a-tour-of-go

Overview ¶

Appending to a slice It is common to append new elements to a slice, and so Go provides a built-in append function. The documentation of the built-in package describes append.

func append(s []T, vs ...T) []T The first parameter s of append is a slice of type T, and the rest are T values to append to the slice.

The resulting value of append is a slice containing all the elements of the original slice plus the provided values.

If the backing array of s is too small to fit all the given values a bigger array will be allocated. The returned slice will point to the newly allocated array.

(To learn more about slices, read the Slices: usage and internals article.)

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.

Basic types Go's basic types are

bool

string

int int8 int16 int32 int64 uint uint8 uint16 uint32 uint64 uintptr

byte // alias for uint8

rune // alias for int32

// represents a Unicode code point

float32 float64

complex64 complex128 The example shows variables of several types, and also that variable declarations may be "factored" into blocks, as with import statements.

The int, uint, and uintptr types are usually 32 bits wide on 32-bit systems and 64 bits wide on 64-bit systems. When you need an integer value you should use int unless you have a specific reason to use a sized or unsigned integer type.

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.

Modify the example to overfill the buffer and see what happens.

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 and slices, channels must be created before use:

ch := make(chan int) By default, sends and receives block until the other side is ready. This allows goroutines to synchronize without explicit locks or condition variables.

The example code sums the numbers in a slice, distributing the work between two goroutines. Once both goroutines have completed their computation, it calculates the final result.

Constants Constants are declared like variables, but with the const keyword.

Constants can be character, string, boolean, or numeric values.

Constants cannot be declared using the := syntax.

Default Selection 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
}

Stacking defers Deferred function calls are pushed onto a stack. When a function returns, its deferred calls are executed in last-in-first-out order.

To learn more about defer statements read this blog post.

Defer A defer statement defers the execution of a function until the surrounding function returns.

The deferred call's arguments are evaluated immediately, but the function call is not executed until the surrounding function returns.

The empty interface The interface type that specifies zero methods is known as the empty interface:

interface{} An empty interface may hold values of any type. (Every type implements at least zero methods.)

Empty interfaces are used by code that handles values of unknown type. For example, fmt.Print takes any number of arguments of type interface{}.

Errors Go programs express error state with error values.

The error type is a built-in interface similar to fmt.Stringer:

type error interface {
    Error() string
}

(As with fmt.Stringer, the fmt package looks for the error interface when printing values.)

Functions often return an error value, and calling code should handle errors by testing whether the error equals nil.

i, err := strconv.Atoi("42")

if err != nil {
    fmt.Printf("couldn't convert number: %v\n", err)
    return
}

fmt.Println("Converted integer:", i) A nil error denotes success; a non-nil error denotes failure.

Exported names In Go, a name is exported if it begins with a capital letter. For example, Pizza is an exported name, as is Pi, which is exported from the math package.

pizza and pi do not start with a capital letter, so they are not exported.

When importing a package, you can refer only to its exported names. Any "unexported" names are not accessible from outside the package.

Run the code. Notice the error message.

To fix the error, rename math.pi to math.Pi and try it again.

For continued The init and post statements are optional.

For is Go's "while" At that point you can drop the semicolons: C's while is spelled for in Go.

For Go has only one looping construct, the for loop.

The basic for loop has three components separated by semicolons:

the init statement: executed before the first iteration the condition expression: evaluated before every iteration the post statement: executed at the end of every iteration The init statement will often be a short variable declaration, and the variables declared there are visible only in the scope of the for statement.

The loop will stop iterating once the boolean condition evaluates to false.

Note: Unlike other languages like C, Java, or JavaScript there are no parentheses surrounding the three components of the for statement and the braces { } are always required.

Forever If you omit the loop condition it loops forever, so an infinite loop is compactly expressed.

Function closures Go functions may be closures. A closure is a function value that references variables from outside its body. The function may access and assign to the referenced variables; in this sense the function is "bound" to the variables.

For example, the adder function returns a closure. Each closure is bound to its own sum variable.

Function values Functions are values too. They can be passed around just like other values.

Function values may be used as function arguments and return values.

Functions continued When two or more consecutive named function parameters share a type, you can omit the type from all but the last.

In this example, we shortened ¶

x int, y int to

x, y int

Functions A function can take zero or more arguments.

In this example, add takes two parameters of type int.

Notice that the type comes after the variable name.

(For more about why types look the way they do, see the article on Go's declaration syntax.)

Slice literals A slice literal is like an array literal without the length.

This is an array literal:

[3]bool{true, true, false} And this creates the same array as above, then builds a slice that references it:

[]bool{true, true, false}

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. (See the next slide.)

If and else Variables declared inside an if short statement are also available inside any of the else blocks.

(Both calls to pow return their results before the call to fmt.Println in main begins.)

If with a short statement 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.

