func TestScrapeResults(t *testing.T) {
  getItem = func(url string) {
    defer wg.Done()

    ch <- Item{
      "FooTitle",
      "FooSize",
      "10.00",
      "FooDescription",
    }
  }

  urls := []string{
    "http://foo.com/",
    "http://bar.com/",
    "http://baz.com/",
  }

  result := Scrape(urls)
  first := result.Items[0]

  var suite = []struct {
    response string
    expected string
  }{
    {first.Title, "FooTitle"},
    {first.Size, "FooSize"},
    {first.UnitPrice, "10.00"},
    {first.Description, "FooDescription"},
    {result.Total, "30.00"},
  }

  for _, v := range suite {
    if v.response != v.expected {
      err(v.response, v.expected, t)
    }
  }
}

func err(response, expected string, t *testing.T) {
  t.Errorf("The response:\n '%s'\ndidn't match the expectation:\n '%s'", response, expected)
}

Logging

Using the standard Logger:

info := log.New(os.Stdout, "STUFF: ", log.Ldate|log.Ltime|log.Lshortfile)
info.Println("Starting up!!!")

f, e := os.Create("test.log")
if e != nil {
  log.Fatal("Failed to create log file")
}

logfile := log.New(f, "STUFF: ", log.Ldate|log.Ltime|log.Lshortfile)
logfile.Println("Starting up!!!")

Using Logrus:

package main

import (
  "os"

  log "github.com/Sirupsen/logrus"
)

func main() {
  // Standard stdout ASCII logging
  log.WithFields(log.Fields{
    "animal": "walrus",
  }).Info("A walrus appears")

  // JSON style structured logging
  log.SetFormatter(&log.JSONFormatter{})
  f, e := os.Create("logs")
  if e != nil {
    log.Fatal("Failed to create log file")
  }
  log.SetOutput(f)
  log.WithFields(log.Fields{
    "animal": "walrus",
    "size":   10,
  }).Info("A group of walrus emerges from the ocean")
  /*
      {
        "animal": "walrus",
        "level": "info",
        "msg": "A group of walrus emerges from the ocean",
        "size": 10,
        "time": "2015-12-22T13:58:46Z"
      }
  */
}

Godo

Godo is a build tool in a similar vein to rake or gulp.

The following example is taken from my own project go-requester:

package main

import (
  "fmt"
  "os"

  do "gopkg.in/godo.v2"
)

func tasks(p *do.Project) {
  if pwd, err := os.Getwd(); err == nil {
    do.Env = fmt.Sprintf("GOPATH=%s/vendor::$GOPATH", pwd)
  }

  p.Task("server", nil, func(c *do.Context) {
    c.Start("main.go ./config/page.yaml", do.M{"$in": "./"})
  }).Src("**/*.go")
}

func main() {
  do.Godo(tasks)
}

Import Race Conditions

When you import a package within a Go script, only the public functions and variables are exposed for the caller to utilise. So if you need a package to execute some bootstrapping code at the point of it being loaded, then you’ll need to stick it inside of an init function.

Note: you can have multiple init functions inside a package e.g. one per file within the package namespace

But be careful using init as it can result in a race condition.

I’ve hit an issue where I had something like:

main.go
foo.go (imported by main.go)
bar.go (imported by foo.go)

Each one of these packages had its own init function and ultimately the bar.go’s init function was being run first, followed by the foo.go’s init function and finally followed by the main.go’s init function.

The reason this was an issue was because main.go was loading some environment variables needed by bar.go but those variables weren’t available by the time the bar.go was running as that happened before main.go’s init function had executed.

The solution was to rename all the init functions to Init and explicitly call them to bootstrap the package when needed (i.e. they didn’t automatically bootstrap themselves and so we avoided that race condition).

New vs Make

func new(Type) *Type: allocate memory for custom-user type
func make(Type, size IntegerType) Type: allocate memory for builtin types (Slice, Map, Chan)

package main

import "fmt"

func main() {
  foo := make(map[string]string)
  fmt.Println(foo) // map[]
  foo["k1"] = "bar"
  fmt.Println(foo) // map[k1:bar]
  fmt.Println(foo["k1"]) // bar
  
  type bar [5]int
  b := new(bar)
  fmt.Println(b) // &[0 0 0 0 0]
  b[0] = 1
  fmt.Println(b) // &[1 0 0 0 0]
}

Custom Types

package main

import (
  "bytes"
  "fmt"
)

type path []byte // our custom Type

// method attached to our custom Type
func (p *path) TruncateAtFinalSlash() {
  i := bytes.LastIndex(*p, []byte("/"))

  if i >= 0 {
    *p = (*p)[0:i]
  }
}

func main() {
  pathName := path("/usr/bin/tso") // Conversion from string to path.

  pathName.TruncateAtFinalSlash()

  fmt.Printf("%s\n", pathName)
}

Alternative example:

package main

import "fmt"

type foo [5]int

func main() {
  f := new(foo)
  fmt.Println(f) // &[0 0 0 0 0]
  f[0] = 1
  fmt.Println(f) // &[1 0 0 0 0]
  f.Bar()
  fmt.Println(f) // &[1 2 0 0 0]

  // We can coerce custom types like we can with built-in types
  b := foo([5]int{9, 9, 9})
  fmt.Println(b) // [9 9 9 0 0]
  
  // Check the types
  fmt.Printf("%T\n", b)               // main.foo
  fmt.Printf("%T\n", [5]int{9, 9, 9}) // [5]int
}

func (f *foo) Bar() {
  f[1] = 2
}

Custom Errors

package main

import (
	"errors"
	"fmt"
)

type CustomErr interface {
	error
}

var (
	CustomError = errors.New("Some Custom Error")
)

// we return an 'error' type specifically
// CustomError is indeed an error type, so returning that works
// but also, the interface CustomErr means the CustomError var _is_ a CustomErr type too
func succeed(b bool) (string, error) {
	if b {
		return "success", nil
	}
	return "", CustomError
}

func main() {
	fmt.Println(CustomError) // Some Custom Error

	resp, err := succeed(false)
	if err != nil {
		t, ok := err.(CustomErr)
		if ok {
			fmt.Printf("t is a custom error '%+v' (%T)\n", t, t)
		}
		fmt.Println(err) // Some Custom Error
	}
	fmt.Println(resp) // success (only if succeed(true) is called)
}

Another form of custom error can be seen using a struct:

type errCustom struct {
    message string
    code    int
}

func (e errCustom) Error() string {
    return fmt.Sprintf("error message: %s (code: %d)", e.message, e.code)
}

