Golang Learning Notes

By convention, packages are given lower case, single-word names; there should be no need for underscores or mixedCaps.

Another convention is that the package name is the base name of its source directory; the package in src/encoding/base64 is imported as "encoding/base64" but has name base64, not encoding_base64 and not encodingBase64.

Use the package structure to help you choose good names.

Go doesn’t provide automatic support for getters and setters.

There’s nothing wrong with providing getters and setters yourself, and it’s often appropriate to do so, but it’s neither idiomatic nor necessary to put Get into the getter’s name.

If you have a field called owner (lower case, unexported), the getter method should be called Owner (upper case, exported), not GetOwner.

A setter function, if needed, will likely be called SetOwner.

Both names read well in practice:

  owner := obj.Owner()
  if owner != user {
      obj.SetOwner(user)
  }

By convention, one-method interfaces are named by the method name plus an -er suffix or similar modification to construct an agent noun: Reader, Writer, Formatter, CloseNotifier etc.

There are a number of such names and it’s productive to honor them and the function names they capture.

Read, Write, Close, Flush, String and so on have canonical signatures and meanings.

To avoid confusion, don’t give your method one of those names unless it has the same signature and meaning.

Conversely, if your type implements a method with the same meaning as a method on a well-known type, give it the same name and signature; call your string-converter method String not ToString.

The expressions need not be constants or even integers,

the cases are evaluated top to bottom until a match is found,

and if the switch has no expression it switches on true.

It’s therefore possible—​and idiomatic—​to write an if-else-if-else chain as a switch.

There is no automatic fall through, but cases can be presented in comma-separated lists.

Although they are not nearly as common in Go as some other C-like languages, break statements can be used to terminate a switch early.

Sometimes, though, it’s necessary to break out of a surrounding loop, not the switch, and in Go that can be accomplished by putting a label on the loop and "breaking" to that label.

Of course, the continue statement also accepts an optional label but it applies only to loops.

The return or result "parameters" of a Go function can be given names and used as regular variables, just like the incoming parameters.

When named, they are initialized to the zero values for their types when the function begins;

if the function executes a return statement with no arguments, the current values of the result parameters are used as the returned values.

Go’s defer statement schedules a function call (the deferred function) to be run immediately before the function executing the defer returns.

It’s an unusual but effective way to deal with situations such as resources that must be released regardless of which path a function takes to return.

  func ReadFile(filename string) ([]byte, error) {
      f, err := os.Open(filename)
      if err != nil {
          return nil, err
      }
      defer f.Close()
      return ReadAll(f)
  }

The arguments to the deferred function (which include the receiver if the function is a method) are evaluated when the defer executes, not when the call executes.

Deferred functions are executed in LIFO order (stacking style).

  for i := 0; i < 5; i++ {
  	defer fmt.Printf("%d ", i)
  }

  // Output:
  // 4 3 2 1 0

  // All function values created by this loop "capture"
  // and share the same variable—an addressable storage location,
  // not its value at that particular moment.
  for i := 0; i < 5; i++ {
      defer func() {
          fmt.Print(i, " ")
      }()
  }

  // Output:
  // 5 5 5 5 5

  for i := 0; i < 5; i++ {
      // declares inner i, intialized to outer i
      i := i
      defer func() {
          fmt.Print(i, " ")
      }()
  }

  // Output:
  // 4 3 2 1 0

Go source code is always UTF-8.

A string holds arbitrary bytes.

A string literal, absent byte-level escapes, always holds valid UTF-8 sequences.

Those sequences represent Unicode code points, called runes.

No guarantee is made in Go that characters in strings are normalized.

The type [n]T is an array of n values of type T.

Arrays are values.

In particular, if you pass an array to a function, it will receive a copy of the array, not a pointer to it.

The size of an array is part of its type.

A slice, on the other hand, is a dynamically-sized, flexible view into the elements of an array.

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:

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

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

  a[1:4]

A slice does not store any data, it just describes a section of an underlying array.

A slice hold references to an underlying array, and if you assign one slice to another, both refer to the same 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.

