Go

[[Go/Code Review]]

Learning resources:

Setup

$GOROOT - where your SDK is
$GOPATH - where your go src, pkg, and bin live

Modules

got mod init gitlab.com/viktomas/{name}
go mod init creates a new module, initializing the go.mod file that describes it.
go build , go test , and other package-building commands add new dependencies to go.mod as needed.
go list -m all prints the current module’s dependencies.
go get changes the required version of a dependency (or adds a new dependency).
go mod tidy removes unused dependencies.
TODO read about minimal version selection https://stackoverflow.com/a/70006832/606571

Build

go build -C dir -o output-file this will build the package in dir and will produce output-file if the package is main package. It will follow all the imports and will build those packages as well

Variables

Naming conventions
- camel case - seasonNumber
- acronyms all uppercase - theURL
- first letter case decides public/private
- longer names for longer lifespan
Declaration
- var foo int - declare in different scope than where we assign, or just want to create zero value
- var foo int = 42 - when compiler guesses wrong
- foo := 42 - all the time
Type conversions
- ```
var i int = 42
var j float32
j = float32(i)
```
- for converting numbers to strings, you have to use strconv package, otherwise go will use int number as the unicode number (e.g. converting 42 to *)
- go doesn’t do automatic type conversion because it might loose information
- It’s different from casting, casting is telling the runtime to trust us that there is a different type, conversion transforms one value in memory to a different representation
- You can do explicit type conversion between compatible structs

Primitive types

Boolean
- zero value is false
Numeric types
- zero value is always 0 or equvialent
- Integer int
  - we don’t know the size, it will be at least 32, but it might be 64 bits
  - also can be unsigned int with uint
- Float
  - f := 3.14 - always initialises to float64
String
- collection of UTF-8 characters
- Immutable
- can be converted back and forth with []byte array

Constants

const keyword
naming is the same as variables
they need to be set up at compile time, you can assign the result of a function call to constant
- const gets replaced with literal everywhere
iota
- scoped to a constant block
- every time we use it it’s like using i++ starting with 0
- ```
const (
	a = iota // will be 0
    b // 1
    c // 2
)
```
- be careful, zero value is 0 so uninitialised variable is going to be equal to the first constant
  - solution _ = iota or errorState = iota
- iota should be used only for constants where the numerical representation doesn’t matter and is not stored
  - Don’t use iota for defining constants where its values are explicitly defined (elsewhere). For example, when implementing parts of a specification and the specification says which values are assigned to which constants, you should explicitly write the constant values. Use iota for “internal” purposes only. That is, where the constants are referred to by name rather than by value. That way you can optimally enjoy iota by inserting new constants at any moment in time / location in the list without the risk of breaking everything.
  - Danny van Heumen

Arrays

a := [3]int{97, 85, 93}
- shorthand a := [...]int{97, 85, 93}
len(a) to find out the length
arrays are values
- copied during re-assignment
- passed as values to functions

Slices

a := []int{97, 85, 93}
- same initialisation as an array, but without specifying the length

Slicing operations

a := [...]int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
b := a[:] // slice of all elements
c := a[3:] // slice from 4th element (index 3 inclusive)
d := a[:6] // slice first 6 elements (index 6 exclusive)
e := a[3:6] // slice the 4th, 5th, and 6th elements

remove first element a[1:]
remove last element a[:len(a)-1]

make([]int, len, cap) - initialise slice with len length and cap capacity
append(arr, el, el1, el2) appends element to the slice, it makes the underlying array larger if needs be
- append can use “spread” operator to append another array: a = append(a, []int{1,2,3,4}...)
all stack operations are manipulating one underlying array

Maps

m := map[string]int{"a": 1, "b": 2}
key type has to be testable for equality, struct, map and function can’t be a key
val := myMap["key"]
val, ok := myMap["key"] - we can check if the key exists, ok is convention
you can use len()
delete(myMap, "key")