We would then implement the struct with the specific error context information and return it:

return errCustom{
    message: "whoops",
    code:    500
}

Function Types

package main

import "fmt"

type Foo func(int, string)

func (f Foo) Bar(s string) {
  fmt.Printf("s: %s\n", s)
}

func FooIt(x int, y string) {
  fmt.Printf("x: %d - y: %s\n", x, y)
}

// We HAVE to define the incoming type of "fn"
// Which in this case is a Foo type
func TestIt(fn Foo) {
  fn(99, "problems")
}

// We could do this without defining a func type
// But as you can see, this is a bit ugly
// Plus if we need this function passed around a lot
// then it means a lot of duplicated effort 
// typing the signature over and over
func TestItManually(fn func(int, string)) {
  fn(100, "problems")
}

func main() {
  // Here we're just demonstrating passing around the FooIt function
  // It demonstrates first-class function support in Go
  // But also that we can ensure the function passed around has the expected signature
  TestIt(FooIt)
  TestItManually(FooIt)
  
  x := Foo(FooIt) // Convert our function into a Foo type
  x(0, "hai")     // Now we can execute it as we would FooIt itself
  
  FooIt(1, "bye")
  
  // Notice the types are different
  // FooIt is just a function with a signature (no known type associated with it)
  // Whereas "x" is of known type "Foo"
  fmt.Printf("%T\n", FooIt) // func(int, string)
  fmt.Printf("%T\n", x)     // main.Foo
  
  // But we'll see that the function "x" 
  // which was converted into a Foo type
  // now has access to a Bar method
  // Although FooIt has a matching signature, it's not a Foo type
  // and so it doesn't have a Bar method available
  x.Bar("we have a Bar method")
  
  // We can't even execute:
  // FooIt.Bar("we don't have a Bar method")
  // Because the compiler will stop us
}

Enumerator IOTA

Within a constant declaration, the predeclared identifier iota represents successive untyped integer constants. It is reset to 0 whenever the reserved word const appears in the source.

package main

import "fmt"

const (
  foo = iota // 0
  bar
  _ // skip this value
  baz
)

const (
  beep = iota // 0 (reset)
  boop
)

func main() {
  fmt.Println(foo, bar, baz) // 0 1 3
  fmt.Println(beep, boop)    // 0 1
}

Struct: Var vs Type

A variable of Struct type doesn’t need to be instantiated like a type struct:

package main

import "fmt"

var data struct {
  A string
  B string
}

type data2 struct {
  A string
  B string
}

func main() {
  data.A = "Hai"
  data.B = "Bai"
  
  fmt.Printf(
    "%#v, %+v, %+v", 
    data.A, 
    data.B, 
    data2{A: "abc", B: "def"}
  )
  // "Hai", Bai, {A:abc B:def}
}

Embedded Structs

The first example demonstrates a ’named’ field utilising an embedded Struct:

package main

import "fmt"

type Point struct {
  X, Y int
}

type Circle struct {
  Center Point // named embeded field
  Radius int
}

type Wheel struct {
  Circle Circle // named embeded field
  Spokes int
}

func main() {
  var w Wheel
  w.Circle.Center.X = 8
  w.Circle.Center.Y = 8
  w.Circle.Radius = 5
  w.Spokes = 20

  fmt.Printf("%+v", w)
}

Which prints:

{Circle:{Center:{X:8 Y:8} Radius:5} Spokes:20}

The second example demonstrates an ‘anonymous’ field instead:

package main

import "fmt"

type Point struct {
  X, Y int
}

type Circle struct {
  Point
  Radius int
}

type Wheel struct {
  Circle
  Spokes int
}

func main() {
  var w Wheel
  w.X = 8       // w.Circle.Point.X
  w.Y = 8       // w.Circle.Point.Y
  w.Radius = 5  // w.Circle.Radius
  w.Spokes = 20

  fmt.Printf("%+v", w)
}

Which prints:

{Circle:{Point:{X:8 Y:8} Radius:5} Spokes:20}

Note: anonymous fields don’t work shorthand literal Struct

The following example demonstrates how methods of a composited object can be accessed from the consuming object:

package main

import "fmt"

type Point struct {
  X, Y int
}

func (p Point) foo() {
  fmt.Printf("foo: %+v\n", p)
}

type Circle struct {
  Point
  Radius int
}

type Wheel struct {
  Circle
  Spokes int
}

func main() {
  var w Wheel
  w.X = 8      // w.Circle.Point.X
  w.Y = 8      // w.Circle.Point.Y
  w.foo()      // w.Circle.Point.foo()
  w.Radius = 5 // w.Circle.Radius
  w.Spokes = 20

  fmt.Printf("%+v", w)
}

Which prints:

foo: {X:8 Y:8}
{Circle:{Point:{X:8 Y:8} Radius:5} Spokes:20}

The following example shows how you can mix and match ’named’ and ‘anonymous’ embeds:

package main

import "fmt"

// Point is ...
type Point struct {
	X, Y int
}

func (p Point) foo() {
	fmt.Printf("foo: %+v\n", p)
}

// Circle is ...
type Circle struct {
	p      Point
	Radius int
}

// Wheel is ...
type Wheel struct {
	Circle
	Spokes int
}

func main() {
	var w Wheel
	w.Spokes = 1
	w.Radius = 2
	w.p.X = 3 // w.Circle.p.X
	w.p.Y = 4 // w.Circle.p.X
	w.p.foo()
	fmt.Printf("%+v", w)
}

Here is a more practical example that demonstrates how embedded functionality can make code more expressive:

package main

import (
  "fmt"
  "sync"
)

// Anonymous struct
var cache = struct {
  sync.Mutex
  mapping map[string]string
}{
  mapping: make(map[string]string), // initial zero value for map
}

func setValue() {
  cache.Lock()
  cache.mapping["foo"] = "bar"
  cache.Unlock()
}

func main() {
  setValue()

  cache.Lock()
  v := cache.mapping["foo"]
  cache.Unlock()

  fmt.Printf("v: %s", v)
}

Reference vs Value

Summary: limit passing by reference unless the size of the copied value is a problem (i.e. memory allocations).

Map data structures are passed by reference, rather than a copied value.

Consider the following example:

package main

import "fmt"

func main() {
  m := make(map[string]int)
  fmt.Println("main before, m = ", m)
  foo(m)
  fmt.Println("main after, m = ", m)
}

func foo(m map[string]int) {
  fmt.Println("foo before, m = ", m)
  m["hai"] = 123
  fmt.Println("foo after, m = ", m)
}

The output:

main before, m =  map[]
foo before, m =  map[]
foo after, m =  map[hai:123]
main after, m =  map[hai:123]

Notice how the map is mutated even once the foo function has finished. This is because a reference to the underlying struct memory location was passed and so the changes made were effective everywhere.

