Go Language CookBook With Examples

Chapter 1Intro
1.1 Objectives
After the event, we expect the attendees to leave with the following knowledge, skills and attributes.
1.1.1 Knowledge
  • pros/cons of static typing in Go
  • what makes Go unique
  • what is Go particularly good at
  • what are the challenging parts of Go
1.1.2 Skills
  • know how to do data modeling with Go
  • know how to organize code in packages
  • know how to test code
  • know how to write documentation
  • know how to use JSON marshaling
  • know how to build a web API (depending on exercises)
  • know how to test a web API (depending on exercises)
  • know how to cross compile
  • know how to use the key go tools
1.1.3 Attitudes
  • value the potential of the Go language
  • can argue when to use Go vs using “legacy language”
  • consider using Go for a future project
Chapter 2 The Basics
Go is often referred to as a “simple” programming language, a language that can be learned in a few hours if you already know another language. Go was designed to feel familiar and to stay as simple as possible, the entire language specification fits in just a few pages.
There are a few concepts we are going to explore before writing our first application.
2.1 Variables & inferred typing
The var statement declares a list of variables with the type declared last.
var (
name string
age int
location string
Or even
var (
name, location string
age int
Variables can also be declared one by one:
var name string
var age int
var location string
A var declaration can include initializers, one per variable.
var (
name string = “Prince Oberyn”
age int = 32
location string = “Dorne”
If an initializer is present, the type can be omitted, the variable will take the type of the initializer (inferred typing).
var (
name = “Prince Oberyn”
age = 32
location = “Dorne”
You can also initialize variables on the same line:
var (
name, location, age = “Prince Oberyn”, “Dorne”, 32
Inside a function, the := short assignment statement can be used in place of a var declaration with implicit type.
func main() {
name, location := “Prince Oberyn”, “Dorne”
age := 32
fmt.Printf(“%s (%d) of %s”, name, age, location)
A variable can contain any type, including functions:
func main() {
action := func() {
//doing something
Outside a function, every construct begins with a keyword (var, func, and so on) and the :=construct is not available.
2.2 Constants
Constants are declared like variables, but with the const keyword.
Constants can only be character, string, boolean, or numeric values and cannot be declared using the := syntax. An untyped constant takes the type needed by its context.

const Pi = 3.14
const (
StatusOK = 200
StatusCreated = 201
StatusAccepted = 202
StatusNonAuthoritativeInfo = 203
StatusNoContent = 204
StatusResetContent = 205
StatusPartialContent = 206

package main

import “fmt”

const (
Pi = 3.14
Truth = false
Big = 1 << 62
Small = Big >> 61

func main() {
const Greeting = “ハローワールド”

2.3 Printing Constants and Variables
While you can print the value of a variable or constant using the built-in print and printlnfunctions, the more idiomatic and flexible way is to use the fmt package
func main() {
cylonModel := 6
fmt.Println prints the passed in variables’ values and appends a newline. fmt.Printf is used when you want to print one or multiple values using a defined format specifier.
func main() {
name := “Caprica-Six”
aka := fmt.Sprintf(“Number %d”, 6)
fmt.Printf(“%s is also known as %s”,
name, aka)
Read the fmt package documentation to see the available flags to create a format specifier.
2.4 Packages and imports
Every Go program is made up of packages. Programs start running in package main.

package main

func main() {
print(“Hello, World!n”)

If you are writing an executable code (versus a library), then you need to define a main package and a main() function which will be the entry point to your software.
By convention, the package name is the same as the last element of the import path. For instance, the “math/rand” package comprises files that begin with the statement package rand.
Import statement examples:
import “fmt”
import “math/rand”
Or grouped:
import (
Usually, non standard lib packages are namespaced using a web url. For instance, I ported to Go some Rails logic, including the cryptography code used in Rails 4. I hosted the source code containing a few packages on github, in the following repository http://github.com/mattetti/goRailsYourself
To import the crypto package, I would need to use the following import statement:
import “github.com/mattetti/goRailsYourself/crypto”
2.5 Code location
The snippet above basically tells the compiler to import the crypto package available at the github.com/mattetti/goRailsYourself/crypto path. It doesn’t mean that the compiler will automatically pull down the repository, so where does it find the code?
You need to pull down the code yourself. The easiest way is to use the go get command provided by Go.
$ go get github.com/mattetti/goRailsYourself/crypto
This command will pull down the code and put it in your Go path. When installing Go, we set the GOPATH environment variable and that is what’s used to store binaries and libraries. That’s also where you should store your code (your workspace).
$ ls $GOPATH
bin pkg src
The bin folder will contain the Go compiled binaries. You should probably add the bin path to your system path.
The pkg folder contains the compiled versions of the available libraries so the compiler can link against them without recompiling them.
Finally the src folder contains all the Go source code organized by import path:

$ ls $GOPATH/src
bitbucket.org    code.google.com    github.com   launchpad.net

$ ls $GOPATH/src/github.com/mattetti
goblin     goRailsYourself jet

When starting a new program or library, it is recommended to do so inside the src folder, using a fully qualified path (for instance: github.com/<your username>/<project name>)

2.6 Exported names
After importing a package, you can refer to the names it exports (meaning variables, methods and functions that are available from outside of the package). In Go, a name is exported if it begins with a capital letter. Foo is an exported name, as is FOO. The name foo is not exported.
See the difference between:
import ( “fmt” “math” ) func main() { fmt.Println(math.pi) }
func main() { fmt.Println(math.Pi) }
Pi is exported and can be accessed from outside the page, while pi isn’t available.
cannot refer to unexported name math.pi
Use the provided Go documentation or godoc.org to find exported names.
2.7 Functions, signature, return values, named results
A function can take zero or more typed arguments. The type comes after the variable name.Functions can be defined to return any number of values that are always typed.
package main import “fmt” func add(x int, y int) int { return x + y } func main() { fmt.Println(add(42, 13)) }
In the following example, instead of declaring the type of each parameter, we only declare one type that applies to both.
package main import “fmt” func add(x, y int) int { return x + y } func main() { fmt.Println(add(42, 13)) }
In the following example, the location function returns two string values.
func location(city string) (string, string) { var region string var continent string switch city { case “Los Angeles”, “LA”, “Santa Monica”: region, continent = “California”, “North America” case “New York”, “NYC”: region, continent = “New York”, “North America” default: region, continent = “Unknown”, “Unknown” } return region, continent } func main() { region, continent := location(“Santa Monica”) fmt.Printf(“Matt lives in %s, %s”, region, continent) }
Functions take parameters. In Go, functions can return multiple “result parameters”, not just a single value. They can be named and act just like variables.
If the result parameters are named, a return statement without arguments returns the current values of the results.
func location(name, city string) (region, continent string) { switch city { case “New York”, “LA”, “Chicago”: continent = “North America” default: continent = “Unknown” } return } func main() { region, continent := location(“Matt”, “LA”) fmt.Printf(“%s lives in %s”, region, continent) }
I personally recommend against using named return parameters because they often cause more confusion than they save time or help clarify your code.
2.8 Pointers
Go has pointers, but no pointer arithmetic. Struct fields can be accessed through a struct pointer. The indirection through the pointer is transparent (you can directly call fields and methods on a pointer).
Note that by default Go passes arguments by value (copying the arguments), if you want to pass the arguments by reference, you need to pass pointers (or use a structure using reference values like slices and maps.
To get the pointer of a value, use the & symbol in front of the value; to dereference a pointer, use the *symbol.
Methods are often defined on pointers and not values (although they can be defined on both), so you will often store a pointer in a variable as in the example below:
client := &http.Client{} resp, err := client.Get(“http://gobootcamp.com”)
2.9 Mutability
In Go, only constants are immutable. However because arguments are passed by value, a function receiving a value argument and mutating it, won’t mutate the original value.
package main import “fmt” type Artist struct { Name, Genre string Songs int } func newRelease(a Artist) int { a.Songs++ return a.Songs } func main() { me := Artist{Name: “Matt”, Genre: “Electro”, Songs: 42} fmt.Printf(“%s released their %dth songn”, me.Name, newRelease(me)) fmt.Printf(“%s has a total of %d songs”, me.Name, me.Songs) }
Matt released their 43th song Matt has a total of 42 songs
As you can see the total amount of songs on the me variable’s value wasn’t changed. To mutate the passed value, we need to pass it by reference, using a pointer.
package main import “fmt” type Artist struct { Name, Genre string Songs int } func newRelease(a *Artist) int { a.Songs++ return a.Songs } func main() { me := &Artist{Name: “Matt”, Genre: “Electro”, Songs: 42} fmt.Printf(“%s released their %dth songn”, me.Name, newRelease(me)) fmt.Printf(“%s has a total of %d songs”, me.Name, me.Songs) }
The only change between the two versions is that newRelease takes a pointer to an Artist value and when I initialize our me variable, I used the & symbol to get a pointer to the value.
