gomeetup.slide

# Time to Go

Dimitris Dranidis

CITY College, University of York Europe Campus

1st SKG Gophers Meetup

Friday, May 7, 2021

dranidis@york.citycollege.eu





## Slides and code at

.link https://github.com/dranidis/time-to-go





## Hello world

.play -edit code/hello/hello.go





## Hello Web

Starts a web server at <http://localhost:8080> using the default HTTP router.

.play code/helloweb/helloweb.go





## Go history

- created at Google, 2009
- by Robert Griesemer, Rob Pike, and Ken Thompson
- open source
- First release in 2012, Go 1
- May 2021, Go 1.16





## Go features

- statically typed (with implicit type inference)
- compiled (fast compilation)
- gargage collection
- object-oriented (??)
- functional
- concurrent





## Code organization and tools






## Tools

- `go mod init` - Initialize a module
- `go build` - Build files
- `go install` -- Install binaries in pkg and bin
- `go test ./...` Run all tests and report results
- `go fmt` Format code
- `go doc`-Get help on a command
- `go ...`





## Some language features





## Features

Statically typed with implicit type inference at compile time

    i := 3 // same with var i int = 3

Multiple assignments

    a, b = b, a

Multiple return values

    func foo() (int, int) {
      ...
      return a, b
    }
    x, y := foo()

Package visibility

    func Foo() {...} // visible outside the package
    func foo() {...} // private to the package










## Variadic functions

Functions that accept a variable number of arguments

.play -edit code/variadic/variadic.go





## Deferred execution

You can execute a function call after the termination of the surrounding function.
Arguments are evaluated immediately.

.play -edit code/defer/defer.go

Good for cleanup, e.g. closing files.





## Pointers

If you are familiar with C/C++ no problem!




## Pointers

.play -edit code/pointers/pointers.go /Decl/,/END OMIT/



## Arrays and Slices

## Arrays
Arrays have fixed length. Go's arrays are values. Not pointers like in C. When passed as a parameter to
a function a copy of the array is created.

.play -edit code/arrays/arrays.go



## Slices 

Slices have no fixed size. They are dynamically resized.

A slice is a struct with an array, a length and a capacity.


```
type slice struct {
    elements unsafe.Pointer
    len      int
    cap      int
}
```

A slice can be created with:

```
slice := make([]int, 5)
```

or with a slice literal:

```
slice := []{1,2,3}

## Slices

.play -edit code/slices/slices.go /workOnSlice/,/END/


## Functional aspects





## Functions as values

.play -edit code/eval/eval.go /M OMIT/,/END OMIT/





## Currying

A function returning a function (closure)

.play -edit code/currying/currying.go





## Go is Object-Oriented (?)





## Struct types instead of classes

.play -edit code/square/square.go /Square OMIT/,/END OMIT/








## Interfaces

Interfaces are collections of method signatures.

.code code/interface/interface.go /interface/,/}/

Interfaces are **contracts**. Any user-defined type providing the methods of an interface **implicitly** implements the interface.

.code code/interface/interface.go /Square/,/INT/

`*Square` implements `Shape`





## Interfaces (example)

#.code player/player.go /INTER OMIT/,/END OMIT/
.play code/interface/interface.go /Shape/,/END/





## Composition (Embedding) instead of Inheritance (It's not!)

.play -edit code/oop/shape.go /type Rectangle/,/END OMIT/







## Error Handling





## Errors as function results - Instead of exceptions

Errors (implicitly) implement the error interface, by defining the Error() method.

.code -edit code/error/error.go /ERROR OMIT/,/END OMIT/

Functions may return errors

.code -edit code/error/error.go /DIV OMIT/,/END OMIT/






## Checking for errors

.play -edit code/error/error.go /CHECK OMIT/,/END OMIT/








## Testing





## Testing

Test files end in `_test.go`.
They can be in the same package with the source file or in a different package.  
No assertions.  Full manual control. Test functions have the form

```
 func TestFoo(t *testing.T) {
     ...
 }
```

`go test ./...` executes all tests in all folders and subfolders.

// .code -edit code/factorial/factorial_test.go /TEST OMIT/,/END OMIT/
.code -edit code/factorial/factorial_test.go 







## Building a REST API





## REST API Routes

Using a 3rd party HTTP router and dispatcher <https://github.com/gorilla/mux>

#.play -edit code/restapi/restapi.go

.play -edit code/restapi/restapi.go /ROUTE OMIT/,/END OMIT/
.link http://localhost:8080/api/products
.link http://localhost:8080/api/product/cof





## REST API - JSON Encoding

.code -edit code/restapi/restapi.go /PROD OMIT/,/END OMIT/





## REST API - Getting a product

.code -edit code/restapi/restapi.go /GET OMIT/,/END OMIT/
.code -edit code/restapi/restapi.go /RESPOND OMIT/,/END OMIT/






## Concurrency






## Goroutines

.play -edit code/goroutines/goroutines.go /CODE OMIT/,/END OMIT/







## Communication

Go follows the CSP (Communicating Sequential Processes) semantics.

Communication is synchronous (blocking)

Write a value to a channel

.code code/clock/clock.go /WRITE OMIT/,/END OMIT/

Read a value from a channel (and assign to a variable)

.code -edit code/clock/clock.go /READ OMIT/,/END OMIT/

Spawning a goroutine

.code -edit code/clock/clock.go /GO OMIT/,/END OMIT/

 




## Example of synchronous channel communication

.play -edit code/clock/clock.go /CODE OMIT/,/ENDCODE OMIT/






## Tasks

Mock Tasks

.code -edit code/tasks/tasks_s.go /Task OMIT/,/END OMIT/


Work on task

.code -edit code/tasks/tasks_s.go /Work OMIT/,/}/






## Worker

Channels

.code -edit code/tasks/tasks_s.go /CHAN OMIT/,/END OMIT/

Worker

.code -edit code/tasks/tasks_s.go /WORKER OMIT/,/END OMIT/







## Worker pool (cont)

.play -edit code/tasks/tasks_s.go /MAIN OMIT/,/END OMIT/





## Consume from many producers

Producer

.code -edit code/producer/producer.go /PROD OMIT/,/END OMIT/





## Consumer (Select statement)

.code -edit code/producer/producer.go /CONS OMIT/,/END OMIT/

.play -edit code/producer/producer.go /func main/,/}/