In fact, anything with make is a reference, as well as any explicit interface.

Now consider the next example, which is a slice of integers:

package main

import "fmt"

func main() {
	m := []int{1, 2, 3}
	fmt.Println("main before, m = ", m)
	foo(m)
	fmt.Println("main after, m = ", m)
}

func foo(m []int) {
	fmt.Println("foo before, m = ", m)
	m = append(m, 4)
	fmt.Println("foo after, m = ", m)
}

The output:

main before, m =  [1 2 3]
foo before, m =  [1 2 3]
foo after, m =  [1 2 3 4]
main after, m =  [1 2 3]

Notice how the slice isn’t showing as mutated after the foo function has finished. This is because a copy of the slice was passed to the function for mutating.

In the following example we pass a struct by value (i.e. a copy of the person variable is passed):

package main

import "fmt"

type Person struct {
    firstName string
    lastName  string
}

func changeName(p Person) {
    p.firstName = "Bob"
}

func main() {
    person := Person {
        firstName: "Alice",
        lastName: "Dow",
    }

    changeName(person)

    fmt.Println(person)
}

In order to have that type of change we would need to pass a pointer reference:

package main

import "fmt"

type Person struct {
    firstName string
    lastName  string
}

func changeName(p *Person) {
    p.firstName = "Bob"
}

func main() {
    person := Person {
        firstName: "Alice",
        lastName: "Dow",
    }

    changeName(&person)

    fmt.Println(person)
}

See all methods on `<Type>`

errType := reflect.TypeOf(err)
for i := 0; i < errType.NumMethod(); i++ {
  method := errType.Method(i)
  fmt.Println(method.Name)
}

Convert Struct into JSON

package main

import (
  "encoding/json"
  "fmt"
  "os"
  "time"
)

func main() {
  type Message struct {
    Sequence  int    `json:"sequence"`
    Title     string `json:"title"`
    Timestamp time.Time   `json:"timestamp"`
  }
  msg := Message{1, "Foobar", time.Now()}
  b, err := json.Marshal(msg)
  if err != nil {
    fmt.Println("error:", err)
  }
  os.Stdout.Write(b)
}

Pretty Printing JSON String

package main

import (
  "encoding/json"
  "fmt"
  "os"
)

func main() {
  type ColorGroup struct {
    ID     int
    Name   string
    Colors []string
  }
  group := ColorGroup{
    ID:     1,
    Name:   "Reds",
    Colors: []string{"Crimson", "Red", "Ruby", "Maroon"},
  }
  b, err := json.MarshalIndent(group, "", "    ")
  if err != nil {
    fmt.Println("error:", err)
  }
  os.Stdout.Write(b)
}

Convert Struct into YAML

package main

import (
  "fmt"

  "gopkg.in/yaml.v2"
)

type ComponentYaml struct {
  Id  string `yaml:"id"`
  Url string `yaml:"url"`
}

type ComponentsYamlList struct {
  Components []ComponentYaml `yaml:"components"`
}

func main() {
  var y ComponentsYamlList

  yaml.Unmarshal([]byte("components:\n  - id: google\n    url: http://google.com\n  - id: integralist\n    url: http://integralist.co.uk"), &y)

  fmt.Println(y)
}

Unknown YAML Structure

package main

import (
  "encoding/json"
  "fmt"
  "os"

  "gopkg.in/yaml.v2"
)

var yml = []byte(`
- key: foo
  value: bar
  secret: false
- key: beep
  value: boop
  secret: true
`)

type Data struct {
  Items []map[string]interface{}
}

func main() {
  y := []map[string]interface{}{}

  if err := yaml.Unmarshal(yml, &y); err == nil {
    fmt.Printf("%#v\n", y)
  } else {
    fmt.Println(err.Error())
  }

  myYaml := Data{Items: y}

  json.NewEncoder(os.Stdout).Encode(myYaml.Items)
}

Sorting Structs

package main

import (
  "fmt"
  "sort"
)

type vals []Value

type Value struct {
  Key string
  Value string
  Secure bool
}

// Satisfy the Sort interface
func (v vals) Len() int      { return len(v) }
func (v vals) Swap(i, j int) { v[i], v[j] = v[j], v[i] }
func (v vals) Less(i, j int) bool { 
  return v[i].Key < v[j].Key 
}

func main() {
  orig := vals{
    {"CK", "BV", false},
    {"DK", "AV", true},
    {"AK", "CV", false},
    {"BK", "DV", true},
  }
  
  fmt.Printf("%+v\n\n", orig)
  sort.Sort(orig)
  fmt.Printf("%+v\n\n", orig)
}

Here is a similar version that sorts by name and age:

package main

import (
  "fmt"
  "sort"
)

type person struct {
  Name string
  Age  int
}

type byName []person

func (p byName) Len() int {
  return len(p)
}
func (p byName) Less(i, j int) bool {
  return p[i].Name < p[j].Name
}
func (p byName) Swap(i, j int) {
  p[i], p[j] = p[j], p[i]
}

type byAge []person

func (p byAge) Len() int {
  return len(p)
}
func (p byAge) Less(i, j int) bool {
  return p[i].Age < p[j].Age
}
func (p byAge) Swap(i, j int) {
  p[i], p[j] = p[j], p[i]
}

func main() {
  kids := []person{
    {"Jill", 9},
    {"Jack", 10},
  }

  sort.Sort(byName(kids))
  fmt.Println(kids)

  sort.Sort(byAge(kids))
  fmt.Println(kids)
}

Which results in:

[{Jack 10} {Jill 9}]
[{Jill 9} {Jack 10}]

Read Users Input

reader := bufio.NewReader(os.Stdin)
fmt.Print("Enter text: ")
text, _ := reader.ReadString('\n')
fmt.Println(text)

