Friday, 17 July 2020

Functions in Go


To declare a function in Go, use the keyword func followed by the function name, any
parameters, and then any return values.

feeds, err := RetrieveFeeds()
if err != nil {
   log.Fatal(err)
}

NOTE: You can omit the parentheses () from an if statement in Golang, but the curly braces {} are mandatory!

This function belongs to the search package and returns two values. The first return value is a slice
of Feed type values. A slice is a reference type that implements a dynamic array. You use
slices in Go to work with lists of data.

The second return value is an error.  
Functions can have multiple return values. It’s common to declare functions that return a value and an error value just like the RetrieveFeeds function. If an error occurs, never trust the other values being returned from the function. They should always be ignored, or else you run the risk of the code generating more errors or panics.

Why doesn't Go allow nested function declarations (functions inside functions)? - Stack Overflow


func plus(a int, b int) int {...}

Can Functions be passed as parameters in Go? Yes.

type MyFunc func(int) string

func Foo(fn MyFunc) {
    str := fn(123)
}

Variable can be of a function type and be assigned a function:

sayHello := func() {
   fmt.Println("hello")
}

go sayHello()


Go by Example: Variadic Functions

func sum(nums... int){...}

Three dots (ellipsis) notation

Specifying function's default value for an argument is NOT supported.
Default value in Go's method

When to return pointer to local variable?

Return pointer to local struct

Go performs pointer escape analysis. If the pointer escapes the local stack, the object is allocated on the heap. If it doesn't escape the local function, the compiler is free to allocate it on the stack (although it makes no guarantees; it depends on whether the pointer escape analysis can prove that the pointer stays local to this function).

Golang documentation states that it's perfectly legal to return a pointer to local variable.
Compiler sees you return the address and just makes it on the heap for you.

Escape Analysis in Go


Naming


Your custom function can't have the same name as the name of some imported package.

Function Overloading


Does the Go language have function/method overloading? (Answer: NO)
Optional parameters, default parameter values and method overloading

The idiomatic way to emulate optional parameters and method overloading in Go is to write several methods with different names.

Alternative Patterns for Method Overloading in Go
Functional options for friendly APIs <-- MUST READ (!)

Deferred execution

go - Multiple defers vs deferred anonymous function - Stack Overflow

defer func() {
  err := f.Close()
  if err != nil {
    log.Println("close:", err)
  }
  err = os.Remove(f.Name())
  if err != nil {
    log.Println("remove:", err)
  }
}()

Built-in Functions


make()


panic()


Go by Example: Panic
When to call panic() : golang
Panic in Golang - GeeksforGeeks


Closures


Go by Example: Closures

What exactly does “closing over” mean?

 "A closes over B" == "B is a free variable of A", where free variables are those that appear in a function's body, but not in its signature.


What will be the output of the following snippet?

var wg sync.WaitGroup

for _, letter := range []string{"a", "b", "c"} {
   wg.Add(1)
   go func() {
      defer wg.Done()
      fmt.Println(letter)
   }()
}

wg.Wait()


Go closure variable scope

Closures in Go capture variables by reference. That means the inner function holds a reference to the  i variable in the outer scope, and each call of it accesses this same variable.


Closure (computer programming)

Comapre to JS Clsoures.
Lexical scoping:
This is an example of lexical scoping, which describes how a parser resolves variable names when functions are nested. The word "lexical" refers to the fact that lexical scoping uses the location where a variable is declared within the source code to determine where that variable is available. Nested functions have access to variables declared in their outer scope.


How golang's “defer” capture closure's parameter?



// Launch the goroutine
go func(matcher Matcher, feed *Feed) {
   Match(matcher, feed, searchTerm, results) 
   waitGroup.Done()
}(matcher, feed)


