🐹 Go

Notes from the book Learning Go, by Jon Bodner.

Setup, Environment

Difference between go get and go install:

See here. Basically go get downloads the source to $GOPATH/src along with the dependencies, and the latter compiles.

go install is recommended. Ignore go get.

Hey

hey for load testing http services. Install with:

go install github.com/rakyll/hey@latest

Vi

Install vim-go: it comes with a massive set of tools.

goimports

goimports for some neat stuff that gofmt doesn't do. Install with:

go install golang.org/x/tools/cmd/goimports@latest

And run with:

goimports -l -w .

Vi: Just save it.

Linting with golint

Install: go install golang.org/x/lint/golint@latest

Run: golint ./...

Vi: :GoLint

SA with govet

Run: go vet ./...

Vi: :GoVet

This does not catch subtle bugs around shadow variables. So consider installing shadow as well:

Install: go install golang.org/x/tools/go/analysis/passes/shadow/cmd/shadow@latest

Run: shadow ./...

Combine golint, govet with golangci-lint

This tool runs 10 different linters by default and support dozens others.

Install: see official docs

Run: golangci-lint run

vim-go notes

• Code completion is with Ctrl-x Ctrl-o
• :Tagbar is bound to F8
• :GoDef is bound to gd
• :GoDefStack shows you how deep you've jumped, GoDefPop or Ctrl-T just pops to the last hop

Sample makefile

.DEFAULT_GOAL := build

fmt:
        go fmt ./...
.PHONY:fmt

lint: fmt
        golint ./...
.PHONY:lint

vet: fmt
        go vet ./...
        shadow ./...
.PHONY:vet

build: vet
        go build hello.go
.PHONY:build

Basic Types

Type Conversions

Very strict here unlike JS / python. One can't just treat non-empty strings like "asd" as true for example. No other type can be converted to a bool, even explicitly. The way to do that is to use comparison operators (==, !=, >=, <=).

e.g. x == 0 will return true or false.

var versus :=

Most verbose way:

var x int = 10

Since the default type of an integer is int, we can omit that:

var x = 10

And since the default value is 0, we can also do this:

var x int

Multiple variables can be declared like this:

var x, y int = 10, 20

Even if types differ:

var x, y = 10, "hello"

For even more in one go, do this:

var (
    x int
    y = 20
    z int = 30
    d, e = 50, "hello"
    f, g string
)

Inside a function, you can use := when you want go to infer the type.

var x = 10

becomes

x := 10

Or

x, y := 10, "Hello"

And so on.. The main difference here is that you can reassign to existing variables too.

Recommendations / Idioms:

• Don't declare variables at the package level, especially if they change later. Keep only immutable variables there, and the rest inside functions.
• Don't use := with type conversions, just use var there.
• If initialization to 0 is expected, use var x int instead of x := 0.

const

Only works at compile time, so can only be assigned to:

• Numeric literals
• true and false
• Strings
• Runes (A rune is a single unicode character I think)
• Other built-ins: complex, real, imag, len, cap
• Expressions that consist of operators and the preceding values

So there's no way of specifying that a value calculated at runtime is immutable. Nor are there immutable arrays, slices, maps or structs.

const x = 10

let's you later do this:

var y int = x
var z float64 = x
var d byte = x

As you'd expect, doing something like const x int64 = 0 can only let you assign that const to another int64.

Unused variables

Not applicable to const's since these are detected and stubbed out from the final binary. For normal vars though, its a compilation error to declare a var and not use it.

Composite Types

Arrays

Rigid, don't use directly.

var x [3]int // Default 0's

var x = [3]int{10, 20, 30}

Sparse arrays: 0's are filled for the positions not specified:

var x = [12]int{1,5:4,6,10:100,15} // this creates {1,0,0,0,0,4,6,0,0,0,100,15}

var x = [...]int{10,20,30} // tell go to fill in the size

var x [2][3]int // multi-dimensional. Poor matrix support, avoid.