HTTP Middleware

This code was modified from @matryer:

package main

import (
  "fmt"
  "log"
  "net/http"
  "os"
)

type data struct {
  Greeting string
  Punct    string
  Who      string
}

func (s data) ServeHTTP(w http.ResponseWriter, r *http.Request) {
  fmt.Fprint(w, s.Greeting, s.Punct, s.Who)
}

type adapter func(http.Handler) http.Handler

func adapt(h http.Handler, adapters ...adapter) http.Handler {
  // Ideally you'd do this in reverse
  // to ensure the order of the middleware
  // matches their specified order
  for _, adapter := range adapters {
    h = adapter(h)
  }
  return h
}

func notify(logger *log.Logger) adapter {
  return func(h http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
      logger.Println("before")
      defer logger.Println("after")
      h.ServeHTTP(w, r)
    })
  }
}

func doSomething() adapter {
  return func(h http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
      fmt.Println("before")
      defer fmt.Println("after")
      h.ServeHTTP(w, r)
    })
  }
}

func main() {
  http.Handle("/hello", &data{"Hello", " ", "Gophers!"})

  logger := log.New(os.Stdout, "server: ", log.Lshortfile)

  http.Handle("/hello-with-middleware", adapt(
    &data{"Hello", " ", "Gophers!"},
    notify(logger), // runs second
    doSomething(), // runs first
  ))

  http.ListenAndServe("localhost:4000", nil)
}

This code will run a web server with two valid endpoints:

/hello
/hello-with-middleware

The client sees the same output but the latter endpoint produces the following stdout output:

before
server: middleware.go:35: before
server: middleware.go:38: after
after

Sessions

package main

import (
  "fmt"
  "net/http"
  "time"
)

const cookiePrefix = "integralist-example-cookie-"

func main() {
  http.HandleFunc("/", login)
  http.HandleFunc("/admin", admin)
  http.HandleFunc("/logout", logout)
  http.ListenAndServe("localhost:4000", nil)
}

func login(w http.ResponseWriter, r *http.Request) {
  if r.Method == "GET" {
    fmt.Fprintf(w, `
<html>
  <body>
    <form method="POST">
      Username: <input type="text" name="username">
      <br />
      Password: <input type="password" name="password">
      <br />
      <input type="submit" value="Login">
  </body>
</html>
`)
  }

  if r.Method == "POST" {
    username := r.FormValue("username")
    password := r.FormValue("password")

    if username == "admin" && password == "password" {
      http.SetCookie(w, &http.Cookie{
        Name:  cookiePrefix + "user",
        Value: username,
      })
      http.Redirect(w, r, "/admin", 302)
    } else {
      fmt.Fprintf(w, `
<html>
  <body>
    Login details were incorrect. Sorry, <a href="/">try again</a>
  </body>
</html>
`)
    }
  }
}

func logout(w http.ResponseWriter, r *http.Request) {
  http.SetCookie(w, &http.Cookie{
    Name:    cookiePrefix + "user",
    Value:   "",
    Expires: time.Now(),
  })

  http.Redirect(w, r, "/", 302)
}

func admin(w http.ResponseWriter, r *http.Request) {
  cookie, err := r.Cookie(cookiePrefix + "user")
  if err != nil {
    http.Redirect(w, r, "/", 401) // Unauthorized
    return
  }

  fmt.Fprintf(w, `
<html>
  <body>
    Logged into admin area as: %s<br><br>
    <a href="/logout">Logout</a>
  </body>
</html>
`, cookie.Value)
}

HTTPS TLS Request

package requester

import (
  "crypto/tls"
  "crypto/x509"
  "flag"
  "io/ioutil"
  "log"
  "net/http"
)

var (
  certFile = flag.String("cert", "/etc/pki/tls/certs/client.crt", "A PEM eoncoded certificate file.")
  keyFile  = flag.String("key", "/etc/pki/tls/private/client.key", "A PEM encoded private key file.")
  caFile   = flag.String("CA", "/etc/ca/cloud-ca.pem", "A PEM eoncoded CA's certificate file.")
)

func SecureClient() *http.Client {
  // Load client cert
  cert, err := tls.LoadX509KeyPair(*certFile, *keyFile)
  if err != nil {
    log.Fatal(err)
  }

  // Load CA cert
  caCert, err := ioutil.ReadFile(*caFile)
  if err != nil {
    log.Fatal(err)
  }
  caCertPool := x509.NewCertPool()
  caCertPool.AppendCertsFromPEM(caCert)

  // Setup HTTPS client
  tlsConfig := &tls.Config{
    Certificates:       []tls.Certificate{cert},
    RootCAs:            caCertPool,
    InsecureSkipVerify: true,
  }
  tlsConfig.BuildNameToCertificate()
  transport := &http.Transport{TLSClientConfig: tlsConfig}
  client := &http.Client{Transport: transport}

  return client
}

And to use it…

client := requester.SecureClient()

// GET
resp, err := client.Get(someEndpoint)

// POST
req, err := http.NewRequest("POST", someEndpoint, bytes.NewBuffer(jsonStr))
req.Header.Set("Content-Type", "application/json")
resp, err := client.Do(req)

HTTP GET Web Page

package main

import (
  "fmt"
  "io/ioutil"
  "net/http"
  "os"
)

func main() {
  response, err := http.Get("http://www.integralist.co.uk/")
  if err != nil {
    fmt.Println(err.Error())
    os.Exit(1)
  }

  defer response.Body.Close()

  contents, err := ioutil.ReadAll(response.Body)
  if err != nil {
    fmt.Println(err.Error())
    os.Exit(1)
  }

  fmt.Println(string(contents))
}

Note: don’t use this ☝️ as it has timeout set to infinity by default

Safer option is to create your own http.Client struct…

package main

import (
	"fmt"
	"io/ioutil"
	"net"
	"net/http"
	"os"
	"sync"
	"time"
)

func defaultClient(wg *sync.WaitGroup) {
	// http.Get calls http.DefaultClient
	// https://golang.org/src/net/http/client.go?s=12086:12134#L359
	// https://golang.org/pkg/net/http/#Client
	response, err := http.Get("https://www.integralist.co.uk/")
	if err != nil {
		fmt.Println(err.Error())
		os.Exit(1)
	}

	defer response.Body.Close()

	contents, err := ioutil.ReadAll(response.Body)
	if err != nil {
		fmt.Println(err.Error())
		os.Exit(1)
	}

	fmt.Println("\ndefaultClient...")
	fmt.Println(response.Status)
	fmt.Println(string(contents[0:10]))

	wg.Done()
}

func customClient(wg *sync.WaitGroup) {
	var netClient = &http.Client{
		Timeout: time.Second * 10,
	}
	response, err := netClient.Get("https://www.integralist.co.uk/")
	if err != nil {
		fmt.Println(err.Error())
		os.Exit(1)
	}

	defer response.Body.Close()

	contents, err := ioutil.ReadAll(response.Body)
	if err != nil {
		fmt.Println(err.Error())
		os.Exit(1)
	}

	fmt.Println("\ncustomClient...")
	fmt.Println(response.Status)
	fmt.Println(string(contents[0:10]))