Structs

type Doctor struct {
  number int
  actorName string
  companions []string
}
d := Doctor{
  number: 3,
  actorName: "John Doe",
  companions: []string{"a", "b"}
}
//or positional syntax (not recommended)
d = Doctor{3, "John Doe", []string{"a", "b"}}

Anonymous struct: a := struct{name: string}{name: "Tomas"}
Structs are passed by value
Embedding type Bird struct {Animal, canFly: bool}
- I need to be aware of the embedding when creating an instance: bird := Bird{Animal: Animal{}, canFly: true}
- Embedding is a good idea when we want consumers of our libraries to embed a complicated structure rather than extend an interface.

Embedding

You can embed a struct in another struct, then you can access the child fields and methods through the parent:

type user struct {
  name string
  email string
}

type admin struct {
  user
  accesLevel string
}

If statement

Initializer:

if val, ok := myMap["key"]; ok {
  fmt.Println(val)
}

The variables are scoped to the if block

Short circuiting - when evaluating || go stops evaluating after the first true, && stops evaluating after the first false

Switch statement

switch <tag> {
case 1, 5, 10:
  fmt.Println("one, five, or ten")
default: 
  fmt.Println("not one")
}

You can use initialiser as well
No braces, any statements between the case and the next case are part of the block
Tagless syntax:
- each case can have a full comparison case i < 10:
Implicit break between cases, you can force immediate execution of the next case (without checking the condition) by using the fallthrough keyword
Type switch: switch i.(type) {}
Break early with break

For statement

for i:=0;i<10;i++
for i, j := 0, 0; i<5; i, j =i+1, j+1
TODO i++ is a statement, not an expression - investigate more about this design choice
we can omit the initialiser for ; i<10;i++ or even the continuation statement for ;i<10; (which is effectively a while loop)
- syntax sugar for while loop: for i<10 (omitting the semicolons)
for { break } for when we don’t know when to finish
continue - skip this iteration and start over with the next loop iteration

we can label which statement we want to break out of

Label:
for i := 0;i<5;i++ {
  for j := 0;j<5;j++ {
    if i*j > 10 {
      break Label
    }
    fmt.Println(i*j)
  }
}

range : for k,v := range s {} to loop over arrays, slices, and maps

Defer

defer moves statement after the function is done, but before it returns any results to the calling function
```
a := "start"
defer fmt.Println(a)
a = "end"
```
- Will print “start” because defer evaluates the argument the moment it is called
panic happens after defer statements have been executed
we can pass anonymous function to the defer statement

Panic/Recover

panic(error)

err := recover()

defer func(){
  err := recover()
  handleError()
}()

Pointers

var a *int - declare pointer
& - “address of” operator - a := &b
* - “dereferencing” operator a := *b
- lower precedence than the . operator => (*a).val
pointer arithmetic available in the unsafe package
new(myStruct) - we get back a pointer to an empty struct
compiler automatically dereferences before using the. operator
- (*a).val == a.val
When function returns pointer to a local variable, the Go runtime has to copy the value on the heap

Functions

You can pass arguments by reference or by value
maps and slices are always passed by reference
func sum(values ...int) int {} Variadic parameter
- values is a slice
You can return local variable as a pointer
func sum(values ...int) (result int){}
- named return variable result
func divide(a float64, b float64) (float64, error)
- we can return error object as the second parameter
Anonymous functions
- best practice is to pass in the variable from outer scope as arguments
  - ```
  func(a){
    fmt.Println(a)
  }()
```
- var divide func(float64, float64)(float64, error)

Methods

functions that are executed on a know context (structs, primitive types)
func (g greeter) greet() {}
- g greeter is a value receiver
  - Use value receiver if all methods can use value receivers - that means they don’t need to mutate data and don’t work with synchronisation primitives
- g *greeter is a pointer receiver
  - Use pointer receiver if even a single method needs to use pointer receiver
- Mixing the two is bad, because the type dictates whether it’s a value or a reference (user is a reference, time is a value) then you should respect the type and use its semantics even if you would otherwise use the other option