Find the size with len(arr).

Limitations:

• The size is part of the type, so an [3]int is not the same type as a [4]int. So you CANNOT use a variable to specify the size of an array, because types must be resolved at compile time. Big limitation of arrays.
• One CANNOT use a type conversion to convert arrays of different sizes to identical types. Because of this, you can't write a function that works with arrays of unspecified sizes, nor can you assign arrays of different sizes to the same variable.

Really just don't use this. Arrays are backing stores for slices and this is what you'd want to use.

Slices

Length is /not/ part of the type. You don't specify the size when declaring:

var x = []int{10, 20, 30} //Note that in an array you'd do [...] instead

Similarly:

var x = []int{1, 5:4, 6, 10:100, 15}

var x [][]int

Empty slice:

var x []int // Since no value is assigned, x is assigned nil

Use append to grow a slice:

var x []int
x = append(x, 10)
x = append(x, 11, 12)

To flatten one slice and append its values to another, use ...:

y := []int{20, 30, 40}
x = append(x, y...)

capacity

The Go runtime will increase the size of a slice when the number of elements exceeds the default allocation, e.g. by doubling it or increasing by 25%, based on the number of elements that are already there.

The built-in function cap returns the current capacity of the slice, and make is used to create a new slice.

make

Like malloc. Will create a slice of fixed capacity and length, and initialize to 0.

x := make([]int, 5)

To specify initial capacity as well:

x := make([]int, 6, 10)

x := make([]int, 0, 10) // 0 length but 10 capacity

Runtime panic if you initialize a slice with a variable for the capacity and it turns out to be lesser than length. Or compile time if you do it with literals.

idiomatic way of declaring slices

• var data []int // if you expect it to stay nil. Has 0 length
• var x = []int{} // empty slice literal i.e. non-nil
• data := []int{2, 4, 6, 8} // if you feel the values aren't going to change
• Use make when you have a good idea of the size.
• When unsure, use a zero length slice with a specified capacity so that append works easily.

slicing

Similar to python. But does NOT copy the data. You get two variables that share the same data. This gets really messy when you append to a sliced slice :( The sub-slice shares the capacity of the main slice, so unused capacity in the original slice is used when you do the append.

copy

Safe way of creating an independent slice.

x := []int{1,2,3,4}
y := make([]int, 4}
num := copy(y, x) // Returns number of elements copied. Y is the target slice.
fmt.Println(y, num) // Prints [1 2 3 4] 4

Strings, runes, bytes

UTF-8, unless specified.

len() gives you the raw byte length not the grapheme length.

byte is the underlying structure, rune is the UTF code point, and string is what the user sees.

Maps

Make a map where key is a string, value is an int:

    m0 := map[string]int{}
    m0["asd"] = 1

Set values up front:

    m1 := map[string]int{
        "hello": 1,
        "world": 2,
    }

for the above, extract and test like this:

    v, ok := m1["hello"]
    fmt.Println(v, ok)
    // Returns, 1 true
    v, ok := m1["does-not-exist"]
    fmt.Println(v, ok)
    // Returns, 0 false

i.e. Value stored in the map is 1 and there is indeed a value present for that map (stored in the ok variable)

delete(m1, "hello") deletes that key/value.

Sets

Don't exist natively but you can make a map with bool values to get a similar data structure.

    myset := map[int]bool{}
    vals := []int{1, 3, 5, 7, 1, 10, 11, 3}

    for _, v := range vals {
        myset[v] = true
    }

Use third-party libraries for Union, Intersection, etc.

Structs

    type person struct {
        name  string
        empid int
    }
    var e1 person
    e1.name = "arun"
    e1.empid = 12345
    // or
    e2 := person{"sid", 5678}

Anonymous struct

    pet := struct {
        name string
        age  int
    }{name: "shadow", age: 10}

Blocks

Anything outside a function is in the package block.