	wg.Done()
}

func customTransport(wg *sync.WaitGroup) {
	var netTransport = &http.Transport{
		Dial: (&net.Dialer{
			Timeout: 5 * time.Second,
		}).Dial,
		TLSHandshakeTimeout: 5 * time.Second,
	}

	var netClient = &http.Client{
		Timeout:   time.Second * 10,
		Transport: netTransport,
	}

	response, err := netClient.Get("https://www.integralist.co.uk/")
	if err != nil {
		fmt.Println(err.Error())
		os.Exit(1)
	}

	defer response.Body.Close()

	contents, err := ioutil.ReadAll(response.Body)
	if err != nil {
		fmt.Println(err.Error())
		os.Exit(1)
	}

	fmt.Println("\ncustomTransport...")
	fmt.Println(response.Status)
	fmt.Println(string(contents[0:10]))

	wg.Done()
}

func main() {
	var wg sync.WaitGroup
	wg.Add(3)
	go defaultClient(&wg)
	go customClient(&wg)
	go customTransport(&wg)
	wg.Wait()
}

Custom HTTP Request Methods

Go doesn’t provide abstractions for all the various HTTP request types, so for things like PUT you have to implement it yourself. The following is an example that creates a secure (TLS/HTTPS) PUT abstraction…

func SecurePut(url, contentType string, configFile io.Reader) (*http.Response, error) {
  client := &http.Client{Transport: configureTLS()}
  req, err := http.NewRequest("PUT", url, configFile)
  if err != nil {
    return nil, err
  }
  req.Header.Add("Content-Type", contentType)
  resp, err := client.Do(req)

  return resp, err
}

func configureTLS() *http.Transport {
  certFilePath := "path/to/cert"
  keyFilePath := "path/to/privateKey"
  caPath := "path/to/ca"

  // Load client cert
  cert, err := tls.LoadX509KeyPair(certFilePath, keyFilePath)
  if err != nil {
    msg := fmt.Sprintf("Error loading developer cert, path: \"%s\"", certFilePath)
    output.Error(msg)
  }

  // Load CA cert
  caCert, err := ioutil.ReadFile(caPath)
  if err != nil {
    msg := fmt.Sprintf("Error loading CA cert, path: \"%s\"", caPath)
    output.Error(msg)
  }
  caCertPool := x509.NewCertPool()
  caCertPool.AppendCertsFromPEM(caCert)

  // Setup HTTPS client
  tlsConfig := &tls.Config{
    Certificates:       []tls.Certificate{cert},
    RootCAs:            caCertPool,
    InsecureSkipVerify: true,
  }
  tlsConfig.BuildNameToCertificate()

  return &http.Transport{TLSClientConfig: tlsConfig}
}

Pointers

package main

import "fmt"

// Point stores co-ordinates
type Point struct {
  x int
  y int
}

// If receiver (Point) isn't set to a pointer (*Point) 
// then the struct's field value won't be updated outside the method
func (p *Point) scaleBy(factor int) {
  fmt.Printf("scaleBy (before modification): %+v\n", p)

  // Don't need to derefence (*) struct fields
  // Compiler will perform an implicit &p for you
  // You only need to dereference in standard functions when a argument pointer is required (see below Array Pointer example)
  p.x *= factor
  p.y *= factor

  fmt.Printf("scaleBy (after modification): %+v\n", p)
}

func main() {
  // Doesn't matter if we do or don't get the address space (&) for foo/bar's Point
  foo := &Point{1, 2}
  bar := &Point{6, 8}

  fmt.Printf("Main foo.x: %+v\n", foo.x)
  fmt.Printf("Main bar.x: %+v\n", bar.x)

  foo.scaleBy(5)
  bar.scaleBy(5)

  fmt.Printf("Main foo.x: %+v\n", foo.x)
  fmt.Printf("Main foo.y: %+v\n", foo.y)

  fmt.Printf("Main bar.x: %+v\n", bar.x)
  fmt.Printf("Main bar.y: %+v\n", bar.y)
}

Note: compiler can only apply implicit dereference for variables and struct fields. This wouldn’t work Point{1, 2}.scaleBy(5)

Results in the following output:

Main foo.x: 1
Main bar.x: 6
scaleBy (before modification): &{x:1 y:2}
scaleBy (after modification): &{x:5 y:10}
scaleBy (before modification): &{x:6 y:8}
scaleBy (after modification): &{x:30 y:40}
Main foo.x: 5
Main foo.y: 10
Main bar.x: 30
Main bar.y: 40

Array Pointer

Deference an Array pointer so you can mutate the original Array values:

package main

import "fmt"

func main() {  
    x := [3]int{1,2,3}

    func(arr *[3]int) {
        (*arr)[0] = 7
        fmt.Println(arr) //prints &[7 2 3]
    }(&x)

    fmt.Println(x) //prints [7 2 3]
}

Alternatively you can utilise a Slice instead of an Array, as the slice ‘header’ already has a ‘pointer’ to an underlying Array:

package main

import "fmt"

func main() {  
    x := []int{1,2,3}

    func(arr []int) {
        arr[0] = 7
        fmt.Println(arr) //prints [7 2 3]
    }(x)

    fmt.Println(x) //prints [7 2 3]
}

Mutating Values

package main

import "fmt"

type Compose string

func (c *Compose) Details() string {
  *c = "beep boop"
  return fmt.Sprintf("Here are your details: %v", *c)
}

func main() {
  var c Compose
  c = "hai"
  fmt.Printf("c: %+v\n", c) // c
  fmt.Println(c.Details())
  fmt.Printf("c: %+v\n", c) // beep boop
}

Type Assertion

if e, ok := err.(net.Error); ok && e.Timeout() {
  //
}

type argError struct {
    arg  int
    prob string
}

func (e *argError) Error() string {
    return fmt.Sprintf("%d - %s", e.arg, e.prob)
}

if ae, ok := e.(*argError); ok {
  //
}

Line Counting

Demonstrates how to use bufio package to scan a file and read it line by line, and then how to increment a map integer value using the shortcut map[key]++. Finally, demonstrates nested maps and ranging over them:

package main

import (
  "bufio"
  "fmt"
  "os"
)

func main() {
  counts := make(map[string]map[string]int)
  files := os.Args[1:]
  if len(files) == 0 {
    countLines(os.Stdin, "n/a", counts)
  } else {
    for _, arg := range files {
      f, err := os.Open(arg)
      if err != nil {
        fmt.Fprintf(os.Stderr, "dup2: %v\n", err)
        continue
      }
      countLines(f, arg, counts)
      f.Close()
    }
  }
  for key, nestedMap := range counts {
    fmt.Printf("Text: %s\n", key)
    for filename, count := range nestedMap {
      fmt.Printf("\tFile: %s\n\tCount: %d\n", filename, count)
    }
    fmt.Println("")
  }
}

func countLines(f *os.File, filename string, counts map[string]map[string]int) {
  input := bufio.NewScanner(f)
  for input.Scan() {
    if val, ok := counts[input.Text()]; ok {
      val[filename]++
    } else {
      inner := make(map[string]int)
      inner[filename]++
      counts[input.Text()] = inner
    }
  }
}