Interface

Interface should represent behaviour, ~~Animal~~ -> Barker
Interface value is internally represented as a pair of pointers:
- {:height 306, :width 291}
- Go interfaces, the tricky parts from Tim Ruffles explains it really well

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

Naming convention is that if there’s only one method, we name the interface with the name of the method + er e.g. Write => Writer
When I want to add methods to a type, I have to have a control over it in my package. type Counter int

Composing same as struct embedding

type WriterCloser interface {
  Writer
  Closer
}

t, ok := val.(TypeToConvertTo) - type conversion for interfaces
interface{} - empty interface
Receiver trickery:
- if the interface is stored as pointer, it can use both value and pointer receivers
- if the interface is stored as a value, it can only use value receivers

Printing

%v - default printer for the value
%+v - prints keys as well
%#v - prints the full go syntax (invokes the go.Stringer interface if the value implements it)
%[1]v - refers to the first argument (1 indexed)

Errors

Internally, error is implemented with errorString in the errors package
fmt.Errorf("Error happened %w") - %w format verb wraps the error so it can be later unwrapped:

When you handle multiple errors with the same treatment (e.g. prepending the same message) you can use defer with named return value:

Before: collapsed:: true

func DoSomeThings(val1 int, val2 string) (string, error) {
    val3, err := doThing1(val1)
    if err != nil {
        return "", fmt.Errorf("in DoSomeThings: %w", err)
    }
    val4, err := doThing2(val2)
    if err != nil {
        return "", fmt.Errorf("in DoSomeThings: %w", err)
    }
    result, err := doThing3(val3, val4)
    if err != nil {
        return "", fmt.Errorf("in DoSomeThings: %w", err)
    }
    return result, nil
}

After: collapsed:: true

func DoSomeThings(val1 int, val2 string) (_ string, err error) {
    defer func() {
        if err != nil {
            err = fmt.Errorf("in DoSomeThings: %w", err)
        }
    }()
    val3, err := doThing1(val1)
    if err != nil {
        return "", err
    }
    val4, err := doThing2(val2)
    if err != nil {
        return "", err
    }
    return doThing3(val3, val4)
}

Error values
- You can define a “sentinel” error as var ErrBadRequest := errors.New("Bad Request")
- This is useful if there is no need for additional context

Using type to provide more context

course material

Type assertion switch:

if err != nil {
  switch e := err.(type) {
  case *UnmarshalTypeError:
    fmt.Printf("UnmarshalTypeError: Value[%s] Type[%v]\n", e.Value, e.Type)
  case *InvalidUnmarshalError:
    fmt.Printf("InvalidUnmarshalError: Type[%v]\n", e.Type)
  default:
    fmt.Println(err)
}

Performance

Garbage collection
Performance drains
- Performance is most affected by these categories (in order)
  - Latency - IO Latency - network, disc, memory access
  - Memory allocations (see garbage collection above)
  - data access
  - algorithm efficiencies

Profiling

add this snippet to your main

    f, err := os.Create("cpuprofile")
	if err != nil {
		fmt.Fprintf(os.Stderr, "could not create CPU profile: %v\n", err)
		os.Exit(1)
	}
	if err := pprof.StartCPUProfile(f); err != nil {
		fmt.Fprintf(os.Stderr, "could not start CPU profile: %v\n", err)
		os.Exit(1)
	}
	defer pprof.StopCPUProfile()

Then inspect the result with
- ```
go tool pprof -http=:7777 cpuprofile
```

Design values

Write less code, for every 20 lines of code you’ll introduce one bug

Go

Setup

Modules

Build

Variables

Naming conventions

Declaration

Type conversions

Primitive types

Boolean

Numeric types

Integer int

Float

String