scala cookbook chapter 9. functional programming

For instance, beginning with the most explicit form, you can print each element in the list using this anonymous function with the foreach method:

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x.foreach ((i: Int) => println(i ))

As before, the Int declaration isn’t required:

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x.foreach ((i) => println( i))

Because there is only one argument, the parentheses around the i parameter aren’t needed:

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x.foreach (i => println(i ))

Because i is used only once in the body of the function, the expression can be further simplified with the _ wildcard:

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x.foreach (println( _))

Finally, if a function literal consists of one statement that takes a single argument, you need not explicitly name and specify the argument, so the statement can finally be
reduced to this:

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x.foreach (println)

You can declare a function literal in at least two different ways. I generally prefer the following approach, which implicitly infers that the following function’s return type is Boolean:

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val f = (i: Int) => { i % 2 == 0 }

However, if you prefer to explicitly declare the return type of a function literal, or want to do so because your function is more complex, the following examples show different forms you can use to explicitly declare that your function returns a Boolean:

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val f: (Int) => Boolean = i => { i % 2 == 0 }
val f: Int => Boolean = i => { i % 2 == 0 }
val f: Int => Boolean = i => i % 2 == 0
val f: Int => Boolean = _ % 2 == 0


val add = (x: Int, y : Int) => { x + y }
val add = (x: Int, y : Int) => x + y
// explicit approach
val add: (Int, Int) => Int = ( x,y ) => { x + y }
val add: (Int, Int) => Int = ( x,y ) => x + y