The way Scala deals with functions is pretty interesting. If you want to use them as you would use Java functions then they’re not that complicated. You have to learn the syntax, a little about the Scala type system, and bada-bing, you’re in business. But if you start exploring the way types are implemented in Scala you find some interesting stuff.

First, we’ll briefly describe the basics.  Here are a few very simple Scala function definitions and invocations in the scala interpreter:

scala> def method1() = { println("method1") }
method1: ()Unit

scala> def method2(str: String) = { println("method2: " + str) }
method2: (String)Unit

scala> def method3(str: String): Int = {
     |   println("method3: " + str); str.length;
     | }
method3: (String)Int

scala> def method4(f: (String) => Int) = {
     |   printf("method4: " + f("method4"))
     | }
method4: ((String) => Int)Unit

scala> method1

scala> method2("abc")
method2: abc

scala> method3("abcdefg")
method3: abcdefg
res13: Int = 7

scala> method4(method3)
method3: method4
method4: 7

Very basic:  method1 takes no parameters and returns nothing, method2 takes a single parameter of type String and returns nothing, method3 takes a String parameter and returns an Int, and method4 takes a parameter of type “function that takes a String parameter and returns Int” and returns nothing.

Why are we able to declare functions like this in Scala?  Didn’t I read somewhere that Scala is very object oriented?  Didn’t I read that everything is an object?  Why do we have these bare naked functions defined outside of objects?  The reason is that in Scala, everything really is an object, even functions!  That method1 we defined?  That’s an object.  When we type “def method1() = {…}” we actually declared an instance of a special class.  I’ll declare method1 again, but with the underlying object exposed:

scala> val method1 = new Function0[Unit] {
     |   def apply: Unit = { println("method1") }
     | }
method1: java.lang.Object with () => Unit = <function>

scala> method1
res1: java.lang.Object with () => Unit = <function>

scala> method1.apply

scala> method1()

We instantiate an instance of trait Function0[Unit] and implement its one abstract method, called apply, and assign it to a val named method1.  Now you can see method1 is actually just a plain old Scala object.  When we type in “method1″ and hit enter, the interpreter just tells us the resulting value of the statement which is an Object with trait Function0.  Hmm, that didn’t work.  Next we try calling the apply method on the object.  That works!  But it’s just a regular call to a member method.  But when we type “method1()” then Scala knows that we want to use this object as a function, and that we’re not refering to the object itself.  When you declare a function using “def” Scala assumes that when you refer to the method you want the apply method invoked, and that you don’t want to return the function object.  Neat.

That Function0[Unit], by the way, defines a function that takes 0 parameters and returns Unit (which is to say nothing as in Java void (not to be confused with Nothing)).  If you want a function that takes two parameters, an Int and a String, and returns a List of Doubles, you would use Function2[Int, String, List[Double]].  So class FunctionX takes (X+1) type parameters, the first X of which define the function parameter types, and the last of which defines the return type.

So what if we go the other way?  What if we declare a method and then store it in a val?  In this case, Scala gets very picky.  Watch this:

scala> def method2 = { println("method2") }
method2: Unit

scala> val m2: () => Unit = method2
<console>:5: error: type mismatch;
 found   : Unit
 required: () => Unit
       val m2: () => Unit = method2

scala> def method2() = { println("method2") }
method2: ()Unit

scala> val m2: () => Unit = method2
m2: () => Unit = <function>

scala> def method2 = { println("method2") }
method2: Unit

scala> val m2: () => Unit = method2 _
m2: () => Unit = <function>

Some strange stuff happens here.  First we just define a function called method2.  Nothing fancy.  Then we try to assign it to a val of type () => Unit.  It fails.  See the error message?  Found : Unit.  It parses it all wrong.  Scala thinks we’re trying to call method2 and assign the result to m2.  How can we set things straight?  Well, one way is to slightly change the way we define method2.  The only difference in the first and second definition is the addition of an empty parameter list, that empty pair parentheses.  For some reason, when we define the method in this apparently equivalent fashion, Scala rightly interprets our intentions and allows us to assign to m2.  There is another way, though.  In the third definition of method2, we’ve again removed the parentheses.  But this time we assign it successfully to val m2 by following method2 with an underscore.  The underscore just causes Scala to treat method2 as a Function0 object, rather than attempting to invoke it.

So a function is an object.  Who cares?  We’re still just calling functions.  Ah, but use your imagination.  You can do all kinds of tricks once you realize that a function is just an object.  For example:

scala> class TestClass {
     |   def f1(): Unit = { println("f1!!!"); func = f2 }
     |   def f2(): Unit = { println("f2!!!"); func = f3 }
     |   def f3(): Unit = { println("f3!!!"); func = f1 }
     |   var func: () => Unit = f1
     |   def test = { func() }
     | }
defined class TestClass

scala> val tc = new TestClass
tc: TestClass = TestClass@1eff71e

scala> tc.test

scala> tc.test

scala> tc.test

scala> tc.test

See what’s happening here?  We can store a reference to a function object, call the function it refers to, and re-assign it.  So the method “test” actually calls a different function each time.

Can you guess why I added the test method instead of just calling func directly?  func is declared with the var keyword, so if I entered “tc.func” instead of “tc.func()” then the interpreter would think I was refering to the function object.  Just so there’s no confusion, I wrapped the call “func()” inside a regular def-defined function called test.

Let’s see, what other neat tricks can we do?  Here’s something interesting:

scala> def printAll(str1: String, str2: String, str3: String): Unit = {
     |   println( str1 + ":" + str2 + ":" + str3 )
     | }
printAll: (String,String,String)Unit

scala> def fillInStr1(func: (String,String,String) => Unit, str1: String): (String,String) => Unit = {
     |   new Function2[String,String,Unit] {
     |     def apply(str2: String, str3: String) = {
     |       func(str1, str2, str3)
     |     }
     |   }
     | }
fillInStr1: ((String, String, String) => Unit,String)(String, String) => Unit

scala> val newPrint = fillInStr1(printAll _, "test123")
newPrint: (String, String) => Unit = <function>

scala> newPrint("abc","xyz")

scala> newPrint("123","456")

First, we define printAll.  It’s just a function that prints out its 3 string parameters.  The next method, fillInStr1, is the interesting one.  The method signature is kind of complex.  It takes 2 parameters, func and str1.  func is a function taking 3 String parameters and returning nothing.  str1 is just a String.  fillInStr1 returns a function taking 2 String parameters and returning nothing.

Inside fillInStr, it just creates an instance of Function2, a function that takes 2 parameters.  This function object is defined so that the apply method calls the func function and passes str1 as the first parameter.  The other two parameters are the parameters of the Function2′s apply method.  Do you see what it’s doing?  It’s taking a function on 3 strings, and transforming it into a function on only 2 strings.  We can call fillInStr1 by passing in printAll (note the underscore), and a string.  What we get back is a function that behaves just like printAll, except with the first parameter already filled in.  Neat trick!

In fact, this trick is so neat that it has a name and is actually built into the language.  This little demonstration is a very simple, non-generalized application of a concept called currying.  The Code Commit blog has an excellent article on function currying in Scala if you’d like to know more about it.

This, of course, isn’t all there is to functions.  There’s a lot more!  But now you know enough to go out there and start experimenting.  See what tricks you can do, what problems you can solve with Scala’s versatile and powerful function objects.