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

  []bool{true, true, false}

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[:]

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.

The zero value of a slice is nil.

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

It is common to append new elements to a slice, and so Go provides a built-in append function.

  func append(s []T, vs ...T) []T

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.

  var s []int

  // append works on nil slices.
  s = append(s, 0)

  // The slice grows as needed.
  s = append(s, 1)

  // We can add more than one element at a time.
  s = append(s, 2, 3, 4)

Maps are a convenient and powerful built-in data structure that associate values of one type (the key) with values of another type (the element or value).

The key can be of any type for which the equality operator is defined, such as integers, floating point and complex numbers, strings, pointers, interfaces (as long as the dynamic type supports equality), structs and arrays.

Slices cannot be used as map keys, because equality is not defined on them.

Like slices, maps hold references to an underlying data structure.

The zero value of a map is nil.

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

var m map[string]int // <nil>
m = map[string]int{
    "hello": 100,
    "world": 200,
}

The make function returns a map of the given type with an optional capacity hint as arguments, initialized and ready for use.

// m := make(map[string]int, 100)
m := make(map[string]int)

// insert or update an element
m["Answer"] = 42

// delete an element:
// The delete function doesn’t return anything, and will do nothing if the specified key doesn’t exist.
delete(m, "Answer")

// retrieve an element
// If the requested key doesn’t exist, we get the value type’s zero value.
v := m["Answer"]

// test that a key is present with a two-value assignment
v, ok := m["Answer"]

Functions are values too.

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

Go functions may be closures.

Go does not have classes.

A method is a function with a special receiver argument.

There are two reasons to use a pointer receiver.

In general, all methods on a given type should have either value or pointer receivers, but not a mixture of both.

The rule about pointers vs. values for receivers is that value methods can be invoked on pointers and values, but pointer methods can only be invoked on pointers.

  package bufio // import "bufio"

  func (b *Reader) Read(p []byte) (n int, err error)

  func (b *Writer) Write(p []byte) (nn int, err error)

Just as some functions allow nil pointers as arguments, so do some methods for their receiver, especially if nil is a meaningful zero value of the type, as with maps and slices.

When you define a type whose methods allow nil as a receiver value, it’s worth pointing this out explicitly in its documentation comment.

A type implements an interface by implementing its methods.

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

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.

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.

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

type I interface {
	M()
}

type T struct{}

func (t *T) M() {
	if t == nil {
		fmt.Println("<nil>")
		return
	}
}

func main() {
	var i I
	var t *T
	i = t
	i.M()
	fmt.Printf("(%v, %T)\n", i, i)

	i = &T{}
	i.M()
	fmt.Printf("(%v, %T)\n", i, i)

	// Output:
	// <nil>
	// (<nil>, *main.T)
	// (&{}, *main.T)
}

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.

var i I
fmt.Printf("(%v, %T)\n", i, i)
i.M()
// (<nil>, <nil>)
// panic: runtime error: invalid memory address or nil pointer dereference

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 convenience, the predeclared type any is an alias for the empty interface.

$ go doc builtin.any
package builtin // import "builtin"

type any = interface{}
    any is an alias for interface{} and is equivalent to interface{} in all
    ways.

func recover() any

In their most general form, an interface element may also be an arbitrary type term T, or a term of the form ~T specifying the underlying type T, or a union of terms t1|t2|…|tn.

By construction, an interface’s type set never contains an interface type.

// An interface representing only the type int.
type I0 interface {
	int
}

// An interface representing all types with underlying type int.
type I1 interface {
	~int
}

// An interface representing all types with underlying type int that implement the String method.
type I2 interface {
	~int
	String() string
}

// An interface representing an empty type set: there is no type that is both an int and a string.
type I3 interface {
	int
	string
}

In a term of the form ~T, the underlying type of T must be itself, and T cannot be an interface.

type MyInt int

type MyI interface {
	~[]byte  // the underlying type of []byte is itself
	~MyInt   // illegal: the underlying type of MyInt is not MyInt
	~error   // illegal: error is an interface
}

Union elements denote unions of type sets:

// The Float interface represents all floating-point types
// (including any named types whose underlying types are
// either float32 or float64).
type Float interface {
	~float32 | ~float64
}

If a type exists only to implement an interface and will never have exported methods beyond that interface, there is no need to export the type itself.