Another place where you need to be careful is when calling methods on values as explained a bit later
Chapter 3 Types
3.1 Basic types
bool string Numeric types: uint either 32 or 64 bits int same size as uint uintptr an unsigned integer large enough to store the uninterpreted bits of a pointer value uint8 the set of all unsigned 8-bit integers (0 to 255) uint16 the set of all unsigned 16-bit integers (0 to 65535) uint32 the set of all unsigned 32-bit integers (0 to 4294967295) uint64 the set of all unsigned 64-bit integers (0 to 18446744073709551615) int8 the set of all signed 8-bit integers (-128 to 127) int16 the set of all signed 16-bit integers (-32768 to 32767) int32 the set of all signed 32-bit integers (-2147483648 to 2147483647) int64 the set of all signed 64-bit integers (-9223372036854775808 to 9223372036854775807) float32 the set of all IEEE-754 32-bit floating-point numbers float64 the set of all IEEE-754 64-bit floating-point numbers complex64 the set of all complex numbers with float32 real and imaginary parts complex128 the set of all complex numbers with float64 real and imaginary parts byte alias for uint8 rune alias for int32 (represents a Unicode code point)
Example of some of the built-in types:
package main import ( “fmt” “math/cmplx” ) var ( goIsFun bool = true maxInt uint64 = 1<<64 – 1 complex complex128 = cmplx.Sqrt(-5 + 12i) ) func main() { const f = “%T(%v)n” fmt.Printf(f, goIsFun, goIsFun) fmt.Printf(f, maxInt, maxInt) fmt.Printf(f, complex, complex) }
bool(true) uint64(18446744073709551615) complex128((2+3i))
3.2 Type conversion
The expression T(v) converts the value v to the type T. Some numeric conversions:
var i int = 42 var f float64 = float64(i) var u uint = uint(f)
Or, put more simply:
i := 42 f := float64(i) u := uint(f)
Go assignment between items of different type requires an explicit conversion which means that you manually need to convert types if you are passing a variable to a function expecting another type.
3.3 Type assertion
If you have a value and want to convert it to another or a specific type (in case of interface{}), you can use type assertion. A type assertion takes a value and tries to create another version in the specified explicit type.
In the example below, the timeMap function takes a value and if it can be asserted as a map of interfaces{} keyed by strings, then it injects a new entry called “updated_at” with the current time value.
package main import ( “fmt” “time” ) func timeMap(y interface{}) { z, ok := y.(map[string]interface{}) if ok { z[“updated_at”] = time.Now() } } func main() { foo := map[string]interface{}{ “Matt”: 42, } timeMap(foo) fmt.Println(foo) }
The type assertion doesn’t have to be done on an empty interface. It’s often used when you have a function taking a param of a specific interface but the function inner code behaves differently based on the actual object type. Here is an example:
package main import “fmt” type Stringer interface { String() string } type fakeString struct { content string } // function used to implement the Stringer interface func (s *fakeString) String() string { return s.content } func printString(value interface{}) { switch str := value.(type) { case string: fmt.Println(str) case Stringer: fmt.Println(str.String()) } } func main() { s := &fakeString{“Ceci n’est pas un string”} printString(s) printString(“Hello, Gophers”) }
Another example is when checking if an error is of a certain type:
if err != nil { if msqlerr, ok := err.(*mysql.MySQLError); ok && msqlerr.Number == 1062 { log.Println(“We got a MySQL duplicate :(“) } else { return err } }
3.4 Structs
A struct is a collection of fields/properties. You can define new types as structs or interfaces (Chapter 7). If you are coming from an object-oriented background, you can think of a struct to be a light class that supports composition but not inheritance. Methods are discussed at length in Chapter 6
You don’t need to define getters and setters on struct fields, they can be accessed automatically.However, note that only exported fields (capitalized) can be accessed from outside of a package.
A struct literal sets a newly allocated struct value by listing the values of its fields. You can list just a subset of fields by using the “Name:” syntax (the order of named fields is irrelevant when using this syntax). The special prefix & constructs a pointer to a newly allocated struct.
package main import ( “fmt” “time” ) type Bootcamp struct { // Latitude of the event Lat float64 // Longitude of the event Lon float64 // Date of the event Date time.Time } func main() { fmt.Println(Bootcamp{ Lat: 34.012836, Lon: -118.495338, Date: time.Now(), }) }
Declaration of struct literals:
package main import “fmt” type Point struct { X, Y int } var ( p = Point{1, 2} // has type Point q = &Point{1, 2} // has type *Point r = Point{X: 1} // Y:0 is implicit s = Point{} // X:0 and Y:0 ) func main() { fmt.Println(p, q, r, s) }
Accessing fields using the dot notation:
package main import ( “fmt” “time” ) type Bootcamp struct { Lat, Lon float64 Date time.Time } func main() { event := Bootcamp{ Lat: 34.012836, Lon: -118.495338, } event.Date = time.Now() fmt.Printf(“Event on %s, location (%f, %f)”, event.Date, event.Lat, event.Lon) }
3.5 Initializing
Go supports the new expression to allocate a zeroed value of the requested type and to return a pointer to it.
x := new(int)
As seen in (Section 3.4) a common way to “initialize” a variable containing a struct or a reference to one, is to create a struct literal. Another option is to create a constructor. This is usually done when the zero value isn’t good enough and you need to set some default field values for instance.
Note that following expressions using new and an empty struct literal are equivalent and result in the same kind of allocation/initialization:
package main import ( “fmt” ) type Bootcamp struct { Lat float64 Lon float64 } func main() { x := new(Bootcamp) y := &Bootcamp{} fmt.Println(*x == *y) }
Note that slices (Section 4.2), maps (Section 4.4) and channels (Section 8.2) are usually allocated using make so the data structure these types are built upon can be initialized.
3.6 Composition vs inheritance
Coming from an OOP background a lot of us are used to inheritance, something that isn’t supported by Go. Instead you have to think in terms of composition and interfaces.
The Go team wrote a short but good segment on this topic.
Composition (or embedding) is a well understood concept for most OOP programmers and Go supports it, here is an example of the problem it’s solving:
package main import “fmt” type User struct { Id int Name string Location string } type Player struct { Id int Name string Location string GameId int } func main() { p := Player{} p.Id = 42 p.Name = “Matt” p.Location = “LA” p.GameId = 90404 fmt.Printf(“%+v”, p) }
The above example demonstrates a classic OOP challenge, our Player struct has the same fields as the User struct but it also has a GameId field. Having to duplicate the field names isn’t a big deal, but it can be simplified by composing our struct.
type User struct { Id int Name, Location string } type Player struct { User GameId int }
We can initialize a new variable of type Player two different ways.
Using the dot notation to set the fields:
package main import “fmt” type User struct { Id int Name, Location string } type Player struct { User GameId int } func main() { p := Player{} p.Id = 42 p.Name = “Matt” p.Location = “LA” p.GameId = 90404 fmt.Printf(“%+v”, p) }
The other option is to use a struct literal:
package main import “fmt” type User struct { Id int Name, Location string } type Player struct { User GameId int } func main() { p := Player{ User{Id: 42, Name: “Matt”, Location: “LA”}, 90404, } fmt.Printf( “Id: %d, Name: %s, Location: %s, Game id: %dn”, p.Id, p.Name, p.Location, p.GameId) // Directly set a field defined on the Player struct p.Id = 11 fmt.Printf(“%+v”, p) }
When using a struct literal with an implicit composition, we can’t just pass the composed fields. We instead need to pass the types composing the struct. Once set, the fields are directly available.
Because our struct is composed of another struct, the methods on the User struct is also available to the Player. Let’s define a method to show that behavior:
package main import “fmt” type User struct { Id int Name, Location string } func (u *User) Greetings() string { return fmt.Sprintf(“Hi %s from %s”, u.Name, u.Location) } type Player struct { User GameId int } func main() { p := Player{} p.Id = 42 p.Name = “Matt” p.Location = “LA” fmt.Println(p.Greetings()) }
As you can see this is a very powerful way to build data structures but it’s even more interesting when thinking about it in the context of interfaces. By composing one of your structure with one implementing a given interface, your structure automatically implements the interface.
Here is another example, this time we will look at implementing a Job struct that can also behave as a logger.
Here is the explicit way:
package main import ( “log” “os” ) type Job struct { Command string Logger *log.Logger } func main() { job := &Job{“demo”, log.New(os.Stderr, “Job: “, log.Ldate)} // same as // job := &Job{Command: “demo”, // Logger: log.New(os.Stderr, “Job: “, log.Ldate)} job.Logger.Print(“test”) }
Our Job struct has a field called Logger which is a pointer to another type (log.Logger)
When we initialize our value, we set the logger so we can then call its Print function by chaining the calls: job.Logger.Print()
But Go lets you go even further and use implicit composition. We can skip defining the field for our logger and now all the methods available on a pointer to log.Logger are available from our struct:
package main import ( “log” “os” ) type Job struct { Command string *log.Logger } func main() { job := &Job{“demo”, log.New(os.Stderr, “Job: “, log.Ldate)} job.Print(“starting now…”) }
Note that you still need to set the logger and that’s often a good reason to use a constructor (custom constructors are used when you need to set a structure before using a value, see (Section 11.5) ).What is really nice with the implicit composition is that it allows to easily and cheaply make your structs implement interfaces. Imagine that you have a function that takes variables implementing an interface with the Print method. By adding *log.Logger to your struct (and initializing it properly), your struct is now implementing the interface without you writing any custom methods.