Anything you call with an import from another file is in the file block.

And things within functions are in their blocks.

There is also a universe block that contains all the built-in types and functions like true and int and make.

    x := 10
    if x > 5 {
        fmt.Println(x)  // 10, from outer scope
        x := 5          // new shadow variable
        fmt.Println(x)  // 5
    }
    fmt.Println(x) // You'd want it to be 5, but it's 10

The use of := makes it easy to miss this, which reuses variables only in the current block.

Consider installing shadow which catches this kind of thing:

code/learninggo/ch04 via  v1.16.3 ❯ shadow blocks.go
/home/arunsrin/code/learninggo/ch04/blocks.go:9:3: declaration of "x" shadows declaration at line 6
code/learninggo/ch04 via  v1.16.3 ❯

if

Fairly obvious. Also, let's you define a variable inside the if condition that you can then use in the rest of the block.

for

Simple, C-style:

    for i := 0; i < 5; i++ {
        fmt.Println(i)
    }

• Must use := to initiatlize, var will not work.
• No parens

Condition-only style:

    for i > 0 {
        fmt.Println(i)
        i--
    }

Infinite style:

    for {
        fmt.Println("loop forever")
    }

• Use with break and continue

for-range style:

    x := []int{100, 200, 300}
    for i, v := range x {
        fmt.Println(i, v)
    }

• i gives the iteration, and v the value
• When looping through a map, i gives the key instead
• Use _ if you don't plan to use that variable in the loop
• Iteration over maps is random, no fixed order is guaranteed
• If you loop over a string, each element will be a rune, not a byte
  • The i value will jump by that many bytes to indicate that a non-ascii UTF symbol was detected
• for copies i and v and gives it to you. So modifying it will not modify the upstream value you're iterating through.

switch

Example:

    words := []string{"a", "cow", "gopher", "smile", "octopus",
        "anthropologist"}
    for _, word := range words {
        switch size := len(word); size {
        case 1, 2, 3, 4:
            fmt.Println(word, " is a short word")
        case 5:
            wordLen := len(word)
            fmt.Println(word, " is the right length: ", wordLen)
        case 6, 7, 8, 9:
        default:
            fmt.Println(word, " is a long word")
        }
    }

• No break needed
• Each switch case is a scope, introduce new variables there and they will be accessible only in that case
• In an empty case, nothing happens. There is no fall-through to the next case
  • Rather, use commas to combine multiple cases that have the same logic
• break in a switch case will only break out of that case. Use labels to actually break out of the outer loop

Blank switch

In the previous example, size compared with each case statement, and, if equal, that case is executed. In a blank switch, you could run any condition in the case, not just equality

Functions

• No named or optional functional parameters.

Variadic functions

Exception is when it's at the end:

func blah(base int, rest ...int)

• Use it in the function as a normal slice
• It can be skipped or called with any number of args

func main() {
    blah(1, 2)
    blah(1)
    lotsOfArgs := []int{2, 3, 4}
    blah(1, lotsOfArgs...) //Note trailing ... for slice
}

func blah(base int, rest ...int) {
    fmt.Println("Received this many rest args", len(rest))
}

Mutliple return values

Example

func div(num int, denom int) (int, error) {...}

Fairly common pattern to return the actual response followed by an error type

Named return values

Example

func div(num int, denom int) (result int, err error) {...}

Advantage is that you are pre-declaring that those variables are present in the function and initialized to default values.

On the calling side, feel free to use any other name.

Blank returns

Example

return

It returns the last value of a named return variable.. not idiomatic and prone to cause confusion.

Passing functions

Here's a simple example:

func main() {
    var opMap = map[string]func(int, int) int{
        "+": add,
        "-": sub,
    }
    opFunc := opMap["+"]
    res := opFunc(5, 10)
    fmt.Println(res)
}

func add(i int, j int) int { return i + j }
func sub(i int, j int) int { return i - j }

Function Type Declarations

type opFuncType func(int, int) int

More to come later but it's essentially like above.