Reading File in Chunks

package main

import (
  "fmt"
  "log"
  "os"
)

func main() {
  // Create file (truncates file if it already exists)
  file, err := os.Create("created.txt")
  if err != nil {
    log.Fatal(err)
  }

  // Populate byte slice with some content
  b := make([]byte, 0)
  for i := 0; i < 5; i++ {
    b = append(b, '!')
    b = append(b, '\n')
    // notice single quotes for Rune rather than double quote for String
  }
  for i := 0; i < 5; i++ {
    b = append(b, '?')
    b = append(b, '\n')
    // notice single quotes for Rune rather than double quote for String
  }
  for i := 0; i < 5; i++ {
    b = append(b, '%')
    b = append(b, '\n')
    // notice single quotes for Rune rather than double quote for String
  }

  // Write file contents
  bytesWritten, err := file.Write(b)
  if err != nil {
    log.Fatal(err)
  }
  fmt.Printf("Bytes written: %+v\n", bytesWritten)

  // Although getting the bytes written was useful for us
  // in this example, you might need to get total bytes
  // which can be done by copying file contents into dev/null
    // io.Copy(ioutil.Discard, resp.Body)

  // Get current offset
  // 1st arg is how much to seek forward/backwards by
  // 2nd arg is relative to different settings
  // 		0 == relative to start of file
  // 		1 == current offset
  // 		2 == relative to end of file
  currentOffset, err := file.Seek(0, 1)
  if err != nil {
    log.Fatal(err)
  }
  fmt.Printf("Current offset: %d\n", currentOffset)
  file.Seek(-currentOffset, 1) // Return to start of file for next Read

  // Read buffered view of file
  data := make([]byte, 10, bytesWritten) // create slice with underlying Array capacity set to total file bytes size
  eof := false
  for !eof {
    count, err := file.Read(data)
    if err != nil {
      eof = true
    }
    fmt.Printf("read %d bytes: %q\n", count, data[:count])
  }
}

Time

now := time.Now()
fmt.Println(now)
expiration := now.Add(time.Hour * 24 * 30)
fmt.Println("Thirty days from now will be : ", expiration)

Here we measure time:

package main

import (
  "fmt"
  "time"
)

// Sleep requires a Duration
// time has set of constants we can use (lowest is 1 Duration)
// Second constant is an abstraction over the other constants
func main() {
  start := time.Now()
  time.Sleep(time.Duration(5) * time.Second) // sleep 5 seconds
  secs := time.Since(start).Seconds()

  fmt.Printf("Time spent: %f seconds", secs)
}

Here is a basic example that pauses execution of a channel until the timer has expired (you would use this over a timer.Sleep because you can cancel a timer before it has expired):

package main

import (
  "fmt"
  "time"
)

func main() {
  timer := time.NewTimer(time.Second * 2)

  <-timer.C // pauses for two seconds

  fmt.Println("Timer expired")
}

Example of cancelling the timer:

package main

import (
  "fmt"
  "time"
)

func main() {
  timer := time.NewTimer(time.Second * 2)

  // Expensive process run in a separate thread
  go func() {
    <-timer.C
    fmt.Println("Timer expired")
  }()

  stop := timer.Stop() // cancel the timer
  fmt.Println(stop)    // true
}

We can do a similar thing with Timers but in a different way:

package main

import (
  "fmt"
  "time"
)

func main() {
  ticker := time.NewTicker(time.Millisecond * 500)

  // Repetitive process
  go func() {
    // Range over the channel rather than pull from it
    for t := range ticker.C {
      fmt.Println("Tick:", t)
    }
  }()

  // Stop ticker after three ticks/intervals
  time.Sleep(time.Millisecond * 1500)
  ticker.Stop()
}

We can combine all these items together with a select statement like so:

package main

import "time"
import "fmt"

func main() {
  timeChan := time.NewTimer(time.Second).C
  tickChan := time.NewTicker(time.Millisecond * 400).C

  // Used to signify we're done with this program
  doneChan := make(chan bool)

  // Sleep for two seconds, then notify the channel we're done
  go func() {
    time.Sleep(time.Second * 2)
    doneChan <- true
  }()

  for {
    select {
    case <-timeChan:
      fmt.Println("Timer expired")
    case <-tickChan:
      fmt.Println("Ticker ticked")
    case <-doneChan:
      fmt.Println("Done")
      return
    }
  }
}

The output of this program would be something like:

Ticker ticked
Ticker ticked
Timer expired
Ticker ticked
Ticker ticked
Done

Starting and Stopping things with Channels

I would imagine that for most cases you’ll want to use a time.NewTimer as seen in previous examples if you want to stop a goroutine that’s processing a long running program. The following example is more for stopping a goroutine that’s running code at a set interval (although using time.NewTicker would probably be more appropriate):

package main

import (
  "fmt"
  "time"
)

func main() {
  quit := make(chan bool)

  // Run a piece of code at a set interval
  go func() {
    for {
      select {
      case <-quit:
        return
      default:
        fmt.Println("Not ready to stop this goroutine")
        time.Sleep(time.Millisecond * 100)
      }
    }
  }()

  // Do other stuff for two seconds
  time.Sleep(time.Second * 2)

  // Quit goroutine
  quit <- true

  fmt.Println("Goroutine was stopped")
}

Starting a goroutine:

package main

import (
  "fmt"
  "time"
)

func main() {
  // Use a struct type channel as it clarifies your intent
  // Which is this channel is used for 'signalling'
  start := make(chan struct{})

  for i := 0; i < 10000; i++ {
    go func() {
      <-start // wait for the start channel to be closed
      fmt.Println("do stuff")
    }()
  }

  // at this point, all goroutines are ready to go
  // we just need to tell them to start by
  // closing the start channel
  close(start)

  fmt.Println("Let's pause briefly to give goroutines time to execute")

  time.Sleep(time.Millisecond * 10)
}

Stopping a goroutine:

package main

import (
  "fmt"
  "time"
)

func main() {
  // Use a struct type channel as it clarifies your intent
  // Which is this channel is used for 'signalling'
  done := make(chan struct{})

  // Long running process put onto a thread
  go func() {
    fmt.Println("Inside thread doing expensive processing")
    time.Sleep(time.Second * 5)
    close(done)
  }()

  fmt.Println("Do other things")

  // Wait for long running process to finish
  <-done

  fmt.Println("Do more things")
}

Channel Pipelines

The principle of a pipeline, is to take data from one function and pass it into another function, that receiving function will process the received data and then that result is returned and subsequently passed onto another function… rinse and repeat for however long your pipeline needs to be.