3.7 Exercise
Looking at the User / Player example, you might have noticed that we composed Player using User but it might be better to compose it with a pointer to a User struct. The reason why a pointer might be better is because in Go, arguments are passed by value and not reference. If you have a small struct that is inexpensive to copy, that is fine, but more than likely, in real life, our User struct will be bigger and should not be copied. Instead we would want to pass by reference (using a pointer). (Section 2.9 & Section 6.3) discuss more in depth how calling a method on a type value vs a pointer affects mutability and memory allocation)
Modify the code to use a pointer but still be able to initialize without using the dot notation.
package main import “fmt” type User struct { Id int Name, Location string } func (u *User) Greetings() string { return fmt.Sprintf(“Hi %s from %s”, u.Name, u.Location) } type Player struct { *User GameId int } func main() { // insert code }
Question: We defined the Greetings method on a pointer to a User type. How come we were able to call it directly on the value?
3.7.1 Solution
package main import “fmt” type User struct { Id int Name, Location string } func (u *User) Greetings() string { return fmt.Sprintf(“Hi %s from %s”, u.Name, u.Location) } type Player struct { *User GameId int } func NewPlayer(id int, name, location string, gameId int) *Player { return &Player{ User: &User{id, name, location}, GameId: gameId, } } func main() { p := NewPlayer(42, “Matt”, “LA”, 90404) fmt.Println(p.Greetings()) }
Answer: That is because methods defined on a pointer are also automatically available on the value itself. The example didn’t use a pointer on purpose, so the dot notation was working right away. In the pointer solution, a zero value player is composed of a nil pointer of type User and therefore, we can’t call a field on a nil pointer.
Chapter 4 Collection Types
4.1 Arrays
The type [n]T is an array of n values of type T.
The expression:
var a [10]int
declares a variable a as an array of ten integers.
An array’s length is part of its type, so arrays cannot be resized. This seems limiting, but don’t worry; Go provides a convenient way of working with arrays.
package main import “fmt” func main() { var a [2]string a[0] = “Hello” a[1] = “World” fmt.Println(a[0], a[1]) fmt.Println(a) }
You can also set the array entries as you declare the array:
package main import “fmt” func main() { a := [2]string{“hello”, “world!”} fmt.Printf(“%q”, a) }
Finally, you can use an ellipsis to use an implicit length when you pass the values:
package main import “fmt” func main() { a := […]string{“hello”, “world!”} fmt.Printf(“%q”, a) }
4.1.1 Printing arrays
Note how we used the fmt package using Printf and used the %q “verb” to print each element quoted.
If we had used Println or the %s verb, we would have had a different result:
package main import “fmt” func main() { a := [2]string{“hello”, “world!”} fmt.Println(a) // [hello world!] fmt.Printf(“%sn”, a) // [hello world!] fmt.Printf(“%qn”, a) // [“hello” “world!”] }
4.1.2 Multi-dimensional arrays
You can also create multi-dimensional arrays:
package main import “fmt” func main() { var a [2][3]string for i := 0; i < 2; i++ { for j := 0; j < 3; j++ { a[i][j] = fmt.Sprintf(“row %d – column %d”, i+1, j+1) } } fmt.Printf(“%q”, a) // [[“row 1 – column 1” “row 1 – column 2” “row 1 – column 3”] // [“row 2 – column 1” “row 2 – column 2” “row 2 – column 3”]] }
Trying to access or set a value at an index that doesn’t exist will prevent your program from compiling, for instance, try to compile the following code:
package main func main() { var a [2]string a[3] = “Hello” }
You will see that the compiler will report the following error:
Invalid array index 3 (out of bounds for 2-element array)
That’s because our array is of length 2 meaning that the only 2 available indexes are 0 and 1. Trying to access index 3 results in an error that tells us that we are trying to access an index that is out of bounds since our array only contains 2 elements and we are trying to access the 4th element of the array.
Slices, the type that we are going to see next is more often used, due to the fact that we don’t always know in advance the length of the array we need.
4.2 Slices
Slices wrap arrays to give a more general, powerful, and convenient interface to sequences of data.Except for items with explicit dimension such as transformation matrices, most array programming in Go is done with slices rather than simple arrays.
Slices hold references to an underlying array, and if you assign one slice to another, both refer to the same array. If a function takes a slice argument, changes it makes to the elements of the slice will be visible to the caller, analogous to passing a pointer to the underlying array.
A slice points to an array of values and also includes a length. Slices can be resized since they are just a wrapper on top of another data structure.
[]T is a slice with elements of type T.
package main import “fmt” func main() { p := []int{2, 3, 5, 7, 11, 13} fmt.Println(p) // [2 3 5 7 11 13] }
4.2.1 Slicing a slice
Slices can be re-sliced, creating a new slice value that points to the same array.
The expression
evaluates to a slice of the elements from lo through hi-1, inclusive. Thus
is empty and
has one element.
Note: lo and hi would be integers representing indexes.
package main import “fmt” func main() { mySlice := []int{2, 3, 5, 7, 11, 13} fmt.Println(mySlice) // [2 3 5 7 11 13] fmt.Println(mySlice[1:4]) // [3 5 7] // missing low index implies 0 fmt.Println(mySlice[:3]) // [2 3 5] // missing high index implies len(s) fmt.Println(mySlice[4:]) // [11 13] }
4.2.2 Making slices
Besides creating slices by passing the values right away (slice literal), you can also use make. You create an empty slice of a specific length and then populate each entry:
package main import “fmt” func main() { cities := make([]string, 3) cities[0] = “Santa Monica” cities[1] = “Venice” cities[2] = “Los Angeles” fmt.Printf(“%q”, cities) // [“Santa Monica” “Venice” “Los Angeles”] }
It works by allocating a zeroed array and returning a slice that refers to that array.
4.2.3 Appending to a slice
Note however, that you would get a runtime error if you were to do that:
cities := []string{} cities[0] = “Santa Monica”
As explained above, a slice is seating on top of an array, in this case, the array is empty and the slice can’t set a value in the referred array. There is a way to do that though, and that is by using the append function:
package main import “fmt” func main() { cities := []string{} cities = append(cities, “San Diego”) fmt.Println(cities) // [San Diego] }
You can append more than one entry to a slice:
package main import “fmt” func main() { cities := []string{} cities = append(cities, “San Diego”, “Mountain View”) fmt.Printf(“%q”, cities) // [“San Diego” “Mountain View”] }
And you can also append a slice to another using an ellipsis:
package main import “fmt” func main() { cities := []string{“San Diego”, “Mountain View”} otherCities := []string{“Santa Monica”, “Venice”} cities = append(cities, otherCities…) fmt.Printf(“%q”, cities) // [“San Diego” “Mountain View” “Santa Monica” “Venice”] }
Note that the ellipsis is a built-in feature of the language that means that the element is a collection.We can’t append an element of type slice of strings ([]string) to a slice of strings, only strings can be appended. However, using the ellipsis () after our slice, we indicate that we want to append each element of our slice. Because we are appending strings from another slice, the compiler will accept the operation since the types are matching.
You obviously can’t append a slice of type []int to another slice of type []string.
4.2.4 Length
At any time, you can check the length of a slice by using len:
package main import “fmt” func main() { cities := []string{ “Santa Monica”, “San Diego”, “San Francisco”, } fmt.Println(len(cities)) // 3 countries := make([]string, 42) fmt.Println(len(countries)) // 42 }
4.2.5 Nil slices
The zero value of a slice is nil. A nil slice has a length and capacity of 0.
package main import “fmt” func main() { var z []int fmt.Println(z, len(z), cap(z)) // [] 0 0 if z == nil { fmt.Println(“nil!”) } // nil! }
4.2.6 Resources
For more details about slices:
4.3 Range
The range form of the for loop iterates over a slice (Section 4.2) or a map (Section 4.4). Being able to iterate over all the elements of a data structure is very useful and range simplifies the iteration.
package main import “fmt” var pow = []int{1, 2, 4, 8, 16, 32, 64, 128} func main() { for i, v := range pow { fmt.Printf(“2**%d = %dn”, i, v) } }
Which will print:
2**0 = 1 2**1 = 2 2**2 = 4 2**3 = 8 2**4 = 16 2**5 = 32 2**6 = 64 2**7 = 128
You can skip the index or value by assigning to _. If you only want the index, drop the “, value” entirely.