That would make the map in the above example much simpler:

var opMap = map[string]opFuncType {...}

Anonymous functions

    i := 10
    func(j int) {
        fmt.Println("inside anon function", j)
    }(i)

Useful while launching goroutines or with defer

Closures

Functions inside functions. They can use and modify variables from the outer function.

    type Person struct {
        name string
        age  int
    }
    people := []Person{
        {"asha", 7},
        {"sid", 5},
    }

    fmt.Println(people)
    sort.Slice(people, func(i int, j int) bool {
        return people[i].age < people[j].age
    })
    fmt.Println(people)

Here, sort.Slice() gets a function with 2 parameters i and j but it has access to people as well.

You can even return a function from another function:

func main() {
    f1 := makeMult(10)
    fmt.Println(f1(20))
}

func makeMult(base int) func(int) int {
    return func(factor int) int {
        return base * factor
    }
}

defer

Here is a simple cat implementation. It uses defer to close file handles. defer is used to run something at the end of a function, whether it ran successfully or not.

package main

import (
    "io"
    "log"
    "os"
)

func main() {
    // simple `cat` implementation
    if len(os.Args) < 2 {
        log.Fatal("No file specified")
    }
    f, err := os.Open(os.Args[1])
    if err != nil {
        log.Fatal(err)
    }
    defer f.Close()

    data := make([]byte, 2048)
    for {
        count, err := f.Read(data)
        os.Stdout.Write(data[:count])
        if err != nil {
            if err != io.EOF {
                log.Fatal(err)
            }
            break
        }
    }
}

Note that if that function that you defer returns some values, there is no way to actually read and use them.

Another good pattern is to write your own cleanup function and then pass it to defer. E.g. if interacting with a DB, you may want to either commit or rollback everything at the end.

Another pattern is to return a closure function alongside the usual content and error. That way the caller can call it themselves in their defer.

Example:

func getFile(n string) (f os.File*, func(), error) {...}

So that the caller can do this:

    f, cleanup, err := getFile("/etc/passwd")
    // check for err
    defer cleanup()
    // do the rest

call by value

Anything you pass as a parameter is copied. Making modifications to it will NOT stick, if you pass a struct or int or string.

If you pass a map or slice, you can modify the content but not the size.

Pointers

Example:

var y int32 = 10
pointerx := &y  //pointerx's content is now y's address
var pointerz *string  //nil pointer, doesn't point to anything

Differences from other languages:

• No memory management
• No pointer arithmetic

& is an address operator that returns the address of that variable.

* is the indirection operator. Using it returns the value in that address. Called derefencing.

Example:

    var x int = 10
    pointerY := &x
    var z int = x + *pointerY
    fmt.Println(z)

Always do a null check!

    var nilPointer *int
    if nilPointer != nil {
        fmt.Println(*nilPointer) // panic here
    }

new creates a new null pointer, but is not used much:

    var x = new(int)

You cannot point to builtins like ints and strings directly. You will need to assign them to a variable and then make a pointer of that variable.

Pointers in structs are a pain for this reason, you cannot assign a string or int directly to them. A good pattern is to have a helper function that basically takes each type and returns a pointer to that type:

func stringp(s string) *string {
    return &s
}

Use Pointers to indicate mutability in a function. Since go is call by value, a copy is always made of the paramter passed to the function, and any change you make inside are not going to reflect on the outside.

The above note is for primitives, structs, and arrays. More to come on maps and slices.

Anyway, a better pattern is to just return the modified content rather than modify it via pointers.

Commonly used in json parsing though.

Json parsing

    type person struct {
        Name string `json:"name"`
        Age  int    `json:"age"`
    }
    var f person
    err := json.Unmarshal(
        []byte(`{"name":"arunsrin","age":38}`), &f)
    fmt.Println(f, err)

The json library expects an interface{} in the second parameter since it cannot anticipate the type. So the value passed to it has to be a pointer. The lack of generics has led to the above pattern become a norm in this use case.