(Try using v in the last return statement.)

Go's if statements are like its for loops; the expression need not be surrounded by parentheses ( ) but the braces { } are required.

Images Package image defines the Image interface:

package image

type Image interface {
    ColorModel() color.Model
    Bounds() Rectangle
    At(x, y int) color.Color
}

Note: the Rectangle return value of the Bounds method is actually an image.Rectangle, as the declaration is inside package image.

(See the documentation for all the details.)

The color.Color and color.Model types are also interfaces, but we'll ignore that by using the predefined implementations color.RGBA and color.RGBAModel. These interfaces and types are specified by the image/color package

Imports This code groups the imports into a parenthesized, "factored" import statement.

You can also write multiple import statements, like:

import "fmt" import "math" But it is good style to use the factored import statement.

Type parameters Go functions can be written to work on multiple types using type parameters. The type parameters of a function appear between brackets, before the function's arguments.

func Index[T comparable](s []T, x T) int This declaration means that s is a slice of any type T that fulfills the built-in constraint comparable. x is also a value of the same type.

comparable is a useful constraint that makes it possible to use the == and != operators on values of the type. In this example, we use it to compare a value to all slice elements until a match is found. This Index function works for any type that supports comparison.

Methods and pointer indirection (2) The equivalent thing happens in the reverse direction.

Functions that take a value argument must take a value of that specific type:

var v Vertex fmt.Println(AbsFunc(v)) // OK fmt.Println(AbsFunc(&v)) // Compile error! while methods with value receivers take either a value or a pointer as the receiver when they are called:

var v Vertex fmt.Println(v.Abs()) // OK p := &v fmt.Println(p.Abs()) // OK In this case, the method call p.Abs() is interpreted as (*p).Abs().

Methods and pointer indirection Comparing the previous two programs, you might notice that functions with a pointer argument must take a pointer:

var v Vertex ScaleFunc(v, 5) // Compile error! ScaleFunc(&v, 5) // OK while methods with pointer receivers take either a value or a pointer as the receiver when they are called:

var v Vertex v.Scale(5) // OK p := &v p.Scale(10) // OK For the statement v.Scale(5), even though v is a value and not a pointer, the method with the pointer receiver is called automatically. That is, as a convenience, Go interprets the statement v.Scale(5) as (&v).Scale(5) since the Scale method has a pointer receiver.

Interface values with nil underlying values If the concrete value inside the interface itself is nil, the method will be called with a nil receiver.

In some languages this would trigger a null pointer exception, but in Go it is common to write methods that gracefully handle being called with a nil receiver (as with the method M in this example.)

Note that an interface value that holds a nil concrete value is itself non-nil.

Interface values Under the hood, interface values can be thought of as a tuple of a value and a concrete type:

(value, type) An interface value holds a value of a specific underlying concrete type.

Calling a method on an interface value executes the method of the same name on its underlying type.

Interfaces are implemented implicitly A type implements an interface by implementing its methods. There is no explicit declaration of intent, no "implements" keyword.

Implicit interfaces decouple the definition of an interface from its implementation, which could then appear in any package without prearrangement.

Interfaces An interface type is defined as a set of method signatures.

A value of interface type can hold any value that implements those methods.

Note: There is an error in the example code on line 22. Vertex (the value type) doesn't implement Abser because the Abs method is defined only on *Vertex (the pointer type).

Creating a slice with make Slices can be created with the built-in make function; this is how you create dynamically-sized arrays.

The make function allocates a zeroed array and returns a slice that refers to that array:

a := make([]int, 5) // len(a)=5 To specify a capacity, pass a third argument to make:

b := make([]int, 0, 5) // len(b)=0, cap(b)=5

b = b[:cap(b)] // len(b)=5, cap(b)=5 b = b[1:] // len(b)=4, cap(b)=4

Map literals continued If the top-level type is just a type name, you can omit it from the elements of the literal.

Map literals Map literals are like struct literals, but the keys are required.

Maps A map maps keys to values.

The zero value of a map is nil. A nil map has no keys, nor can keys be added.

The make function returns a map of the given type, initialized and ready for use.

Methods continued You can declare a method on non-struct types, too.

In this example we see a numeric type MyFloat with an Abs method.

You can only declare a method with a receiver whose type is defined in the same package as the method. You cannot declare a method with a receiver whose type is defined in another package (which includes the built-in types such as int).

Methods are functions Remember: a method is just a function with a receiver argument.

Here's Abs written as a regular function with no change in functionality.

Pointers and functions Here we see the Abs and Scale methods rewritten as functions.

Again, try removing the * from line 16. Can you see why the behavior changes? What else did you need to change for the example to compile?

(If you're not sure, continue to the next page.)

Pointer receivers You can declare methods with pointer receivers.

This means the receiver type has the literal syntax *T for some type T. (Also, T cannot itself be a pointer such as *int.)