package main import “fmt” func main() { pow := make([]int, 10) for i := range pow { pow[i] = 1 << uint(i) } for _, value := range pow { fmt.Printf(“%dn”, value) } }
4.3.1 Break & continue
As if you were using a normal for loop, you can stop the iteration anytime by using break:
package main import “fmt” func main() { pow := make([]int, 10) for i := range pow { pow[i] = 1 << uint(i) if pow[i] >= 16 { break } } fmt.Println(pow) // [1 2 4 8 16 0 0 0 0 0] }
You can also skip an iteration by using continue:
package main import “fmt” func main() { pow := make([]int, 10) for i := range pow { if i%2 == 0 { continue } pow[i] = 1 << uint(i) } fmt.Println(pow) // [0 2 0 8 0 32 0 128 0 512] }
4.3.2 Range and maps
Range can also be used on maps (Section 4.4), in that case, the first parameter isn’t an incremental integer but the map key:
package main import “fmt” func main() { cities := map[string]int{ “New York”: 8336697, “Los Angeles”: 3857799, “Chicago”: 2714856, } for key, value := range cities { fmt.Printf(“%s has %d inhabitantsn”, key, value) } }
Which will output:
New York has 8336697 inhabitants Los Angeles has 3857799 inhabitants Chicago has 2714856 inhabitants
4.3.3 Exercise
Given a list of names, you need to organize each name within a slice based on its length.
package main var names = []string{“Katrina”, “Evan”, “Neil”, “Adam”, “Martin”, “Matt”, “Emma”, “Isabella”, “Emily”, “Madison”, “Ava”, “Olivia”, “Sophia”, “Abigail”, “Elizabeth”, “Chloe”, “Samantha”, “Addison”, “Natalie”, “Mia”, “Alexis”} func main() { // insert your code here }
After you implement your solution, you should get the following output (slice of slice of strings):
[[] [] [Ava Mia] [Evan Neil Adam Matt Emma] [Emily Chloe] [Martin Olivia Sophia Alexis] [Katrina Madison Abigail Addison Natalie] [Isabella Samantha] [Elizabeth]]
4.3.4 Solution
package main import “fmt” var names = []string{“Katrina”, “Evan”, “Neil”, “Adam”, “Martin”, “Matt”, “Emma”, “Isabella”, “Emily”, “Madison”, “Ava”, “Olivia”, “Sophia”, “Abigail”, “Elizabeth”, “Chloe”, “Samantha”, “Addison”, “Natalie”, “Mia”, “Alexis”} func main() { var maxLen int for _, name := range names { if l := len(name); l > maxLen { maxLen = l } } output := make([][]string, maxLen) for _, name := range names { output[len(name)-1] = append(output[len(name)-1], name) } fmt.Printf(“%v”, output) }
There are a few interesting things to note. To avoid an out of bounds insert, we need our outputslice to be big enough. But we don’t want it to be too big. That’s why we need to do a first pass through all the names and find the longest. We use the longest name length to set the length of the output slice length. Slices are zero indexed, so when inserting the names, we need to get the length of the name minus one.
4.4 Maps
Maps are somewhat similar to what other languages call “dictionaries” or “hashes”.
A map maps keys to values. Here we are mapping string keys (actor names) to an integer value (age).
package main import “fmt” func main() { celebs := map[string]int{ “Nicolas Cage”: 50, “Selena Gomez”: 21, “Jude Law”: 41, “Scarlett Johansson”: 29, } fmt.Printf(“%#v”, celebs) }
map[string]int{“Nicolas Cage”:50, “Selena Gomez”:21, “Jude Law”:41, “Scarlett Johansson”:29}
When not using map literals like above, maps must be created with make (not new) before use. The nil map is empty and cannot be assigned to.
Assignments follow the Go convention and can be observed in the example below.
package main import “fmt” type Vertex struct { Lat, Long float64 } var m map[string]Vertex func main() { m = make(map[string]Vertex) m[“Bell Labs”] = Vertex{40.68433, -74.39967} fmt.Println(m[“Bell Labs”]) }
When using map literals, if the top-level type is just a type name, you can omit it from the elements of the literal.
package main import “fmt” type Vertex struct { Lat, Long float64 } var m = map[string]Vertex{ “Bell Labs”: {40.68433, -74.39967}, // same as “Bell Labs”: Vertex{40.68433, -74.39967} “Google”: {37.42202, -122.08408}, } func main() { fmt.Println(m) }
4.4.1 Mutating maps
Insert or update an element in map m:
m[key] = elem
Retrieve an element:
elem = m[key]
Delete an element:
delete(m, key)
Test that a key is present with a two-value assignment:
elem, ok = m[key]
If key is in m, ok is true. If not, ok is false and elem is the zero value for the map’s element type.Similarly, when reading from a map if the key is not present the result is the zero value for the map’s element type.
4.4.2 Resources
4.4.3 Exercise
Implement WordCount.
package main import ( “code.google.com/p/go-tour/wc” ) func WordCount(s string) map[string]int { return map[string]int{“x”: 1} } func main() { wc.Test(WordCount) }
It should return a map of the counts of each “word” in the string s. The wc.Test function runs a test suite against the provided function and prints success or failure.
You might find strings.Fields helpful.
4.4.4 Solution
package main import ( “code.google.com/p/go-tour/wc” “strings” ) func WordCount(s string) map[string]int { words := strings.Fields(s) count := map[string]int{} for _, word := range words { count[word]++ } return count } func main() { wc.Test(WordCount) }
Chapter 5 Control flow
The if statement looks as it does in C or Java, except that the ( ) are gone and the { } are required. Like for, the if statement can start with a short statement to execute before the condition.Variables declared by the statement are only in scope until the end of the if. Variables declared inside an if short statement are also available inside any of the else blocks.
if answer != 42 { return “Wrong answer” }
if err := foo(); err != nil { panic(err) }
5.2 For Loop
Go has only one looping construct, the for loop. The basic for loop looks as it does in C or Java, except that the ( ) are gone (they are not even optional) and the { } are required. As in C or Java, you can leave the pre and post statements empty.
sum := 0 for i := 0; i < 10; i++ { sum += i }
sum := 1 for ; sum < 1000; { sum += sum }
sum := 1 for sum < 1000 { sum += sum }
for { // do something in a loop forever }
5.3 Switch case statement
Most programming languages have some sort of switch case statement to allow developers to avoid doing complex and ugly series of if else statements.
package main import ( “fmt” “time” ) func main() { now := time.Now().Unix() mins := now % 2 switch mins { case 0: fmt.Println(“even”) case 1: fmt.Println(“odd”) } }
There are a few interesting things to know about this statement in Go:
  • You can only compare values of the same type.
  • You can set an optional default statement to be executed if all the others fail.
  • You can use an expression in the case statement, for instance you can calculate a value to use in the case:
package main import “fmt” func main() { num := 3 v := num % 2 switch v { case 0: fmt.Println(“even”) case 3 – 2: fmt.Println(“odd”) } }
package main import “fmt” func main() { score := 7 switch score { case 0, 1, 3: fmt.Println(“Terrible”) case 4, 5: fmt.Println(“Mediocre”) case 6, 7: fmt.Println(“Not bad”) case 8, 9: fmt.Println(“Almost perfect”) case 10: fmt.Println(“hmm did you cheat?”) default: fmt.Println(score, ” off the chart”) } }
  • You can execute all the following statements after a match using the fallthrough statement:
package main import “fmt” func main() { n := 4 switch n { case 0: fmt.Println(“is zero”) fallthrough case 1: fmt.Println(“is <= 1”) fallthrough case 2: fmt.Println(“is <= 2”) fallthrough case 3: fmt.Println(“is <= 3”) fallthrough case 4: fmt.Println(“is <= 4”) fallthrough case 5: fmt.Println(“is <= 5”) fallthrough case 6: fmt.Println(“is <= 6”) fallthrough case 7: fmt.Println(“is <= 7”) fallthrough case 8: fmt.Println(“is <= 8”) fallthrough default: fmt.Println(“Try again!”) } }
is <= 4 is <= 5 is <= 6 is <= 7 is <= 8 Try again!
You can use a break statement inside your matched statement to exit the switch processing:
package main import ( “fmt” “time” ) func main() { n := 1 switch n { case 0: fmt.Println(“is zero”) fallthrough case 1: fmt.Println(“<= 1”) fallthrough case 2: fmt.Println(“<= 2”) fallthrough case 3: fmt.Println(“<= 3”) if time.Now().Unix()%2 == 0 { fmt.Println(“un pasito pa lante maria”) break } fallthrough case 4: fmt.Println(“<= 4”) fallthrough case 5: fmt.Println(“<= 5”) } }
<= 1 <= 2 <= 3 un pasito pa lante maria
5.4 Exercise
You have 50 bitcoins to distribute to 10 users: Matthew, Sarah, Augustus, Heidi, Emilie, Peter, Giana, Adriano, Aaron, Elizabeth The coins will be distributed based on the vowels contained in each name where:
a: 1 coin e: 1 coin i: 2 coins o: 3 coins u: 4 coins
and a user can’t get more than 10 coins. Print a map with each user’s name and the amount of coins distributed. After distributing all the coins, you should have 2 coins left.