There is a perf hit if you use pointers for small data. Becomes an advantage only for large structs (~mb).

Maps vs slices

A map is internally implemented as a pointer to a struct. So when passing maps to functions and modifying them inside, you would see a change on the outside as well.

So avoid using maps, especially for public consumption.

Slices are more complicated when passed as parameters to functions:

• Change in values are reflected
• But appending to a slice is not affected

This is because when passed to the function, a copy of the length, capacity and pointer are made. So a change in content is because of the pointer pointing to the same common content. But an append results in an increase in length and capacity of the copy and not the original.

Using slices as buffers

A good pattern is to make a single slice of fixed size and use it in a loop while processing I/O. This is better than allocating memory for each chunk, which leads to the GC having to do a lot of work.

Here is an example:

package main

import (
    "fmt"
    "os"
)

func main() {
    file, _ := os.Open("/etc/passwd")
    defer file.Close()

    data := make([]byte, 100)
    for {
        count, err := file.Read(data)
        if err != nil {
            break
        }
        if count == 0 {
            break
        }
        process(data[:count])
    }
}

func process(d []byte) {
    fmt.Print(string(d))
}

As you can see, data is declared once and reused repeatedly when it is passed to process().

While the length cannot be modified, process() can change the content that was sent to it.

Types, methods, interfaces

An abstract type specifies what a type should do, not how it is done.

A concrete type specifies what and how.

Here's a function attached to a struct:

func main() {

    c := ComplexNumber{3.55, 10}
    fmt.Println(c.toString())

}

type ComplexNumber struct {
    Real      float32
    Imaginary float32
}

func (c ComplexNumber) toString() string {
    return fmt.Sprintf("%0.1f + %0.1f i", c.Real, c.Imaginary)
}

You can attach the same method name to different types. The bit between func and the name toString() is called the receiver spec. You should use a pointer receiver if you intend to mutate it (or handle nil instanes). Otherwise use a simple value receiver.

Also the function has to be in the same package level. You can't take a type from some package and override with a function in yours.

Modified version of the above with 1 pointer receiver and the other normal receiver:

func main() {

    c := ComplexNumber{3.55, 10}
    fmt.Println(c.toString())
    c.increment() //observe that you didn't have to do &c.increment() here. Go does it automatically
    fmt.Println(c.toString())

}

type ComplexNumber struct {
    Real      float32
    Imaginary float32
}

func (c ComplexNumber) toString() string {
    return fmt.Sprintf("%0.1f + %0.1f i", c.Real, c.Imaginary)
}

func (c *ComplexNumber) increment() {
    c.Real++
    c.Imaginary++
}

iota

Equivalent of enums. Seems pretty crippled, ignore.

Initializes to 0 and auto-increments from there on.

    type MailCategory int
    const (
        Uncategorized MailCategory = iota
        Personal
        Spam
        Social
        Advertisements
    )
    x := Spam
    fmt.Println(x)

Inheritance

Not exactly. But you can embed a type in another. So the former's methods can be accessed in the outer type seamlessly.

Interfaces

Example:

    type Stringer interface {
        String() string
    }

Here, an implementation must implement a String() method.

Idiom is to end the name with er e.g. Stringer(), Closer.

A concrete type does not need to declare that it implements a particular interface, it implicitly happens. Like python's duck typing.

!!note Idiom: Accept interfaces, return structs

interfaces and nil

For an interface to be nil, both the type and the value must be nil.

When the type is non-nil, it apparently is not straightforward to tell if the value is nil. Reflection helps here.

empty interfaces

They map to anything in go, and main use case is for open-ended stuff like json parsing.

    var i interface{} //empty interface
    i = 10
    i = "hello"
    i = struct {
      FirstName string
      LastName string
      } {"A", "S"}

So in the json parsing case you'd see something like this:

    data := map[string]interface{}{}

The first {} is for making an empty interface, the second {} is for instantiating a map instance.