In the below example (copied from here) demonstrates how a set of functions accept a channel and return a channel and so channels is the ‘data’ that is passed around the pipeline functions:

package main

import "fmt"
import "sync"

func ingest() <-chan []string {
  out := make(chan []string)
  go func() {
    out <- []string{"aaaa", "bbb"}
    out <- []string{"cccccc", "dddddd"}
    out <- []string{"e", "fffff", "g"}
    close(out)
  }()
  return out
}

func process(concurrency int, in <-chan []string) <-chan int {
  var wg sync.WaitGroup
  wg.Add(concurrency)

  out := make(chan int)

  work := func() {
    for data := range in {
      for _, word := range data {
        out <- len(word)
      }
    }
    wg.Done()

  }

  go func() {
    for i := 0; i < concurrency; i++ {
      go work()
    }

  }()

  go func() {
    wg.Wait()
    close(out)
  }()
  return out
}

func store(in <-chan int) <-chan struct{} {
  done := make(chan struct{})
  go func() {
    defer close(done)
    for data := range in {
      fmt.Println(data)
    }
  }()
  return done
}

func main() {
  // stage 1 ingest data from source
  in := ingest()

  // stage 2 - process data
  reduced := process(4, in)

  // stage 3 - store
  <-store(reduced)
}

Templating

Here is a basic program that uses a Struct for its data source:

package main

import (
  "log"
  "os"
  "text/template"
)

type dataSource struct {
  Baz int
}

func (ds dataSource) Foo() string {
  return "I am foo"
}

func (ds dataSource) Bar() string {
  return "I am bar"
}

const templ = `
  Foo: {{.Foo}}
  Piping: {{.Bar | printf "Bar: %s"}}
  Function: {{.Baz | qux}}
`

func qux(baz int) int {
  return baz * 2
}

// template.Must handles parsing errors better
var setupTemplate = template.Must(
  template.New("whatever").
    Funcs(template.FuncMap{"qux": qux}).
    Parse(templ),
)

func main() {
  ds := dataSource{5}

  if err := setupTemplate.Execute(os.Stdout, ds); err != nil {
    log.Fatal(err)
  }
}

Note: printf is a built-in function for templating and is functionally equivalent to fmt.Sprintf

Program output:

Foo: I am foo
Piping: Bar: I am bar
Function: 10

Here is a HTML templating version:

package main

import (
  "html/template"
  "log"
  "os"
)

var data struct {
  A string        // untrusted plain text
  B template.HTML // trusted HTML
}

const templ = `<p>A: {{.A}}</p><p>B: {{.B}}</p>`

func main() {
  t := template.Must(template.New("escape").Parse(templ))

  data.A = "<b>Hello</b>"
  data.B = "<b>Hello</b>"

  if err := t.Execute(os.Stdout, data); err != nil {
    log.Fatal(err)
  }
}

The output would be:

<p>A: &lt;b&gt;Hello&lt;/b&gt;</p>
<p>B: <b>Hello</b></p>

Error handling

The following code outputs:

This is our custom error with some more context prefixed: oh noes!

Here’s the code:

package main

import (
  "errors"
  "fmt"
)

type errWithContext struct {
  err error
  msg string
}

func (e errWithContext) Error() string {
  return e.msg + ": " + e.err.Error()
}

func triggerError() (bool, error) {
  return false, errors.New("oh noes!")
}

func main() {
  var e *errWithContext

  _, err := triggerError()
  if err != nil {
    e = &errWithContext{
      err,
      "This is our custom error with some more context prefixed",
    }
  }

  fmt.Print(e.Error())
}

Socket Programming

There are two main types of sockets:

STREAM sockets (e.g. TCP)
DATAGRAM sockets (e.g. UDP)

Note: a “unix domain socket” is actually a physical file
it’s useful for local (same host) data communication

The principle steps behind a socket is:

Create the socket
Bind the socket to an address (e.g. 127.0.0.1:80)
Listen for socket connections
Accept the socket connection

There are two main packages in our below example: server.go and client.go.

Run both of them in separate terminals (e.g. go run ...)

Then for the client.go type your message followed by a new line, for example:

Hello World
Message from server: HELLO WORLD

Whilst in the server.go terminal you should see:

Starting TCP server...
Message Received: Hello World

The code for this program is as follows:

server.go

package main

import (
  "bufio"
  "fmt"
  "net"
  "strings"
)

func main() {
  fmt.Println("Starting TCP server...")

  // Listen on all network interfaces (e.g. 0.0.0.0)
  // Documentation: godoc net Listener | less
  listener, _ := net.Listen("tcp", ":8081")

  // Accept connection on the port we specified (see above)
  connection, _ := listener.Accept()

  // Handle incoming connections forever
  for {
    // Listen for message to process ending in newline (\n)
    // Note: single quotes needed for type byte (double quotes is a string)
    message, _ := bufio.NewReader(connection).ReadString('\n')

    // Output message received
    fmt.Println("Message Received:", string(message))

    // Do something with the message (e.g. uppercase it)
    newmessage := strings.ToUpper(message)

    // Send new string back to client
    connection.Write([]byte(newmessage + "\n"))
  }
}

client.go

package main

import (
  "bufio"
  "fmt"
  "net"
  "os"
)

func main() {
  // Open socket connection to a locally runnning TCP server
  connection, _ := net.Dial("tcp", "127.0.0.1:8081")

  // Handle incoming responses
  for {
    // Read the input
    reader := bufio.NewReader(os.Stdin)

    // Message to be sent
    // Note: single quotes needed for type byte (double quotes is a string)
    // Documentation: godoc bufio ReadString | less
    // ReadString reads until the first occurrence of the delimiter \n in the input
    text, _ := reader.ReadString('\n')

    // Send message to open Socket
    fmt.Fprintf(connection, text+"\n")

    // Listen for response
    // Note: single quotes needed for type byte (double quotes is a string)
    message, _ := bufio.NewReader(connection).ReadString('\n')

    fmt.Println("Message from server: " + message)
  }
}

Comparing Maps

This code demonstrates how to be careful about false positives!