The output should look something like that:
map[Matthew:2 Peter:2 Giana:4 Adriano:7 Elizabeth:5 Sarah:2 Augustus:10 Heidi:5 Emilie:6 Aaron:5] Coins left: 2
Note that Go doesn’t keep the order of the keys in a map, so your results might not look exactly the same but the key/value mapping should be the same.
Here is some starting code:
package main import “fmt” var ( coins = 50 users = []string{ “Matthew”, “Sarah”, “Augustus”, “Heidi”, “Emilie”, “Peter”, “Giana”, “Adriano”, “Aaron”, “Elizabeth”, } distribution = make(map[string]int, len(users)) ) func main() { fmt.Println(distribution) fmt.Println(“Coins left:”, coins) }
5.5 Solution
package main import “fmt” var ( coins = 50 users = []string{ “Matthew”, “Sarah”, “Augustus”, “Heidi”, “Emilie”, “Peter”, “Giana”, “Adriano”, “Aaron”, “Elizabeth”, } distribution = make(map[string]int, len(users)) ) func main() { coinsForUser := func(name string) int { var total int for i := 0; i < len(name); i++ { switch string(name[i]) { case “a”, “A”: total++ case “e”, “E”: total++ case “i”, “I”: total = total + 2 case “o”, “O”: total = total + 3 case “u”, “U”: total = total + 4 } } return total } for _, name := range users { v := coinsForUser(name) if v > 10 { v = 10 } distribution[name] = v coins = coins – v } fmt.Println(distribution) fmt.Println(“Coins left:”, coins) }
Chapter 6 Methods
While technically Go isn’t an Object Oriented Programming language, types and methods allow for an object-oriented style of programming. The big difference is that Go does not support type inheritance but instead has a concept of interface.
In this chapter, we will focus on Go’s use of methods and interfaces.
Note: A frequently asked question is “what is the difference between a function and a method”. A method is a function that has a defined receiver, in OOP terms, a method is a function on an instance of an object.
Go does not have classes. However, you can define methods on struct types.
The method receiver appears in its own argument list between the func keyword and the method name. Here is an example with a User struct containing two fields: FirstName and LastName of string type.
package main import ( “fmt” ) type User struct { FirstName, LastName string } func (u User) Greeting() string { return fmt.Sprintf(“Dear %s %s”, u.FirstName, u.LastName) } func main() { u := User{“Matt”, “Aimonetti”} fmt.Println(u.Greeting()) }
Note how methods are defined outside of the struct, if you have been writing Object Oriented code for a while, you might find that a bit odd at first. The method on the User type could be defined anywhere in the package.
6.1 Code organization
Methods can be defined on any file in the package, but my recommendation is to organize the code as shown below:
package models // list of packages to import import ( “fmt” ) // list of constants const ( ConstExample = “const before vars” ) // list of variables var ( ExportedVar = 42 nonExportedVar = “so say we all” ) // Main type(s) for the file, // try to keep the lowest amount of structs per file when possible. type User struct { FirstName, LastName string Location *UserLocation } type UserLocation struct { City string Country string } // List of functions func NewUser(firstName, lastName string) *User { return &User{FirstName: firstName, LastName: lastName, Location: &UserLocation{ City: “Santa Monica”, Country: “USA”, }, } } // List of methods func (u *User) Greeting() string { return fmt.Sprintf(“Dear %s %s”, u.FirstName, u.LastName) }
In fact, you can define a method on any type you define in your package, not just structs. You cannot define a method on a type from another package, or on a basic type.
To define methods on a type you don’t “own”, you need to define an alias for the type you want to extend:
package main import ( “fmt” “strings” ) type MyStr string func (s MyStr) Uppercase() string { return strings.ToUpper(string(s)) } func main() { fmt.Println(MyStr(“test”).Uppercase()) }
package main import ( “fmt” “math” ) type MyFloat float64 func (f MyFloat) Abs() float64 { if f < 0 { return float64(-f) } return float64(f) } func main() { f := MyFloat(-math.Sqrt2) fmt.Println(f.Abs()) }
6.3 Method receivers
Methods can be associated with a named type (User for instance) or a pointer to a named type (*User). In the two type aliasing examples above, methods were defined on the value types (MyStrand MyFloat).
There are two reasons to use a pointer receiver. First, to avoid copying the value on each method call (more efficient if the value type is a large struct). The above example would have been better written as follows:
package main import ( “fmt” ) type User struct { FirstName, LastName string } func (u *User) Greeting() string { return fmt.Sprintf(“Dear %s %s”, u.FirstName, u.LastName) } func main() { u := &User{“Matt”, “Aimonetti”} fmt.Println(u.Greeting()) }
Remember that Go passes everything by value, meaning that when Greeting() is defined on the value type, every time you call Greeting(), you are copying the User struct. Instead when using a pointer, only the pointer is copied (cheap).
The other reason why you might want to use a pointer is so that the method can modify the value that its receiver points to.
package main import ( “fmt” “math” ) type Vertex struct { X, Y float64 } func (v *Vertex) Scale(f float64) { v.X = v.X * f v.Y = v.Y * f } func (v *Vertex) Abs() float64 { return math.Sqrt(v.X*v.X + v.Y*v.Y) } func main() { v := &Vertex{3, 4} v.Scale(5) fmt.Println(v, v.Abs()) }
In the example above, Abs() could be defined on the value type or the pointer since the method doesn’t modify the receiver value (the vertex). However Scale() has to be defined on a pointer since it does modify the receiver. Scale() resets the values of the X and Y fields.
Chapter 7 Interfaces
An interface type is defined by a set of methods. A value of interface type can hold any value that implements those methods.
Here is a refactored version of our earlier example. This time we made the greeting feature more generic by defining a function called Greet which takes a param of interface type Namer. Namer is a new interface we defined which only defines one method: Name(). So Greet() will accept as param any value which has a Name() method defined.
To make our User struct implement the interface, we defined a Name() method. We can now call Greet and pass our pointer to User type.
package main import ( “fmt” ) type User struct { FirstName, LastName string } func (u *User) Name() string { return fmt.Sprintf(“%s %s”, u.FirstName, u.LastName) } type Namer interface { Name() string } func Greet(n Namer) string { return fmt.Sprintf(“Dear %s”, n.Name()) } func main() { u := &User{“Matt”, “Aimonetti”} fmt.Println(Greet(u)) }
We could now define a new type that would implement the same interface and our Greet function would still work.
package main import ( “fmt” ) type User struct { FirstName, LastName string } func (u *User) Name() string { return fmt.Sprintf(“%s %s”, u.FirstName, u.LastName) } type Customer struct { Id int FullName string } func (c *Customer) Name() string { return c.FullName } type Namer interface { Name() string } func Greet(n Namer) string { return fmt.Sprintf(“Dear %s”, n.Name()) } func main() { u := &User{“Matt”, “Aimonetti”} fmt.Println(Greet(u)) c := &Customer{42, “Francesc”} fmt.Println(Greet(c)) }
7.1 Interfaces are satisfied implicitly
A type implements an interface by implementing the methods.
There is no explicit declaration of intent.
Implicit interfaces decouple implementation packages from the packages that define the interfaces: neither depends on the other.
It also encourages the definition of precise interfaces, because you don’t have to find every implementation and tag it with the new interface name.
package main import ( “fmt” “os” ) type Reader interface { Read(b []byte) (n int, err error) } type Writer interface { Write(b []byte) (n int, err error) } type ReadWriter interface { Reader Writer } func main() { var w Writer // os.Stdout implements Writer w = os.Stdout fmt.Fprintf(w, “hello, writern”) }
Package io defines Reader and Writer so you don’t have to.
7.2 Errors
An error is anything that can describe itself as an error string. The idea is captured by the predefined, built-in interface type, error, with its single method, Error, returning a string:
type error interface { Error() string }
The fmt package’s various print routines automatically know to call the method when asked to print an error.
package main import ( “fmt” “time” ) type MyError struct { When time.Time What string } func (e *MyError) Error() string { return fmt.Sprintf(“at %v, %s”, e.When, e.What) } func run() error { return &MyError{ time.Now(), “it didn’t work”, } } func main() { if err := run(); err != nil { fmt.Println(err) } }
7.3 Exercise: Errors
Copy your Sqrt function from the earlier exercises (Section 5.4) and modify it to return an errorvalue.
Sqrt should return a non-nil error value when given a negative number, as it doesn’t support complex numbers.
Create a new type
type ErrNegativeSqrt float64
and make it an error by giving it a
func (e ErrNegativeSqrt) Error() string
method such that ErrNegativeSqrt(-2).Error() returns
cannot Sqrt negative number: -2.
Note: a call to fmt.Print(e) inside the Error method will send the program into an infinite loop.You can avoid this by converting e first: fmt.Print(float64(e)). Why?
Change your Sqrt function to return an ErrNegativeSqrt value when given a negative number.