goroutine

  • light-weight process that is automatically time-sliced onto one or more operating system threads by the Go runtime.
  • a function that’s launched to run independently from other functions in the program.
  • Use the keyword go to launch and schedule goroutines to run concurrently. 
  • the order the goroutines get executed is unpredictable
  • can be an anonymous function
  • can be launched in a (for-range) loop, for each element of some set: this allows each element to be processed independently in a concurrent fashion
  • There's no goroutine ID available from the runtime
  • not an OS thread (thread managed/scheduled natively by OS)
  • not exactly a green thread (thread managed/scheduled by languages runtime or virtual machine) [Green threads] [Why not Green Threads?]
  • is a special type of coroutine (concurrent subroutines - functions, closures or methods) - the one that is non-preemptive. It cannot be interrupted but instead has multiple points through which it can be suspended or reentered. Go runtime defines this points internally and automatically suspends them when they block and resumes them when they become unblocked.
  • goroutines operate within the same address space as each other, and host functions
  • Go implements M:N scheduler, which means it maps M green threads to N OS threads. Goroutines are then scheduled onto the green threads. When we have more goroutines than green threads available, the scheduler handles the distribution of the goroutines across the available threads and ensures that when these goroutines become blocked, other goroutines can be run.
  • Go follows a model of concurrency called the fork-join model.
  • function which runs as goroutine can have return value(s) and return them but they will never be read => they should not have a return value. If we want to read some value which is calculated in goroutine we should either use channels or variable capturing mechanisms [Catching return values from goroutines].

To launch a function as goroutine:
goroutines operate within the same address space as each other, and
simply host functions,
func foo() {
   fmt.Println("foo()")
}

go foo()

To launch anonymous function as goroutine:

go func() {
   fmt.Println("foo()")
}()



anonymous function

  • a function that’s declared without a name
  • can take parameters


pointer variables

  • are great for sharing variables between functions. They allow functions to access and change the state of a variable that was declared within the scope of a different function and possibly a different goroutine.
  • In Go, all variables are passed by value. Since the value of a pointer variable is the address to the memory being pointed to, passing pointer variables between functions is still considered a pass by value.
Pointers vs. values in parameters and return values


waitGroup.Done()

Once the main task withing the goroutine completes, we execute the code which decrements the WaitGroup count. Once every goroutine finishes calling Done method, the program will know every main task has been done (e.g. element has been processed).

There’s something else interesting about the method call to Done: the WaitGroup
value was never passed into the anonymous function as a parameter, yet the anony-
mous function has access to it. Go supports closures and you’re seeing this in action. In fact, the searchTerm and results variables are also being accessed by the anonymous function via closures.
Thanks to closures, the function can access those variables directly without the need to
pass them in as parameters. The anonymous function isn’t given a copy of these variables; it has direct access to the same variables declared in the scope of the outer function. This is the reason why we don’t use closures for the matcher and feed variables.

---

With all the processing goroutines working, sending results on the results channel
and decrementing the waitGroup counter, we need a way to display those results and
keep the main function alive until all the processing is done. We'll launch yet another anonymous function as a goroutine. This anonymous function takes no parameters and uses closures to access both the waitGroup and results variables. This goroutine calls the method Wait() on the WaitGroup value, which is causing the goroutine to block until the count for the WaitGroup hits zero. Once that happens, the goroutine calls the built-in function close on the channel, which as you’ll see causes the program to terminate.


// Launch a goroutine to monitor when all the work is done.
go func() {
   // Wait for everything to be processed.
   waitGroup.Wait()

   // Close the channel to signal to the Display

   // function that we can exit the program.
   close(results)
}()

---

func Match(matcher Matcher, feed *Feed, searchTerm string, results chan<- *Result) {
   // Perform the search against the specified matcher.
   searchResults, err := matcher.Search(feed, searchTerm)
   if err != nil {
      log.Println(err)
      return
   }
   // Write the results to the channel.
   for _, result := range searchResults {
      results <- result
   }
}

results are written to the channel

BK: not how nil is used instead of null
BK: note how it's so convenient the model of returning function result and error at the same time; no need to pass variable by pointer and no expensive exceptions
BK: note how there are no brackets around conditions in if, for... statements.

init() function


All init functions in any code file that are part of the program will get called before
the main function.

When is the init() function run?

func init() {
  // Change the device for logging to stdout.
   log.SetOutput(os.Stdout)
   ...
}

This init function sets the logger from the standard library to write to the stdout device. By default, the logger is set to write to the stderr device.


main() function


How do I return from func main in Go? - Stack Overflow

func main() { os.Exit(mainReturnWithCode()) }

func mainReturnWithCode() int {
    // do stuff, defer functions, etc.
    return exitcode // a suitable exit code
}

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