Type assertions and type switches

type MyInt int

func main() {
    var i interface{}
    var mine MyInt = 20
    i = mine
    // i2 := i Fails in last line with error `(mismatched types interface {} and int)`
    i2 := i.(MyInt)
    fmt.Println(i2 + 1)
}

Basically here we tell that i2 is of type MyInt.

This is a type assertion not a conversion. Happens only at runtime unlike latter which is compile-type. So use the comma-ok pattern to catch the failure.

func doThings(i interface{}) {
    switch j := i.(type) {
    case nil:
        // i is nil, type of j is interface{}
    case int:
        // j is of type int
    case MyInt:
        // j is of type MyInt
    case io.Reader:
        // j is of type io.Reader
    case bool, rune:
        // i is either bool or rune, so j is of type inteface{}
    default:
        // no idea what i is, so j is of type interface{}
    }
}

People usually shadow the variable i.e. i := i.(type)

In the above we had some estimates of the types. If you don't know the type at all, use Reflection.

WebApp example

Too long to print here. Here it is.

Errors

Always return an error as the last return value. Simple example:

func calcRemainderAndMod(num, denom int) (int, int, error) {
    if denom == 0 {
        return 0, 0, errors.New("denom is 0")
    }
    return num / denom, num % denom, nil
}

Reasons for returning instead of throwing: - Simpler code paths - Force developers to check and handle natively since all variables must be read in go

Alternate way

Rather than errors.New("some message"), one can also do this:

  fmt.ErrorF("%d isn't even", i)

Sentinel Errors

Their names start with Err by convention, and indicate that no further processing is possible.

Example: the zip package has zip.ErrFormat.

Once declared, it is part of the public API of your package. So use carefully, or just pick one that's already there.

Looks messy actually, just don't do it.

Adding more info

Since the error interface is just a string:

type error interface {
  Error() string
}

You can extend it quite easily like so:

struct MyError {
  StatusCode int
  Message string
}

And use it in your code like this:

  return nil, MyError{
    StatusCode: 400,
    Message: fmt.Sprintf("bad request %s", uid)
  }

Wrapping errors

You can chain your information to a downstream error using fmt.Errorf's %w verb.

On the receiving side you can unwrap with errors.Unwrap. But you would'nt usually do this. Instead, use errors.Is and errors.As.

Once you wrap something, you can't compare and check if a certain Sentinel Error occurred. This is where errors.Is helps. It iterates throught the error chain and tells you if a match is found.

  if err != nil {
    if errors.Is(err, os.ErrNotExist) {
      fmt.Println("That file doesn't exist")
    }
  }

You can also implement Is() in your custom Error Type and do your own pattern matching implementation.

errors.As lets you check if a returned error matches a certain type.

err := AFunctionThatReturnsAnError()
var myErr MyErr
if errors.As(err, &myErr) {
  fmt.Println(myErr.Code)
}

Second argument sould either be a pointer to an error, or a pointer to an interface.

Don't use this unless you know what you're doing.

panic and recover

When a panic happens: - current function exits immediately - all its defers run - then all the defers of the calling function run, and so on - finally when main is reached, the program exits with a message and stacktrace

You can call panic in your code, usually with a string.

func main() {
    doPanic("goodbyeee")
}

func doPanic(msg string) {
    panic(msg)
}

You can call a recover inside a defer to halt the panic and continue execution.