7.3.1 Solution
package main import ( “fmt” ) type ErrNegativeSqrt float64 func (e ErrNegativeSqrt) Error() string { return fmt.Sprintf(“cannot Sqrt negative number: %g”, float64(e)) } func Sqrt(x float64) (float64, error) { if x < 0 { return 0, ErrNegativeSqrt(x) } z := 1.0 for i := 0; i < 10; i++ { z = z – ((z*z)-x)/(2*z) } return z, nil } func main() { fmt.Println(Sqrt(2)) fmt.Println(Sqrt(-2)) }
Tip: When doing an inferred declaration of a float, you can omit the decimal value and do the following:
z := 1. // same as // z := 1.0
Chapter 8 Concurrency
Concurrent programming is a large topic but it’s also one of the most interesting aspects of the Go language.
Concurrent programming in many environments is made difficult by the subtleties required to implement correct access to shared variables. Go encourages a different approach in which shared values are passed around on channels and, in fact, never actively shared by separate threads of execution. Only one goroutine has access to the value at any given time. Data races cannot occur, by design. To encourage this way of thinking we have reduced it to a slogan:
Do not communicate by sharing memory; instead, share memory by communicating.
This approach can be taken too far. Reference counts may be best done by putting a mutex around an integer variable, for instance. But as a high-level approach, using channels to control access makes it easier to write clear, correct programs.
Although Go’s approach to concurrency originates in Hoare’s Communicating Sequential Processes (CSP), it can also be seen as a type-safe generalization of Unix pipes.
8.1 Goroutines
A goroutine is a lightweight thread managed by the Go runtime.
go f(x, y, z)
starts a new goroutine running:
f(x, y, z)
The evaluation of f, x, y, and z happens in the current goroutine and the execution of f happens in the new goroutine.
Goroutines run in the same address space, so access to shared memory must be synchronized. The sync package provides useful primitives, although you won’t need them much in Go as there are other primitives.
package main import ( “fmt” “time” ) func say(s string) { for i := 0; i < 5; i++ { time.Sleep(100 * time.Millisecond) fmt.Println(s) } } func main() { go say(“world”) say(“hello”) }
8.2 Channels
Channels are a typed conduit through which you can send and receive values with the channel operator, <-.
ch <- v // Send v to channel ch. v := <-ch // Receive from ch, and // assign value to v.
(The data flows in the direction of the arrow.)
Like maps (Section 4.4) and slices (Section 4.2), channels must be created before use:
ch := make(chan int)
By default, sends and receives block wait until the other side is ready. This allows goroutines to synchronize without explicit locks or condition variables.
package main import “fmt” func sum(a []int, c chan int) { sum := 0 for _, v := range a { sum += v } c <- sum // send sum to c } func main() { a := []int{7, 2, 8, -9, 4, 0} c := make(chan int) go sum(a[:len(a)/2], c) go sum(a[len(a)/2:], c) x, y := <-c, <-c // receive from c fmt.Println(x, y, x+y) }
8.2.1 Buffered channels
Channels can be buffered. Provide the buffer length as the second argument to make to initialize a buffered channel:
ch := make(chan int, 100)
Sends to a buffered channel block only when the buffer is full. Receives block when the buffer is empty.
package main import “fmt” func main() { c := make(chan int, 2) c <- 1 c <- 2 fmt.Println(<-c) fmt.Println(<-c) }
But if you do:
package main import “fmt” func main() { c := make(chan int, 2) c <- 1 c <- 2 c <- 3 fmt.Println(<-c) fmt.Println(<-c) fmt.Println(<-c) }
You are getting a deadlock:
fatal error: all goroutines are asleep – deadlock!
That’s because we overfilled the buffer without letting the code a chance to read/remove a value from the channel.
However, this version using a goroutine would work fine:
package main import “fmt” func main() { c := make(chan int, 2) c <- 1 c <- 2 c3 := func() { c <- 3 } go c3() fmt.Println(<-c) fmt.Println(<-c) fmt.Println(<-c) }
The reason is that we are adding an extra value from inside a go routine, so our code doesn’t block the main thread. The goroutine is being called before the channel is being emptied, but that is fine, the goroutine will wait until the channel is available. We then read a first value from the channel, which frees a spot and our goroutine can push its value to the channel.
8.3 Range and close
A sender can close a channel to indicate that no more values will be sent. Receivers can test whether a channel has been closed by assigning a second parameter to the receive expression: after
v, ok := <-ch
ok is false if there are no more values to receive and the channel is closed.
The loop for i := range ch receives values from the channel repeatedly until it is closed.
Note: Only the sender should close a channel, never the receiver. Sending on a closed channel will cause a panic.
Another note: Channels aren’t like files; you don’t usually need to close them. Closing is only necessary when the receiver must be told there are no more values coming, such as to terminate a range loop.
package main import ( “fmt” ) func fibonacci(n int, c chan int) { x, y := 0, 1 for i := 0; i < n; i++ { c <- x x, y = y, x+y } close(c) } func main() { c := make(chan int, 10) go fibonacci(cap(c), c) for i := range c { fmt.Println(i) } }
8.4 Select
The select statement lets a goroutine wait on multiple communication operations.
A select blocks until one of its cases can run, then it executes that case. It chooses one at random if multiple are ready.
package main import “fmt” func fibonacci(c, quit chan int) { x, y := 0, 1 for { select { case c <- x: x, y = y, x+y case <-quit: fmt.Println(“quit”) return } } } func main() { c := make(chan int) quit := make(chan int) go func() { for i := 0; i < 10; i++ { fmt.Println(<-c) } quit <- 0 }() fibonacci(c, quit) }
8.4.1 Default case
The default case in a select is run if no other case is ready.
Use a default case to try a send or receive without blocking:
select { case i := <-c: // use i default: // receiving from c would block }
package main import ( “fmt” “time” ) func main() { tick := time.Tick(100 * time.Millisecond) boom := time.After(500 * time.Millisecond) for { select { case <-tick: fmt.Println(“tick.”) case <-boom: fmt.Println(“BOOM!”) return default: fmt.Println(” .”) time.Sleep(50 * time.Millisecond) } } }
8.4.2 Timeout
package main import ( “fmt” “log” “net/http” “time” ) func main() { response := make(chan *http.Response, 1) errors := make(chan *error) go func() { resp, err := http.Get(“http://matt.aimonetti.net/”) if err != nil { errors <- &err } response <- resp }() for { select { case r := <-response: fmt.Printf(“%s”, r.Body) return case err := <-errors: log.Fatal(*err) case <-time.After(200 * time.Millisecond): fmt.Printf(“Timed out!”) return } } }
See in playground but note that in playground, you won’t get a response due to sandboxing.
We are using the time.After call as a timeout measure to exit if the request didn’t give a response within 200ms.
8.5 Exercise: Equivalent Binary Trees
There can be many different binary trees with the same sequence of values stored at the leaves. For example, here are two binary trees storing the sequence 1, 1, 2, 3, 5, 8, 13.
A function to check whether two binary trees store the same sequence is quite complex in most languages. We’ll use Go’s concurrency and channels to write a simple solution.
This example uses the tree package, which defines the type:
type Tree struct { Left *Tree Value int Right *Tree }
  1. Implement the Walk function.
  2. Test the Walk function.
The function tree.New(k) constructs a randomly-structured binary tree holding the values k, 2k, 3k, …, 10k.
Create a new channel ch and kick off the walker:
go Walk(tree.New(1), ch)
Then read and print 10 values from the channel. It should be the numbers 1, 2, 3, …, 10.
  1. Implement the Same function using Walk to determine whether t1 and t2 store the same values.
  2. Test the Same function.
Same(tree.New(1), tree.New(1)) should return true, and Same(tree.New(1), tree.New(2)) should return false.
8.5.1 Solution
If you print tree.New(1) you will see the following tree:
((((1 (2)) 3 (4)) 5 ((6) 7 ((8) 9))) 10)
To implement the Walk function, we need two things:
  • walk each side of the tree and print the values
  • close the channel so the range call isn’t stuck.
We need to set a recursive call and for that, we are defining a non-exported recWalk function, the function walks the left side first, then pushes the value to the channel and then walks the right side.This allows our range to get the values in the right order. Once all branches have been walked, we can close the channel to indicate to the range that the walking is over.
package main import ( “code.google.com/p/go-tour/tree” “fmt” ) // Walk walks the tree t sending all values // from the tree to the channel ch. func Walk(t *tree.Tree, ch chan int) { recWalk(t, ch) // closing the channel so range can finish close(ch) } // recWalk walks recursively through the tree and push values to the channel // at each recursion func recWalk(t *tree.Tree, ch chan int) { if t != nil { // send the left part of the tree to be iterated over first recWalk(t.Left, ch) // push the value to the channel ch <- t.Value // send the right part of the tree to be iterated over last recWalk(t.Right, ch) } } // Same determines whether the trees // t1 and t2 contain the same values. func Same(t1, t2 *tree.Tree) bool { ch1 := make(chan int) ch2 := make(chan int) go Walk(t1, ch1) go Walk(t2, ch2) for { x1, ok1 := <-ch1 x2, ok2 := <-ch2 switch { case ok1 != ok2: // not the same size return false case !ok1: // both channels are empty return true case x1 != x2: // elements are different return false default: // keep iterating } } } func main() { ch := make(chan int) go Walk(tree.New(1), ch) for v := range ch { fmt.Println(v) } fmt.Println(Same(tree.New(1), tree.New(1))) fmt.Println(Same(tree.New(1), tree.New(2))) }
The comparison of the two trees is trivial once we know how to extract the values of each tree. We just need to loop through the first tree (via the channel), read the value, get the value from the second channel (walking the second tree) and compare the two values.
