go_primer.slide

Go language primer
Pavel Tišnovský <golang@centrum.cz>
12 Jun 2024
Tags: golang, go

Red Hat, Inc.

* Sources
- [[https://github.com/RedHatOfficial/GoCourse]]
.image ./common/qr_address.png

* Gophers
#The Go gopher was designed by Renee French. (http://reneefrench.blogspot.com/)
#Source https://golang.org/doc/gopher/fiveyears.jpg
#The design and this image is licensed under the Creative Commons 3.0 Attributions license.
.image ./common/fiveyears.jpg _ 900

* Gopher

.image ./common/gopher.jpg _ 600

* Go keywords

   break         case       chan        const           continue
   default       defer      else        fallthrough     for
   func          go         goto        if              import
   interface     map        package     range           return
   select        struct     switch      type            var

* Packages and Imports
- Program building blocks ("libraries")
- Imported by other programs
- Package name is the last element of the import path
- Standard packages: [[https://golang.org/pkg/]]

.code lesson1/packages.go

* Hello world in Go
.play lesson1/hello3.go
.play lesson1/hello.go
.play lesson1/hello2.go

* Syntax
- Evolved from C
- Declarations in postfix
- Exported symbols begin with Capital letter
- [[https://blog.golang.org/gos-declaration-syntax]]

.code lesson1/syntax_c.go
.code lesson1/syntax_go.go

* Functions
.play lesson1/functions.go

* Multiple return values
.play lesson1/functions2.go

* Named return values
.play lesson1/functions3.go

* Variables
.play lesson1/vars.go

* Types
- Similar to C
- No auto casts
- Type inference && default types
- Each type has clear zero value (0, false, "")
.code lesson1/types.go

* Type conversions
- No auto casts
.play lesson1/casts.go

* Constants
.play lesson1/const.go

* Conditions
- Similarly to for loop allows init statement
- Variables declared there don't exist outside of the if block
- No parenthesis around the condition expression
- Blocks required! (re Apple Goto fail)
.play lesson1/if.go

* Else statement
- Variables declared in the init statement do exist in the else block
- Else on the same line as closing bracket
.play lesson1/if2.go

* Switch statement
- Again allows init statement
- Like set of if-else statements
- Evaluated top-bottom, first successful case is executed
- Thus there's no break statement
.play lesson1/switch.go

* Switch statement 2
- Allows non-constant and non-integer values
.play lesson1/switch2.go

* Switch with no condition
- switch without condition is like switch true
.play lesson1/switch3.go

* Loops
- Just for loop which also serves as while loop and endless loop
.play lesson1/for.go

* Error handling
- error is built-in type
[[https://blog.golang.org/error-handling-and-go]]
.play lesson1/ret.go

* Ignoring return values
- '_' idiom can be used to drop return values
.play lesson1/ret2.go

* defer statement
- `defer` is a keyword in the Go programming language
- used to "remember" commands that will be called before `return` or exit
- based on LIFO (stack) of remembered commands
- parameters are evaluated when `defer` is declared (ie. in runtime)
- (not when the specified code is called)
- it is possible to change function return value(s) via `defer`

* Basic usage of defer statement
- function `on_finish()` is called before exit from `main()`
.play lesson2/01_defer_basic_usage.go

* Structs (records)
- a.k.a. records
- user-defined data type (so called named structure)
- or anonymous structure
- dot operator to access struct members
- initialization using {}
- or by using named members (which is more explicit and better)
- structs are comparable
- pass to functions as value or via pointer (by reference)

* Structs and dot operator
.play lesson2/11_struct.go

* Comparison of whole structs is possible!
- `==` and `!=` operators
.play lesson2/14_struct_comparison.go /^func main/,/^}/

* Comparison of whole structs is possible! (cont.)
.play lesson2/14_struct_comparison.go /^func main/,/^}/

* Arrays
- basic data type in the Go programming language
- all array items has the same type
- (well, you can use `interface{}` to allow _dynamic_typing_behaviour_)
- type of array is derived from type of items *and* array size
- (unlike slices)
- index in range 0..length-1
- items indexing via [] (as in most other languages)

* Array copy
- unlike slices, arrays can be copied
.play lesson2/17_array_copy.go