In the following example, division by zero does not kill the program, it continues to the next iteration:

func main() {
    for _, val := range []int{1, 2, 0, 6} {
        div60(val)
    }
}

func div60(i int) {
    defer func() {
        if v := recover(); v != nil {
            fmt.Println(v)
        }
    }()
    fmt.Println(60 / i)
}

Modules, Packages, Imports

Core concepts:

• repositories - where in VCS you'd store your code
• modules - root of a Go library or app
• packages - modules consist of one or more of these

Modules should be globally unique, like Java's packages. Convention is to have the repo e.g. github.com/arunsrin/blah

Have a go.mod in the root directory to declare a module. Use the go mod command to manage this file.

learninggo/ch09/my1stmodule ❯ go mod init github.com/arunsrin/example01/v2
go: creating new go.mod: module github.com/arunsrin/example01/v2
learninggo/ch09/my1stmodule via  v1.16.5 ❯ cat go.mod
module github.com/arunsrin/example01/v2

go 1.16
learninggo/ch09/my1stmodule via  v1.16.5 ❯

A require section declares your dependencies. E.g.

require (
    github.com/learning-go-book/formatter v0.0.0-20200921021027-5abc380940ae
    github.com/shopspring/decimal v1.2.0
)

A replace section let's you override a module's location, and an exclude section prevents a specific version from being used.

Packages

Say you have 2 packages, formatter and math in 2 subfolders in your module. Use them with the full path in your main.go like so:

import (
    "fmt"

    "github.com/learning-go-book/package_example/formatter"
    "github.com/learning-go-book/package_example/math"
)

Your packages should declare the package name as the first line, e.g. package formatter. Usually this is same as your directory name.

Use good naming conventions: - Bad: util.ExtractNames, util.FormatNames - Good: extract.Names, format.Names

Good module conventions

• Create a cmd/ directory, with one sub-folder for each binary built from the module.
• All other go code go into packages inside a pkg/ directory.

Overrides

Example: Both crypto/rand and math/rand exist. So the equivalent of python's import blah as blah2 is:

import (
    crand "crypto/rand"
    "encoding/binary"
    "fmt"
    "math/rand"
)

godoc

Place the comment above the thing being documented, with no new lines in between.

Before the package declaration: package-level comments.

internal Packages

When defines as internal, everything exported by that Package is only visible to other sibling packages.

init Function

Avoid. A function called init() with no parameters and return values. Gets run the first time a package is referenced by another.

Check available versions of a module

go list -m -versions github.com/learning-go-book/simpletax

To downgrade to a specific version:

go get github.com/learning-go-book/[email protected]

Major versions

For all versions apart from 0 and 1, the module path must end in vN where N is the major version. E.g.

"github.com/learning-go-book/simpletax/v2"

In the code itself you can do one of these 2: - Create a sub-directory called v2 or whatever and copy your README, LICENSE fiiles. - Create a branch called v2 or keep version 2 in master and create a branch called v1 for the legacy version.

Vendoring

Keep copies of dependencies inside your module.

go mod vendor

Module Proxy Server

go's mirror. First tries there, and downloads and caches if not present.

Google also maintains a sum database that has version information. Protects from malicious version modifications.

Alternates to this default behaviour: - Use a different proxy server like JFrog's GoCenter - To do this,export GOPROXY="https://gocenter.io,direct" - Disable the behaviour: export GOPROXY=direct - Run your own

Private proxy servers

Set GOPRIVATE:

export GOPRIVATE=*.example.com,company.com/repo

Anything matching the above will be downloaded directly.

Standard Library

Useful stuff from the standard library

strconv.Atoi: convert "5" to 5

From math/rand: rand.Intn(10) returns a random number between 0 and 10. Seed is fixed though so look deeper

os.Args like python's sys.argv

Third Party

Useful stuff from the ecosystem. Search in this page for details.

• go install github.com/rakyll/hey@latest
• go install golang.org/x/tools/cmd/goimports@latest
• go install golang.org/x/lint/golint@latest
• go install golang.org/x/tools/go/analysis/passes/shadow/cmd/shadow@latest

The equivalent of pypi here is pkg.go.dev. It automatically indexes open-source go projects.

References

• Learning Go, by Jon Bodner

Queued

• official code review comments

• Effective Go

• Inside the Map implementation, video