Chapter 9 Tips and Tricks
This section will grow over time but the main goal is to share some tricks experienced developers discovered over time. Hopefully this tips will get new users more productive faster.
9.1 140 char tips
  • leave your object oriented brain at home. Embrace the interface. @mikegehard
  • Learn to do things the Go way, don’t try to force your language idioms into Go. @DrNic
  • It’s better to over do it with using interfaces than use too few of them. @evanphx
  • Embrace the language: simplicity, concurrency, and composition. @francesc
  • read all the awesome docs that they have on golang.org. @vbatts
  • always use gofmt. @darkhelmetlive
  • read a lot of source code. @DrNic
  • Learn and become familiar with tools and utilities, or create your own! They are as vital to your success as knowing the language. @coreyprak
9.2 Alternate Ways to Import Packages
There are a few other ways of importing packages. We’ll use the fmt package in the following examples:
  • import format “fmt” – Creates an alias of fmt. Preceed all fmt package content with format. instead of fmt..
  • import . “fmt” – Allows content of the package to be accessed directly, without the need for it to be preceded with fmt.
  • import _ “fmt” – Suppresses compiler warnings related to fmt if it is not being used, and executes initialization functions if there are any. The remainder of fmt is inaccessible.
9.3 goimports
Goimports is a tool that updates your Go import lines, adding missing ones and removing unreferenced ones.
It acts the same as gofmt (drop-in replacement) but in addition to code formatting, also fixes imports.
9.4 Organization
Go is a pretty easy programming language to learn but the hardest thing for developers at first is how to organize their code. Rails became popular for many reasons and scaffolding was one of them. It gave new developers clear directions and places to put their code and idioms to follow.
To some extent, Go does the same thing by providing developers with great tools like go fmt and by having a strict compiler that won’t compile unused variables or unused import statements.
9.5 Custom Constructors
A question I often hear is when should I use custom constructors like NewJob. My answer is that in most cases you don’t need to. However, whenever you need to set your value at initialization time and you have some sort of default values, it’s a good candidate for a constructor. In the above example, adding a constructor makes a lot of sense so we can set a default logger.
package main import ( “log” “os” ) type Job struct { Command string *log.Logger } func NewJob(command string) *Job { return &Job{command, log.New(os.Stderr, “Job: “, log.Ldate)} } func main() { NewJob(“demo”).Print(“starting now…”) }
9.6 Breaking down code in packages
See this blog post on refactoring Go code, the first part talks about package organization.
9.7 Sets
You might want to find a way to extract unique value from a collection. In other languages, you often have a set data structure not allowing duplicates. Go doesn’t have that built in, however it’s not too hard to implement (due to a lack of generics, you do need to do that for most types, which can be cumbersome).
// UniqStr returns a copy if the passed slice with only unique string results. func UniqStr(col []string) []string { m := map[string]struct{}{} for _, v := range col { if _, ok := m[v]; !ok { m[v] = struct{}{} } } list := make([]string, len(m)) i := 0 for v := range m { list[i] = v i++ } return list }
I used a few interesting tricks that are interesting to know. First, the map of empty structs:
m := map[string]struct{}{}
We create a map with the keys being the values we want to be unique, the associated value doesn’t really matter much so it could be anything. For instance:
m := map[string]bool{}
However I chose an empty structure because it will be as fast as a boolean but doesn’t allocate as much memory.
The second trick can been seen a bit further:
if _, ok := m[v]; !ok { m[v] = struct{}{} }
What we are doing here, is simply check if there is a value associated with the key v in the map m, we don’t care about the value itself, but if we know that we don’t have a value, then we add one.
Once we have a map with unique keys, we can extract them into a new slice of strings and return the result.
Here is the test for this function, as you can see, I used a table test, which is the idiomatic Go way to run unit tests:
func TestUniqStr(t *testing.T) { data := []struct{ in, out []string }{ {[]string{}, []string{}}, {[]string{“”, “”, “”}, []string{“”}}, {[]string{“a”, “a”}, []string{“a”}}, {[]string{“a”, “b”, “a”}, []string{“a”, “b”}}, {[]string{“a”, “b”, “a”, “b”}, []string{“a”, “b”}}, {[]string{“a”, “b”, “b”, “a”, “b”}, []string{“a”, “b”}}, {[]string{“a”, “a”, “b”, “b”, “a”, “b”}, []string{“a”, “b”}}, {[]string{“a”, “b”, “c”, “a”, “b”, “c”}, []string{“a”, “b”, “c”}}, } for _, exp := range data { res := UniqStr(exp.in) if !reflect.DeepEqual(res, exp.out) { t.Fatalf(“%q didn’t match %qn”, res, exp.out) } } }
9.8 Dependency package management
Unfortunately, Go doesn’t ship with its own dependency package management system. Probably due to its roots in the C culture, packages aren’t versioned and explicit version dependencies aren’t addressed.
The challenge is that if you have multiple developers on your project, you want all of them to be on the same version of your dependencies. Your dependencies might also have their own dependencies and you want to make sure everything is in a good state. It gets even tricker when you have multiple projects using different versions of the same dependency. This is typically the case in a CIenvironment.
The Go community came up with a lot of different solutions for these problems. But for me, none are really great so at Splice we went for the simplest working solution we found: gpm
Gpm is a simple bash script, we end up modifying it a little so we could drop the script in each repo.The bash script uses a custom file called Godeps which lists the packages to install.
When switching to a different project, we run the project gpm script to pull down or set the right revision of each package.
In our CI environment, we set GOPATH to a project specific folder before running the test suite so packages aren’t shared between projects.
9.9 Using errors
Errors are very important pattern in Go and at first, new developers are surprised by the amount of functions returning a value and an error.
Go doesn’t have a concept of an exception like you might have seen in other programming languages.Go does have something called panic but as its name suggests they are really exceptional and shouldn’t be rescued (that said, they can be).
The error handling in Go seems cumbersome and repetitive at first, but quickly becomes part of the way we think. Instead of creating exceptions that bubble up and might or might not be handled or passed higher, errors are part of the response and designed to be handled by the caller. Whenever a function might generate an error, its response should contain an error param.
Andrew Gerrand from the Go team wrote a great blog post on errors I strongly recommend you read it.
9.10 Quick look at some compiler’s optimization
You can pass specific compiler flags to see what optimizations are being applied as well as how some aspects of memory management. This is an advanced feature, mainly for people who want to understand some of the compiler optimizations in place.
Let’s take the following code example from an earlier chapter:
package main import “fmt” type User struct { Id int Name, Location string } func (u *User) Greetings() string { return fmt.Sprintf(“Hi %s from %s”, u.Name, u.Location) } func NewUser(id int, name, location string) *User { id++ return &User{id, name, location} } func main() { u := NewUser(42, “Matt”, “LA”) fmt.Println(u.Greetings()) }
Build your file (here called t.go) passing some gcflags:
$ go build -gcflags=-m t.go # command-line-arguments ./t.go:15: can inline NewUser ./t.go:21: inlining call to NewUser ./t.go:10: leaking param: u ./t.go:10: leaking param: u ./t.go:12: (*User).Greetings … argument does not escape ./t.go:15: leaking param: name ./t.go:15: leaking param: location ./t.go:17: &User literal escapes to heap ./t.go:15: leaking param: name ./t.go:15: leaking param: location ./t.go:21: &User literal escapes to heap ./t.go:22: main … argument does not escape
The compiler notices that it can inline the NewUser function defined on line 15 and inline it on line 21. Dave Cheney has a great post about why Go’s inlining is helping your programs run faster.
Basically, the compiler moves the body of the NewUser function (L15) to where it’s being called (L21) and therefore avoiding the overhead of a function call but increasing the binary size.
The compiler creates the equivalent of:
func main() { id := 42 + 1 u := &User{id, “Matt”, “LA”} fmt.Println(u.Greetings()) }
On a few lines, you see the potentially alarming leaking param message. It doesn’t mean that there is a memory leak but that the param is kept alive even after returning. The “leaked params” are:
  • On the Greetings’s method: u (receiver)
  • On the NewUser’s functon: name, location
The reason why u “leaks” in the Greetings method is because it’s being used in the fmt.Sprintffunction call as an argument. name and location are also “leaked” because they are used in theUser’s literal value. Note that id doesn’t leak because it’s a value, only references and pointers can leak.