Exporting just the interface makes it clear the value has no interesting behavior beyond what is described in the interface.

It also avoids the need to repeat the documentation on every instance of a common method.

In such cases, the constructor should return an interface value rather than the implementing type.

A type assertion provides access to an interface value’s underlying concrete value.

t := i.(T)

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)

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.

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
}

Go does not provide the typical, type-driven notion of subclassing, but it does have the ability to “borrow” pieces of an implementation by embedding types within a struct or interface.

  package io // import "io"

  type Reader interface {
      Read(p []byte) (n int, err error)
  }

  type Writer interface {
      Write(p []byte) (n int, err error)
  }

  // ReadWriter is the interface that combines the Reader and Writer interfaces.
  type ReadWriter interface {
      Reader
      Writer
  }

  package bufio // import "bufio"

  type Reader struct {
      // Has unexported fields.
  }

  func (b *Reader) Read(p []byte) (n int, err error)

  type Writer struct {
      // Has unexported fields.
  }

  func (b *Writer) Write(p []byte) (nn int, err error)

  // ReadWriter stores pointers to a Reader and a Writer.
  // It implements io.ReadWriter.
  type ReadWriter struct {
      *Reader
      *Writer
  }

There’s an important way in which embedding differs from subclassing.

Embedding types introduces the problem of name conflicts but the rules to resolve them are simple.

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:

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

// Sends to a buffered channel block only when the buffer is full.
// Receives block when the buffer is empty.
bufChan := make(chan int, 100)

A sender can close a channel to indicate that no more values will be sent.

A channel may be constrained only to send or only to receive by assignment or explicit conversion.

func main() {
	var (
		_ = make(chan int)   // bidirectional
		_ = make(<-chan int) // receive-only
		_ = make(chan<- int) // send-only
	)

	ch := make(chan int)

	// send-only
	go func(ch chan<- int) {
		for i := 0; i < 3; i++ {
			ch <- i
		}
		close(ch)
	}(ch)

	// receive-only
	go func(ch <-chan int) {
		for v := range ch {
			fmt.Println(v)
		}
	}(ch)

	time.Sleep(time.Millisecond)
	// Output:
	// 0
	// 1
	// 2
}

func main() {
	ch1 := make(chan int)
	ch2 := make(chan int, 2) // buffering channel
	quit := make(chan int)

	go func() {
		for i := 1; ; i++ {
			ch1 <- 2 * i
			time.Sleep(time.Duration(rand.Intn(1e3)) * time.Millisecond)
		}
	}()

	go func(ch chan<- int) {
		for i := 1; ; i++ {
			ch <- 2*i + 1
			time.Sleep(time.Duration(rand.Intn(1e3)) * time.Millisecond)
		}
	}(ch2)

	go func() {
		<-time.After(time.Duration(5e3) * time.Millisecond)
		quit <- 0
	}()

	//  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.
	ch3 := make(chan int)
	timeout := time.After(500 * time.Millisecond)

	go func() {
		defer close(ch3)
		for {
			// multiplexing: ch1 + ch2 => ch3
			select {
			case ch3 <- <-ch1:
			case ch3 <- <-ch2:
			case <-timeout:
				fmt.Println("You're too slow.")
				return
			case <-quit:
				fmt.Println("Quit.")
				return
			}
		}
	}()

	for v := range ch3 {
		fmt.Println(v)
	}
}

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

Constants cannot be declared using the := syntax.

Constants are created at compile time, even when defined as locals in functions, and can only be numbers, characters (runes), strings or booleans.

Because of the compile-time restriction, the expressions that define them must be constant expressions, evaluatable by the compiler.