* Slices
- proper data type in the Go programming language
- interface to sequences (better than arrays)
- slices are used more often than _plain_ _old_ _arrays_
- type of slice is derived from type of items
- slices has defined length and capacity (those numbers can be different)
- internally a slice is triple: (pointer to the first element + `len` + `cap`)
- so called "slice operator" `[from:to]`
- `append` function to add a new element to slice (complicated internally)



* Usage of slices

.play lesson2/21_slice_append.go



* Slices and arrays as data source for them

- slice can be created from any array
- but the slice does not contain any data, just a pointer to array element
- so any modify in slice element is reflected in an array as well

.play lesson2/19_slice_copy.go /package main/,/cont//

* Pointers
- always points to an element of some type
- ie. `void` pointers are not supported
- implicit pointer value is `nil`
- address of element can be retrieved using the `&` operator
- access via pointer (reference in some other languages) using the `*` operator



* Basic usage of pointers
- please note the usage of `*p_i++`

.play lesson2/22_pointer_to_int.go

* Maps
- a.k.a. associative array or hash
- container for key-value pairs
- "nil map":

    var m1 map[string]int

- empty map:

    var m2 map[string]int = make(map[string]int)
    m3 := make(map[string]int)



* Maps
- four basic operations: add/put, get, delete, and contains

- add/put items to a map:

    m3["zero"] = 0
    m3["one"] = 1
    m3["two"] = 2

- get item from a map:

    result := m3["two"]

- delete from a map

    delete(m3, "zero")

- contains

    v, exists := m3["two"]



* Loops and the range clause
- `range` keyword used in `for` to loop over array items, map pairs etc.
- provides an item index as well (Java on the other side...)



* Iterating over integer values
.play lesson2/int_range.go

* Iterating over an array
.play lesson2/33_for_range_1.go



* Iterating over slice
.play lesson2/35_for_range_3.go



* Iterating over Unicode glyphs
.play lesson2/36_for_range_4.go



* Iterating over map pairs (key+value)
.play lesson2/37_for_range_map.go


* User-defined data types
- `type` keyword
- Go is very strict in typing (see examples)



* User-defined data types
.play lesson2/38_user_types.go

* Methods on types
- a _method_ is a function that has a defined receiver
- (in OOP: a method is a function on an instance of an object)
- concept of a _receiver_
- usage - method declaration

    func (receiver) method_name(parameters) return_types {
        ...
        ...
        ...
    }

- method is called by

    some_object.method_name(parameters)

- methods can modify (mutate) a receiver
- call by value vs. call by reference (via pointer)



* User type with one method
.play lesson3/09_methods.go

* Interfaces
- set of methods required to implement such interface
- declared via `interface` keyword:

    type XI interface {
        method1()
        method2(int) int
    }

- interface type — variable of interface type, can hold any value implementing an interface
- variable of interface type *I* can hold any value _implementing_ *I*



* Implementing an interface
- a.k.a. _satisfying_ an interface
- no such keyword (`implements` or anything similar)
- in Go: - every type which implements all interface’s method automatically satisfies such interface



* How to satisfy an interface
- artificial example
.play lesson3/13_B_satisfy.go


* Communication via message queues
- Analogy of phone calls
- Phone ringing: blocking call
- Answerphone: non-blocking call (tape length = queue size)
- Multiple receivers: Call centre
- Multiple senders: Conf call

* Threads and goroutines
- Reminder: Threads share memory
- Go routines are light-weighted threads, cheap to spawn
- It should be fine to spawn thousands of goroutines
- Internally multiplexed across thread pool
- Spawned with simple go keyword

* Goroutines
.play lesson4/goroutines.go

* Channels
- Way to transfer data between goroutines
- Type of data is part of the channel type
- By default blocks on both tx/rx (but can be buffered too)
- Same operator to send and receive from channel
- Created with make()
.code lesson4/chan.go

* Channels
- Note that this is racy.. more about that later
.play lesson4/chan2.go

* Channels
- Unbuffered channels do block (and golang kindly detects deadlock)
.play lesson4/chan3.go

* HTTP and friends

.play lesson6/web_server.go

* JSON parsing: unknown format

.play lesson6/web_server_json.go

* JSON parsing: known format

.play lesson6/web_server_json_2.go