X argument does not escape means that the argument doesn’t “escape” the function, meaning that it’s not used outside of the function so it’s safe to store it on the stack.
On the other hand, you can see that &User literal escapes to heap. What it means is that the address of a literal value is used outside of the function and therefore can’t be stored on the stack.The value could be stored on the stack, except a pointer to the value escapes the function, so the value has to be moved to the heap to prevent the pointer referring to incorrect memory once the function returns. This is always the case when calling a method on a value and the method uses one or more fields.
9.11 Expvar
TODO package
9.12 Set the build id using git’s SHA
It’s often very useful to burn a build id in your binaries. I personally like to use the SHA1 of the git commit I’m committing. You can get the short version of the sha1 of your latest commit by running the following git command from your repo:
git rev-parse –short HEAD
The next step is to set an exported variable that you will set at compilation time using the -ldflagsflag.
package main import “fmt” // compile passing -ldflags “-X main.Build <build sha1>” var Build string func main() { fmt.Printf(“Using build: %sn”, Build) }
Save the above code in a file called example.go. If you run the above code, Build won’t be set, for that you need to set it using go build and the -ldflags.
$ go build -ldflags “-X main.Build a1064bc” example.go
Now run it to make sure:
$ ./example Using build: a1064bc
Now, hook that into your deployment compilation process, I personally like Rake to do that, and this way, every time I compile, I think of Jim Weirich.
9.13 How to see what packages my app imports
It’s often practical to see what packages your app is importing. Unfortunately there isn’t a simple way to do that, however it is doable via the go list tool and using templates.
Go to your app and run the following.
$ go list -f ‘{{join .Deps “n”}}’ | xargs go list -f ‘{{if not .Standard}}{{.ImportPath}}{{end}}’
Here is an example with the clirescue refactoring example:
$ cd $GOPATH/src/github.com/GoBootcamp/clirescue $ go list -f ‘{{join .Deps “n”}}’ | xargs go list -f ‘{{if not .Standard}}{{.ImportPath}}{{end}}’ github.com/GoBootcamp/clirescue/cmdutil github.com/GoBootcamp/clirescue/trackerapi github.com/GoBootcamp/clirescue/user github.com/codegangsta/cli
If you want the list to also contain standard packages, edit the template and use:
$ go list -f ‘{{join .Deps “n”}}’ | xargs go list -f ‘{{.ImportPath}}’
9.14 Iota: Elegant Constants
Some concepts have names, and sometimes we care about those names, even (or especially) in our code.
const ( CCVisa = “Visa” CCMasterCard = “MasterCard” CCAmericanExpress = “American Express” )
At other times, we only care to distinguish one thing from the other. There are times when there’s no inherently meaningful value for a thing. For example, if we’re storing products in a database table we probably don’t want to store their category as a string. We don’t care how the categories are named, and besides, marketing changes the names all the time.
We care only that they’re distinct from each other.
const ( CategoryBooks = 0 CategoryHealth = 1 CategoryClothing = 2 )
Instead of 0, 1, and 2 we could have chosen 17, 43, and 61. The values are arbitrary.
Constants are important but they can be hard to reason about and difficult to maintain. In some languages like Ruby developers often just avoid them. In Go, constants have many interesting subtleties that, when used well, can make the code both elegant and maintainable.
9.14.1 Auto Increment
A handy idiom for this in golang is to use the iota identifier, which simplifies constant definitions that use incrementing numbers, giving the categories exactly the same values as above.
const ( CategoryBooks = iota // 0 CategoryHealth // 1 CategoryClothing // 2 )
9.14.2 Custom Types
Auto-incrementing constants are often combined with a custom type, allowing you to lean on the compiler.
type Stereotype int const ( TypicalNoob Stereotype = iota // 0 TypicalHipster // 1 TypicalUnixWizard // 2 TypicalStartupFounder // 3 )
If a function is defined to take an int as an argument rather than a Stereotype, it will blow up at compile-time if you pass it a Stereotype:
func CountAllTheThings(i int) string { return fmt.Sprintf(“there are %d things”, i) } func main() { n := TypicalHipster fmt.Println(CountAllTheThings(n)) } // output: // cannot use TypicalHipster (type Stereotype) as type int in argument to CountAllTheThings
The inverse is also true. Given a function that takes a Stereotype as an argument, you can’t pass it an int:
func SoSayethThe(character Stereotype) string { var s string switch character { case TypicalNoob: s = “I’m a confused ninja rockstar.” case TypicalHipster: s = “Everything was better we programmed uphill and barefoot in the snow on the SUTX 5918” case TypicalUnixWizard: s = “sudo grep awk sed %!#?!!1!” case TypicalStartupFounder: s = “exploit compelling convergence to syndicate geo-targeted solutions” } return s } func main() { i := 2 fmt.Println(SoSayethThe(i)) }
// output: // cannot use i (type int) as type Stereotype in argument to SoSayethThe
There’s a dramatic twist, however. You could pass a number constant, and it would work: “`go func main() { fmt.Println(SoSayethThe(0)) } // output: // I’m a confused ninja rockstar. “` This is because constants in Go are loosely typed until they are used in a strict context. ### Skipping Values Imagine that you’re dealing with consumer audio output. The audio might not have any output whatsoever, or it could be mono, stereo, or surround. There’s some underlying logic to defining no output as 0, mono as 1, and stereo as 2, where the value is the number of channels provided. So what value do you give Dolby 5.1 surround? On the one hand, it’s 6 channel output, but on the other hand, only 5 of those channels are full bandwidth channels (hence the 5.1 designation – with the .1 referring to the low-frequency effects channel). Either way, we don’t want to simply increment to 3. We can use underscores to skip past the unwanted values. “`go type AudioOutput int const ( OutMute AudioOutput = iota // 0 OutMono // 1 OutStereo // 2 _ _ OutSurround // 5 ) “` ### Expressions The iota can do more than just increment. Or rather, iota always increments, but it can be used in expressions, storing the resulting value in the constant. Here we’re creating constants to be used as a bitmask. “`go type Allergen int const ( IgEggs Allergen = 1 << iota // 1 << 0 which is 00000001 IgChocolate // 1 << 1 which is 00000010 IgNuts // 1 << 2 which is 00000100 IgStrawberries // 1 << 3 which is 00001000 IgShellfish // 1 << 4 which is 00010000 ) “` This works because when you have only an identifier on a line in a const group, it will take the previous expression and reapply it, with the incremented iota. In the language of [the spec](http://golang.org/ref/spec#Iota), this is called _implicit repetition of the last non-empty expression list_. If you’re allergic to eggs, chocolate, and shellfish, and flip those bits to the “on” position (mapping the bits right to left), then you get a bit value of 00010011, which corresponds to 19 in decimal. “`go fmt.Println(IgEggs | IgChocolate | IgShellfish) // output: // 19 “` There’s a great example in [Effective Go](https://golang.org/doc/effective_go.html#constants) for defining orders of magnitude: “`go type ByteSize float64 const ( _ = iota // ignore first value by assigning to blank identifier KB ByteSize = 1 << (10 * iota) // 1 << (10*1) MB // 1 << (10*2) GB // 1 << (10*3) TB // 1 << (10*4) PB // 1 << (10*5) EB // 1 << (10*6) ZB // 1 << (10*7) YB // 1 << (10*8) ) “` Today I learned that after zettabyte we get yottabyte. #TIL ### But Wait, There’s More What happens if you define two constants on the same line? What is the value of Banana? 2 or 3? And what about Durian? “`go const ( Apple, Banana = iota + 1, iota + 2 Cherimoya, Durian Elderberry, Fig ) “` The iota increments on the next line, rather than as soon as it gets referenced. “`go // Apple: 1 // Banana: 2 // Cherimoya: 2 // Durian: 3 // Elderberry: 3 // Fig: 4 “` Which is messed up, because now you have constants with the same value. ### So, Yeah There’s a lot more to be said about constants in Go, and you should probably read Rob Pike’s [blog post on the subject](http://blog.golang.org/constants) over on the golang blog. _This was first published by Katrina Owen on the [Splice blog](https://splice.com/blog/iota-elegant-constants-golang/)._
Chapter 10 Exercises
We have 4 exercises to choose from, each exercise focuses on different challenges. Try to tackle the exercise of your choice by pairing with at least one person, ideally try to have 4 people by group.
Fork the original repo (with the instructions), work on it and send a pull request when you are done.
If your group wants to work on a new exercise, please create an assignment and send it to me (Matt) so I can add it to the repository and the following list.
  • Hashing
  • image manipulation
  • Concurrency (channels, go routines)
  • Network interface
  • depending on the commands you implement
  • concurrency
  • consumption of web resources (http/json/XML)
  • sorting
  • data sets
  • data storage
  • Building a web API
  • Testing a web API
  • JSON encoding/decoding
  • Exploring godoc
  • Extending existing packages or
  • Shelling out
Source: ednsquare