In Go, enumerated constants are created using the iota enumerator.

type Weekday int

const (
    Sunday Weekday = iota + 1 // iota: 0 ~ Sunday    : 1
    _                         // iota: 1 ~ iota increased
    // comments               // iota: 1 ~ skip: comment
                              // iota: 1 ~ skip: empty line
    Monday                    // iota: 2 ~ Monday    : 3
    Tuesday                   // iota: 3 ~ Monday    : 4
    Wednesday                 // iota: 4 ~ Monday    : 5
    Thursday                  // iota: 5 ~ Monday    : 6
    Friday                    // iota: 6 ~ Monday    : 7
    Saturday                  // iota: 7 ~ Monday    : 8
)

iota (noun)
/aɪˈəʊtə/
/aɪˈəʊtə/

1. [singular] (usually used in negative sentences) an extremely small amount
    There is not one iota of truth (= no truth at all) in the story.
    I don't think that would help one iota.
2. the 9th letter of the Greek alphabet (I, ι)

ref: https://www.oxfordlearnersdictionaries.com/us/definition/english/iota

Go has two allocation primitives, the built-in functions new and make.

new is a built-in function that allocates memory, but unlike its namesakes in some other languages it does not initialize the memory, it only zeros it.

The built-in function make(T, args) serves a purpose different from new(T).

var p *[]int = new([]int)       // allocates slice structure; *p == nil; rarely useful
var v  []int = make([]int, 100) // the slice v now refers to a new array of 100 ints

// Unnecessarily complex:
var p *[]int = new([]int)
*p = make([]int, 100, 100)

// Idiomatic:
v := make([]int, 100)

Each source file can define its own niladic init function to set up whatever state is required.

Actually each file can have multiple init functions.

init is called after all the variable declarations in the package have evaluated their initializers, and those are evaluated only after all the imported packages have been initialized.

0 for numeric types,

false for the boolean type,

"" (the empty string) for strings,

nil for the pointers, slices, maps, functions, interfaces, channels,

A race condition is a situation in which the program does not give the correct result for some interleaving of the operations of multiple goroutines.

A data race, that is, a particular kind of race condition, occurs whenever two goroutines access the same variable concurrently and at least one of the accesses is a write.

Synchronization is about more than just the order of execution of multiple goroutines; synchronization also affets memory.

The race detector (just add the -race flag to your go build, go run, or go test command) studies this steam of events, looking for cases in which one goroutine reads or writes a shared variables that was most recently written by a different goroutine without an intervening synchronization operation.

The race detector reports all data races that wre actually executed. However, it can only detect race conditions that occur during a run; it cannot prove that none will ever occur.

func main() {
	var wg sync.WaitGroup

	var x, y int

	wg.Add(1)
	go func() {
		defer wg.Done()
		x = 1
		fmt.Printf("y = %d\n", y)
	}()

	wg.Add(1)
	go func() {
		defer wg.Done()
		y = 1
		fmt.Printf("x = %d\n", x)
	}()

	wg.Wait()
}

$ go run -race race.go

x = 0
==================
WARNING: DATA RACE
Write at 0x00c0000a6020 by goroutine 7:
  main.main.func1()
      /tmp/race.go:16 +0x8a

Previous read at 0x00c0000a6020 by goroutine 8:
  main.main.func2()
      /tmp/race.go:24 +0xaa

Goroutine 7 (running) created at:
  main.main()
      /tmp/race.go:14 +0x119

Goroutine 8 (finished) created at:
  main.main()
      /tmp/race.go:21 +0x166
==================
==================
WARNING: DATA RACE
Read at 0x00c0000a6028 by goroutine 7:
  main.main.func1()
      /tmp/race.go:17 +0xaa

Previous write at 0x00c0000a6028 by goroutine 8:
  main.main.func2()
      /tmp/race.go:23 +0x8a

Goroutine 7 (running) created at:
  main.main()
      /tmp/race.go:14 +0x119

Goroutine 8 (finished) created at:
  main.main()
      /tmp/race.go:21 +0x166
==================
y = 1
Found 2 data race(s)
exit status 66

Within a single goroutine, reads and writes must behave as if they executed in the order specified by the program.

