[Golang] Functions 101

Functions are the building blocks of procedural programming. They help in creating modular code where a big job can be segmented into small pieces of code and might have been written by different people separated by both time and space. In fact, function is one kind of first-class citizen types in Go. In other words, we can use functions as values. Although Go is a static language, Go functions are very flexible. The feeling of using Go functions is much like using many dynamic languages. A function is a mapping of zero or more input parameters to zero or more output parameters.

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The advantages of using functions are:

• Reducing duplication of code

• Decomposing complex problems into simpler pieces

• Improving clarity of the code

• Reuse of code

• Information hiding

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Special Functions

Golang has 2 special functions main() and init() . They have some special responsibilities assigned to them

The main() and init() function

The main package is a special package which is used with the programs that are executable and this package contains main() function. The main() function is a special type of function and it is the entry point of the executable programs. It does not take any argument nor return anything. Go automatically call main() function, so there is no need to call main() function explicitly and every executable program must contain single main package and main() function. The init() function is declared implicitly and we cannot reference it from anywhere in the code, but we are allowed to create multiple init() functions in the same program. The init() function can be incredibly powerful and compared to some other languages, is a lot easier to use within your Go programs. These init() functions can be used within a package block and regardless of how many times that package is imported, the init() function will only be called once.

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Function with returns (none, single value or multiple values)

We declare a function using the func keyword. A function has a name, a list of comma-separated input parameters along with their types, the result type(s), and a body.

func function_name(Parameter-list)(Return_type){
    // function body.....
}

Let say we have function repeatWord() which return nothing, so main() just executes the repeatWord() which take a string and int as arguments and prints. Nothing to return

Now we change the function to return more than one values and also return if there are any errors.

You see if the condition below matches, function return error using fmt.Errorf else it send a nil in place of error.

if len(s) <= 0 {
    return "", len(s), fmt.Errorf("Length of string can't less than euql zero")
  }

Overall repeatWord() returns multiple value, along side with error.

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Function as Parameter

A Go function can be passed to other functions as a parameter. Such a function is called a higher-order function.

Here we have simple action functions called repeatWord() and revertWord(). Also we have function called applyFunction() which take a function and string as parameters.

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Function as custom type

Go allows to create reusable functions signatures with the type keyword. They have the same number of arguments with each argument is the same type. They have the same number of return values and each return value is of the same type

With the type keyword, we create a function type which accepts one string parameter and returns a string.

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Function as closure aka anonymous function

It is possible to create functions inside of functions. Go supports anonymous functions, which can form closures. Anonymous functions are useful when you want to define a function inline without having to name it. Closure is a nested function that helps us access the outer function’s variables even after the outer function is closed

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Function Higher Order

Higher order functions are functions that operate on other functions, either by taking them as arguments or by returning them.

Above you can see that concatStringUsingFunctionOfHigerOrder() returns another function, which again return another function. It is kind of Spaghetti Code in my opinion. However above is just an example, I would avoid writing code like this.

Hope this provide little bit more clarity on function and way they can be used in Golang program. Above examples can be found here: go-functions-101

Happy Coding!!