Understanding lambda in Kotlin

Kotlin is very concise language. You can achieve same functionality with fewer characters compared to Java. But as an experienced developer, it was easy for me to switch from Java to Kotlin. Same might not be true for those who are new to programming. I had tough time explaining how Lambda works to a trainee who was new to programming.
For instance, I was trying to explain a simple map function that maps objects of one type to another. I have two simple classes

	class Journal(id: String)
	class DisplayJournal(journal: Journal)

view raw journalclass hosted with ❤ by GitHub

and a simple usage of map function

val listOfDisplayJournals = listOf(Journal("123")).map { DisplayJournal(it) }

view raw map hosted with ❤ by GitHub

I came up with following step by step code to explain what's actually going on. All the code snippets do the exact same thing. Converting imperative to functional code at the end.
Extract the list of journals to new variable.

	val listOfJournals = listOf(Journal("123"))
	val listOfDisplayJournals = listOfJournals.map { DisplayJournal(it) }

view raw extractlist hosted with ❤ by GitHub

The map function is being executed here, we can specify the round brackets to be explicit

	val listOfJournals = listOf(Journal("123"))
	val listOfDisplayJournals = listOfJournals.map() { DisplayJournal(it) }

view raw roundbrac hosted with ❤ by GitHub

The code after map function including the curly braces is actually a lambda. It is the last (and only) parameter for the map function. So we move it inside the round brackets.

	val listOfJournals = listOf(Journal("123"))
	val listOfDisplayJournals = listOfJournals.map ({ DisplayJournal(it) })

view raw insidebrackets hosted with ❤ by GitHub

The variable it is default name of the parameter in the lambda, if the lambda takes exactly one parameter. We can specify the name explicitly in the definition

	val listOfJournals = listOf(Journal("123"))
	val listOfDisplayJournals = listOfJournals.map ({it-> DisplayJournal(it) })

view raw itparam hosted with ❤ by GitHub

The type of the parameter was implicit. As map function is being called on list of Journal, the lambda parameter type is Journal. Below we specify the type for it.

	val listOfJournals = listOf(Journal("123"))
	val listOfDisplayJournals = listOfJournals.map ({it: Journal-> DisplayJournal(it) })

view raw ittype hosted with ❤ by GitHub

Let's rename it variable so that it is more readable

	val listOfJournals = listOf(Journal("123"))
	val listOfDisplayJournals = listOfJournals.map ({journal: Journal-> DisplayJournal(journal) })

view raw renameit hosted with ❤ by GitHub

Now this lambda can be extracted out to a new variable. Below we assign the lambda expression to the new variable and replace the lambda in map function with this new variable

	val listOfJournals = listOf(Journal("123"))
	val convertJournalToDisplayJournal = { journal: Journal -> DisplayJournal(journal) }
	val listOfDisplayJournals = listOfJournals.map (convertJournalToDisplayJournal)

view raw extrlambda hosted with ❤ by GitHub

The new variable type again is implicit above. Let's specify it explicitly. It takes a parameter of type Journal and returns DisplayJournal

	val listOfJournals = listOf(Journal("123"))
	val convertJournalToDisplayJournal: (Journal)->DisplayJournal = { journal: Journal -> DisplayJournal(journal) }
	val listOfDisplayJournals = listOfJournals.map (convertJournalToDisplayJournal)

view raw lambdatype hosted with ❤ by GitHub

As lambda is nothing but a function, let's define it as a function. We add the fun keyword here. Note that Kotlin functions are first-class, which means that I can assign those to a variable.

	val listOfJournals = listOf(Journal("123"))
	val convertJournalToDisplayJournal: (Journal)->DisplayJournal = fun(journal: Journal) {
	DisplayJournal(journal)
	}
	val listOfDisplayJournals = listOfJournals.map (convertJournalToDisplayJournal)

view raw anonfun hosted with ❤ by GitHub

We can also define this function explicitly and use the function reference using ::. Note that in the function definition we have added return keyword which returns a value from the function body

	val listOfJournals = listOf(Journal("123"))
	fun convertJournalToDisplayJournalFn(journal: Journal): DisplayJournal{
	return DisplayJournal(journal)
	}
	val convertJournalToDisplayJournal: (Journal)->DisplayJournal = ::convertJournalToDisplayJournalFn
	val listOfDisplayJournals = listOfJournals.map (convertJournalToDisplayJournal)

view raw lambdatofun hosted with ❤ by GitHub

We can also convert this function to expression body by removing the curly braces and the return keyword

	val listOfJournals = listOf(Journal("123"))
	fun convertJournalToDisplayJournalFn(journal: Journal): DisplayJournal = DisplayJournal(journal)
	val convertJournalToDisplayJournal: (Journal)->DisplayJournal = ::convertJournalToDisplayJournalFn
	val listOfDisplayJournals = listOfJournals.map (convertJournalToDisplayJournal)

view raw exprebody hosted with ❤ by GitHub

The return type that is explicitly defined for the function can be removed in case of expression body

	val listOfJournals = listOf(Journal("123"))
	fun convertJournalToDisplayJournalFn(journal: Journal) = DisplayJournal(journal)
	val convertJournalToDisplayJournal: (Journal)->DisplayJournal = ::convertJournalToDisplayJournalFn
	val listOfDisplayJournals = listOfJournals.map (convertJournalToDisplayJournal)

view raw 1 hosted with ❤ by GitHub