For example, the Scale method here is defined on *Vertex.

Methods with pointer receivers can modify the value to which the receiver points (as Scale does here). Since methods often need to modify their receiver, pointer receivers are more common than value receivers.

Try removing the * from the declaration of the Scale function on line 16 and observe how the program's behavior changes.

With a value receiver, the Scale method operates on a copy of the original Vertex value. (This is the same behavior as for any other function argument.) The Scale method must have a pointer receiver to change the Vertex value declared in the main function.

Choosing a value or pointer receiver There are two reasons to use a pointer receiver.

The first is so that the method can modify the value that its receiver points to.

The second is to avoid copying the value on each method call. This can be more efficient if the receiver is a large struct, for example.

In this example, both Scale and Abs are with receiver type *Vertex, even though the Abs method needn't modify its receiver.

In general, all methods on a given type should have either value or pointer receivers, but not a mixture of both. (We'll see why over the next few pages.)

Methods Go does not have classes. However, you can define methods on types.

A method is a function with a special receiver argument.

The receiver appears in its own argument list between the func keyword and the method name.

In this example, the Abs method has a receiver of type Vertex named v.

Multiple results A function can return any number of results.

The swap function returns two strings.

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.

If key is not in the map, then elem is the zero value for the map's element type.

Note: If elem or ok have not yet been declared you could use a short declaration form:

elem, ok := m[key]

sync.Mutex We've seen how channels are great for communication among goroutines.

But what if we don't need communication? What if we just want to make sure only one goroutine can access a variable at a time to avoid conflicts?

This concept is called mutual exclusion, and the conventional name for the data structure that provides it is mutex.

Go's standard library provides mutual exclusion with sync.Mutex and its two methods:

Lock Unlock We can define a block of code to be executed in mutual exclusion by surrounding it with a call to Lock and Unlock as shown on the Inc method.

We can also use defer to ensure the mutex will be unlocked as in the Value method.

Named return values Go's return values may be named. If so, they are treated as variables defined at the top of the function.

These names should be used to document the meaning of the return values.

A return statement without arguments returns the named return values. This is known as a "naked" return.

Naked return statements should be used only in short functions, as with the example shown here. They can harm readability in longer functions.

Nil interface values A nil interface value holds neither value nor concrete type.

Calling a method on a nil interface is a run-time error because there is no type inside the interface tuple to indicate which concrete method to call.

Nil slices The zero value of a slice is nil.

A nil slice has a length and capacity of 0 and has no underlying array.

Numeric Constants Numeric constants are high-precision values.

An untyped constant takes the type needed by its context.

Try printing needInt(Big) too.

(An int can store at maximum a 64-bit integer, and sometimes less.)

Packages Every Go program is made up of packages.

Programs start running in package main.

This program is using the packages with import paths "fmt" and "math/rand".

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.

Note: The environment in which these programs are executed is deterministic, so each time you run the example program rand.Intn will return the same number.

(To see a different number, seed the number generator; see rand.Seed. Time is constant in the playground, so you will need to use something else as the seed.)

Pointers Go has pointers. A pointer holds the memory address of a value.

The type *T is a pointer to a T value. Its zero value is nil.

var p *int The & operator generates a pointer to its operand.

i := 42 p = &i The * operator denotes the pointer's underlying value.

fmt.Println(*p) // read i through the pointer p *p = 21 // set i through the pointer p This is known as "dereferencing" or "indirecting".

Unlike C, Go has no pointer arithmetic.

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 c 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.

Range continued You can skip the index or value by assigning to _.

for i, _ := range pow for _, value := range pow If you only want the index, you can omit the second variable.

for i := range pow

Range The range form of the for loop iterates over a slice or map.

When ranging over a slice, two values are returned for each iteration. The first is the index, and the second is a copy of the element at that index.

Readers The io package specifies the io.Reader interface, which represents the read end of a stream of data.

The Go standard library contains many implementations of this interface, including files, network connections, compressors, ciphers, and others.

The io.Reader interface has a Read method:

func (T) Read(b []byte) (n int, err error) Read populates the given byte slice with data and returns the number of bytes populated and an error value. It returns an io.EOF error when the stream ends.

The example code creates a strings.Reader and consumes its output 8 bytes at a time.

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.

Short variable declarations Inside a function, the := short assignment statement can be used in place of a var declaration with implicit type.

Outside a function, every statement begins with a keyword (var, func, and so on) and so the := construct is not available.

Slice defaults When slicing, you may omit the high or low bounds to use their defaults instead.

The default is zero for the low bound and the length of the slice for the high bound.

For the array ¶

var a [10]int these slice expressions are equivalent:

a[0:10] a[:10] a[0:] a[:]

Slice length and capacity A slice has both a length and a capacity.