package main

import "fmt"

func equal(x, y map[string]int) bool {
  if len(x) != len(y) {
    // fail fast
    return false
  }

  for k, xv := range x {
    // Verify "missing" key and "present but zero" key value
    if yv, ok := y[k]; !ok || yv != xv {
      return false
    }
    
    /*
    // The following condition would incorrectly return "true" for the below example comparison!
    // This is because the empty value for an int type is a zero, while the actual value of x's key is zero
    if xv != y[k] {
      return false
    }
    */
  }

  return true
}

func main() {
  fmt.Println(
    equal(map[string]int{"A": 0}, map[string]int{"B": 42}),
  )
}

Zip File Contents

package main

import (
  "compress/zlib"
  "io"
  "log"
  "os"
)

func main() {
  var err error

  // This defends against an error preventing `defer` from being called
  // As log.Fatal otherwise calls `os.Exit`
  defer func() {
    if err != nil {
      log.Fatalln("\nDeferred log: \n", err)
    }
  }()

  src, err := os.Create("source.txt")
  if err != nil {
    return
  }
  src.WriteString("source content")
  src.Close()

  dest, err := os.Create("new.txt")
  if err != nil {
    return
  }

  openSrc, err := os.Open("source.txt")
  if err != nil {
    return
  }

  zdest := zlib.NewWriter(dest)
  if _, err := io.Copy(zdest, openSrc); err != nil {
    return
  }

  // Close these explicitly
  zdest.Close()
  dest.Close()

  n, err := os.Open("new.txt")
  if err != nil {
    return
  }

  r, err := zlib.NewReader(n)
  if err != nil {
    return
  }
  defer r.Close()
  io.Copy(os.Stdout, r)

  err = os.Remove("source.txt")
  if err != nil {
    return
  }

  err = os.Remove("new.txt")
  if err != nil {
    return
  }
}

Shell Commands

Here is a simple example that writes the output of a command to a file:

package main

import (
  "os"
  "os/exec"
)

func main() {
  cmd := exec.Command("ls")

  outfile, err := os.Create("./out.txt")
  if err != nil {
    panic(err)
  }
  defer outfile.Close()

  cmd.Stdout = outfile
  cmd.Stderr = outfile

  err = cmd.Start()
  if err != nil {
    panic(err)
  }

  cmd.Wait()
}

Here is an older example that printed the results of a command:

var (
  cmdOut []byte
  err    error
)
cmdName := "spurious"
cmdArgs := []string{"ports", "--json"}
if cmdOut, err = exec.Command(cmdName, cmdArgs...).Output(); err != nil {
  fmt.Fprintln(os.Stderr, "There was an error running 'spurious ports --json' command: ", err)
  os.Exit(1)
}
fmt.Println(string(cmdOut))

New Instance Idiom

package main

import "fmt"

type Sqs struct {
  foo string
}

func (s *Sqs) create() {
  fmt.Println("I'll create stuff")
}

func NewSqs() *Sqs {
  return &Sqs{"bop"}
}

func main() {
  s := NewSqs()
  fmt.Println(s.foo)
  s.create()
}

JSON Connection Draining

When using json.NewDecoder:

func main() {
    http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
        var u User
        if r.Body == nil {
            http.Error(w, "Please send a request body", 400)
            return
        }
        err := json.NewDecoder(r.Body).Decode(&u)
        if err != nil {
            http.Error(w, err.Error(), 400)
            return
        }
        fmt.Println(u.Id)
    })
    log.Fatal(http.ListenAndServe(":8080", nil))
}

…it doesn’t read the response Body completely. So when closing the response you might get an error as a stray \n could be present later on. You’ll need to drain the response instead:

defer func() {
  io.CopyN(ioutil.Discard, r.Body, 512)
  r.Body.Close()
}()

Note: https://github.com/google/go-github/pull/317

Writing your own Marshal/Unmarshal functions

package main

import (
	"bytes"
	"encoding/json"
	"fmt"
	"strconv"
)

func main() {
	a := make(Stuff)
	a[1] = "asdf"
	a[-1] = "qwer"
	fmt.Println("Initial:     ", a)

	stuff, _ := json.Marshal(a)
	fmt.Println("Serialized:  ", string(stuff))

	b := make(Stuff)
	_ = json.Unmarshal(stuff, &b)
	fmt.Println("Deserialized:", b)
}

type Stuff map[int]string

// MarshalJSON works the same as the underlying json.Marshal would
func (this Stuff) MarshalJSON() ([]byte, error) {
	buffer := bytes.NewBufferString("{")
	length := len(this)
	count := 0
	for key, value := range this {
		jsonValue, err := json.Marshal(value)
		if err != nil {
			return nil, err
		}
		buffer.WriteString(fmt.Sprintf("\"%d\":%s", key, string(jsonValue)))
		count++
		if count < length {
			buffer.WriteString(",")
		}
	}
	buffer.WriteString("}")
	return buffer.Bytes(), nil

  // for example you could totally change the behaviour...
  /*
	buffer := bytes.NewBufferString("{")
	buffer.WriteString("}")
	return buffer.Bytes(), nil
	*/
}

// UnmarshalJSON works the same as the underlying json.Unmarshal would
func (this Stuff) UnmarshalJSON(b []byte) error {
	var stuff map[string]string
	err := json.Unmarshal(b, &stuff)
	if err != nil {
		return err
	}
	for key, value := range stuff {
		numericKey, err := strconv.Atoi(key)
		if err != nil {
			return err
		}
		this[numericKey] = value
	}
	return nil
}

This means you can also leverage custom type checking:

See: leveraging interfaces in golang

Introduction

Private Repo Access

Build and Compilation

Gox

Other information

Build Time Dynamic Variables

Package Naming

Dependency Information

Dependency Management

Godeps

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godoc encoding/json Encoder

go doc encoding/json Encoder

Full Browser Documentation

Testing

More Test Examples

Logging

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Import Race Conditions

New vs Make

Custom Types

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Struct: Var vs Type

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See all methods on <Type>

Convert Struct into JSON

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Convert Struct into YAML

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Read Users Input

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Pointers

Array Pointer

Mutating Values

Type Assertion

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Reading File in Chunks

Time

Starting and Stopping things with Channels

Channel Pipelines

Templating

Error handling

Socket Programming

Comparing Maps

Zip File Contents

Shell Commands

New Instance Idiom

JSON Connection Draining

Writing your own Marshal/Unmarshal functions

`godoc encoding/json Encoder`

`go doc encoding/json Encoder`

See all methods on `<Type>`