Learning Go : Part 1

This is a short summary of effective_go - 1

Formatting

• use gofmt to format packages

Commentary

• Go uses c like comments // or /* */

• Every Function/Struct/Variable that you have to export, name it so that first letter is capital

Package name

• short, concise, evocative

• the package in src/encoding/base64 is imported as "encoding/base64" but has name base64, not encoding_base64 and not encodingBase64.

Getters & Setters

• 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.

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

Naming

Interface Naming

• 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.

Variable Naming

• Finally, the convention in Go is to use MixedCaps or mixedCaps rather than underscores to write multi word names

Indentation

• if i < f() {
      g()
  }
  
  if i < f()  // wrong!
  {           // wrong!
      g()
  }

Control Structure

• There is no do or while loop, only a slightly generalised for; switch is much better

• If

• In Go a simple if looks like this:

if x > 0 {
    return y
}

• Mandatory braces encourage writing simple if statements on multiple lines. It's good style to do so anyway, especially when the body contains a control statement such as a return or break. Since if and switch accept an initialisation statement, it's common to see one used to set up a local variable.

if err := file.Chmod(0664); err != nil {
    log.Print(err)
    return err
}

Re-declaration and Re-assignment

• The last example in the previous section demonstrates a detail of how the := short declaration form works. The declaration that calls os.Open reads,

f, err := os.Open(name)

• This statement declares two variables, f and err. A few lines later, the call to f.Stat reads,

d, err := f.Stat()

​ which looks as if it declares d and err. Notice, though, that err appears in both statements. This duplication is legal: err is declared by the first statement, but only re-assigned in the second. This means that the call to f.Stat uses the existing err variable declared above, and just gives it a new value.

• In a := declaration a variable v may appear even if it has already been declared, provided:

  • this declaration is in the same scope as the existing declaration of v (if v is already declared in an outer scope, the declaration will create a new variable §),

  • the corresponding value in the initialisation is assignable to v, and

  • there is at least one other variable that is created by the declaration

Range

• If you're looping over an array, slice, string, or map, or reading from a channel, a range clause can manage the loop.

for key, value := range oldMap {
    newMap[key] = value
}

• If you only need the first item in the range (the key or index), drop the second:

for key := range m {
    if key.expired() {
        delete(m, key)
    }
}

• If you only need the second item in the range (the value), use the blank identifier, an underscore, to discard the first:

sum := 0
for _, value := range array {
    sum += value
}

Switch in Golang

• Go's switch is more general than C's.

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

func unhex(c byte) byte {
    switch {
    case '0' <= c && c <= '9':
        return c - '0'
    case 'a' <= c && c <= 'f':
        return c - 'a' + 10
    case 'A' <= c && c <= 'F':
        return c - 'A' + 10
    }
    return 0
}

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

func shouldEscape(c byte) bool {
    switch c {
    case ' ', '?', '&', '=', '#', '+', '%':
        return true
    }
    return false
}

Type Switch

• A switch can also be used to discover the dynamic type of an interface variable

var t interface{}
t = functionOfSomeType()
switch t := t.(type) {
default:
    fmt.Printf("unexpected type %T\n", t)     // %T prints whatever type t has
case bool:
    fmt.Printf("boolean %t\n", t)             // t has type bool
case int:
    fmt.Printf("integer %d\n", t)             // t has type int
case *bool:
    fmt.Printf("pointer to boolean %t\n", *t) // t has type *bool
case *int:
    fmt.Printf("pointer to integer %d\n", *t) // t has type *int
}

Functions

Multiple return values

func squareRoot (num int) (n float, err error)
// or
func squareRoot (num int) (float, error)
// to return error if num < 0

// named return

Defer

• Go's defer statement schedules a function call (the deferred function) to be run immediately before the function exits.

func fileContent (filename string) (string, error) {
    f, err := os.Open(filename)
    if err != nil {
        return "", err
    }
    // defer will run the function just before exiting the function fileContents
    defer f.Close()
    // something with return   
}

• Deferring a call to a function such as Close has two advantages.

  • First, it guarantees that you will never forget to close the file, a mistake that's easy to make if you later edit the function to add a new return path.

  • Second, it means that the close sits near the open, which is much clearer than placing it at the end of the function.

• For Example :

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

• Deferred functions are executed in LIFO order, so this code will cause 4 3 2 1 0 to be printed when the function returns

• A more plausible example is a simple way to trace function execution through the program. We could write a couple of simple tracing routines like this:

func trace(s string)   { fmt.Println("entering:", s) }
func untrace(s string) { fmt.Println("leaving:", s) }

// Use them like this:
func a() {
    trace("a")
    defer untrace("a")
    // do something....
}