That is, compilers and processors may reorder the reads and writes executed within a single goroutine only when the reordering does not change the behavior within that goroutine as defined by the language specification.

Because of this reordering, the execution order observed by one goroutine may differ from the order perceived by another.

To specify the requirements of reads and writes, we define happens before, a partial order on the execution of memory operations in a Go program.

Within a single goroutine, the happens-before order is the order expressed by the program.

Programs that modify data being simultaneously accessed by multiple goroutines must serialize such access.

Do not communicate by sharing memory; instead, share memory by communicating.

One way to think about this model is to consider a typical single-threaded program running on one CPU.

A goroutine has a simple model: it is a function executing concurrently with other goroutines in the same address space.

Goroutines are multiplexed onto multiple OS threads so if one should block, such as while waiting for I/O, others continue to run.

Like maps, channels are allocated with make, and the resulting value acts as a reference to an underlying data structure.

Receivers always block until there is data to receive.

The sender blocks only until the value has been copied to the buffer;

A buffered channel can be used like a semaphore, for instance to limit throughput.

The assembly line metaphor (pipeline) is useful one for channels and goroutines.

// A concurrent prime sieve

package main

// Send the sequence 2, 3, 4, ... to channel 'ch'.
func Generate(ch chan<- int) {
	for i := 2; ; i++ {
		ch <- i // Send 'i' to channel 'ch'.
	}
}

// Copy the values from channel 'in' to channel 'out',
// removing those divisible by 'prime'.
func Filter(in <-chan int, out chan<- int, prime int) {
	for {
		i := <-in // Receive value from 'in'.
		if i%prime != 0 {
			out <- i // Send 'i' to 'out'.
		}
	}
}

// The prime sieve: Daisy-chain Filter processes.
func main() {
	ch := make(chan int) // Create a new channel.
	go Generate(ch)      // Launch Generate goroutine.
	for i := 0; i < 10; i++ {
		prime := <-ch
		print(prime, "\n")
		ch1 := make(chan int)
		go Filter(ch, ch1, prime)
		ch = ch1
	}
}

Be sure not to confuse the ideas of concurrency—​structuring a program as independently executing components—​and parallelism—​executing calculations in parallel for efficiency on multiple CPUs.

Although the concurrency features of Go can make some problems easy to structure as parallel computations, Go is a concurrent language, not a parallel one, and not all parallelization problems fit Go’s model.

package runtime // import "runtime"

func NumCPU() int
    NumCPU returns the number of logical CPUs usable by the current process.

    The set of available CPUs is checked by querying the operating system at
    process startup. Changes to operating system CPU allocation after process
    startup are not reflected.

func GOMAXPROCS(n int) int
    GOMAXPROCS sets the maximum number of CPUs that can be executing
    simultaneously and returns the previous setting. If n < 1, it does not
    change the current setting. The number of logical CPUs on the local machine
    can be queried with NumCPU. This call will go away when the scheduler
    improves.

There is a built-in function panic that in effect creates a runtime unrecoverable error that will stop the program.

The function takes a single argument of arbitrary type—​often a string—​to be printed as the program dies.

package builtin // import "builtin"

func panic(v interface{})
    The panic built-in function stops normal execution of the current goroutine.
    When a function F calls panic, normal execution of F stops immediately. Any
    functions whose execution was deferred by F are run in the usual way, and
    then F returns to its caller. To the caller G, the invocation of F then
    behaves like a call to panic, terminating G's execution and running any
    deferred functions. This continues until all functions in the executing
    goroutine have stopped, in reverse order. At that point, the program is
    terminated with a non-zero exit code. This termination sequence is called
    panicking and can be controlled by the built-in function recover.

When panic is called, including implicitly for runtime errors such as indexing a slice out of bounds or failing a type assertion,

However, it is possible to use the built-in function recover to regain control of the goroutine and resume normal execution.

A call to recover stops the unwinding and returns the argument passed to panic.