The length of a slice is the number of elements it contains.

The capacity of a slice is the number of elements in the underlying array, counting from the first element in the slice.

The length and capacity of a slice s can be obtained using the expressions len(s) and cap(s).

You can extend a slice's length by re-slicing it, provided it has sufficient capacity. Try changing one of the slice operations in the example program to extend it beyond its capacity and see what happens.

Slices are like references to arrays A slice does not store any data, it just describes a section of an underlying array.

Changing the elements of a slice modifies the corresponding elements of its underlying array.

Other slices that share the same underlying array will see those changes.

Slices of slices Slices can contain any type, including other slices.

Slices An array has a fixed size. A slice, on the other hand, is a dynamically-sized, flexible view into the elements of an array. In practice, slices are much more common than arrays.

The type []T is a slice with elements of type T.

A slice is formed by specifying two indices, a low and high bound, separated by a colon:

a[low : high] This selects a half-open range which includes the first element, but excludes the last one.

The following expression creates a slice which includes elements 1 through 3 of a:

a[1:4]

* Stringers One of the most ubiquitous interfaces is Stringer defined by the fmt package.

type Stringer interface {
    String() string
}

A Stringer is a type that can describe itself as a string. The fmt package (and many others) look for this interface to print values.

Struct Fields Struct fields are accessed using a dot.

Struct Literals A struct literal denotes 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. (And the order of named fields is irrelevant.)

The special prefix & returns a pointer to the struct value.

Pointers to structs Struct fields can be accessed through a struct pointer.

To access the field X of a struct when we have the struct pointer p we could write (*p).X. However, that notation is cumbersome, so the language permits us instead to write just p.X, without the explicit dereference.

Structs A struct is a collection of fields.

Switch evaluation order Switch cases evaluate cases from top to bottom, stopping when a case succeeds.

(For example,

switch i { case 0: case f(): } does not call f if i==0.)

Note: Time in the Go playground always appears to start at 2009-11-10 23:00:00 UTC, a value whose significance is left as an exercise for the reader.

Switch with no condition Switch without a condition is the same as switch true.

This construct can be a clean way to write long if-then-else chains.

Switch A switch statement is a shorter way to write a sequence of if - else statements. It runs the first case whose value is equal to the condition expression.

Go's switch is like the one in C, C++, Java, JavaScript, and PHP, except that Go only runs the selected case, not all the cases that follow. In effect, the break statement that is needed at the end of each case in those languages is provided automatically in Go. Another important difference is that Go's switch cases need not be constants, and the values involved need not be integers.

Type assertions A type assertion provides access to an interface value's underlying concrete value.

t := i.(T) This statement asserts that the interface value i holds the concrete type T and assigns the underlying T value to the variable t.

If i does not hold a T, the statement will trigger a panic.

To test whether an interface value holds a specific type, a type assertion can return two values: the underlying value and a boolean value that reports whether the assertion succeeded.

t, ok := i.(T) If i holds a T, then t will be the underlying value and ok will be true.

If not, ok will be false and t will be the zero value of type T, and no panic occurs.

Note the similarity between this syntax and that of reading from a map.

Type conversions 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) Unlike in C, in Go assignment between items of different type requires an explicit conversion. Try removing the float64 or uint conversions in the example and see what happens.

Type inference When declaring a variable without specifying an explicit type (either by using the := syntax or var = expression syntax), the variable's type is inferred from the value on the right hand side.

When the right hand side of the declaration is typed, the new variable is of that same type:

var i int j := i // j is an int But when the right hand side contains an untyped numeric constant, the new variable may be an int, float64, or complex128 depending on the precision of the constant:

i := 42 // int f := 3.142 // float64 g := 0.867 + 0.5i // complex128 Try changing the initial value of v in the example code and observe how its type is affected.

Type switches A type switch is a construct that permits several type assertions in series.

A type switch is like a regular switch statement, but the cases in a type switch specify types (not values), and those values are compared against the type of the value held by the given interface value.

switch v := i.(type) { case T:

// here v has type T

case S:

// here v has type S

default:

    // no match; here v has the same type as i
}

The declaration in a type switch has the same syntax as a type assertion i.(T), but the specific type T is replaced with the keyword type.

This switch statement tests whether the interface value i holds a value of type T or S. In each of the T and S cases, the variable v will be of type T or S respectively and hold the value held by i. In the default case (where there is no match), the variable v is of the same interface type and value as i.

Variables with initializers A var declaration can include initializers, one per variable.

If an initializer is present, the type can be omitted; the variable will take the type of the initializer.

Variables The var statement declares a list of variables; as in function argument lists, the type is last.

A var statement can be at package or function level. We see both in this example.

Zero values Variables declared without an explicit initial value are given their zero value.

The zero value is:

0 for numeric types, false for the boolean type, and "" (the empty string) for strings.