Scala Functions vs Methods

Scala has both functions and methods. Most of the time we can ignore this distinction, but sometimes we have to deal with the fact that they are not quite the same thing.

In my Scala Syntax Primer I mention that I use the terms method and function interchangeably in the discussion. This is a simplification. In many situations, you can ignore the difference between functions and methods and just think of them as the same thing, but occasionally you may run into a situation in which the difference matters. This is analogous to how most of us treat mass and weight. In our daily lives on the surface of planet Earth, we treat them as interchangeable units, with 1Kg being the same as 2.2 pounds. But they are not quite the same thing: when an astronaut walks on the surface of the moon, his mass (kilograms) has not changed but his weight (pounds) is only about one sixth of what it was on Earth.

In contrast to Kg vs pounds, you are rather more likely to come across a situation in which the distinction between Scala functions and methods is important than you are to be walking on the surface of the moon. So when can you ignore the difference between functions and methods, and when do you need to pay attention to it? You can answer that question once you understand the difference.

A Scala method, as in Java, is a part of a class. It has a name, a signature, optionally some annotations, and some bytecode.

A function in Scala is a complete object. There are a series of traits in Scala to represent functions with various numbers of arguments: Function0, Function1, Function2, etc. As an instance of a class that implements one of these traits, a function object has methods. One of these methods is the apply method, which contains the code that implements the body of the function. Scala has special "apply" syntax: if you write a symbol name followed by an argument list in parentheses (or just a pair of parentheses for an empty argument list), Scala converts that into a call to the apply method for the named object. When we create a variable whose value is a function object and we then reference that variable followed by parentheses, that gets converted into a call to the apply method of the function object.

When we treat a method as a function, such as by assigning it to a variable, Scala actually creates a function object whose apply method calls the original method, and that is the object that gets assigned to the variable. Defining a function object and assigning it to an instance variable this way consumes more memory than just defining the functionally equivalent method because of the additional instance variable and the overhead of another object instance for the function. Thus you would not want every method to be a function; but functions give you a great deal of power that is not available with just methods, and in those situations that power is well worth the additional memory used.

Let's look at some details of this mechanism. Create a file test.scala with this in it:

class test {
    def m1(x:Int) = x+3
    val f1 = (x:Int) => x+3
}

Compile that file with scalac and list the resulting files. Scala creates two files: test.class and test$$anonfun$1.class. That strange extra class file is the anonymous class for the function object that Scala created in response to the function expression assigned to f1. If you use more than one function expression in your test class, there will be more than one anonymous class file, even if you write the same function expression over again.

If you run javap on the test class, you will see this:

Compiled from "test.scala"
public class test extends java.lang.Object implements scala.ScalaObject{
    public test();
    public scala.Function1 f1();
    public int m1(int);
    public int $tag()       throws java.rmi.RemoteException;
}

Running javap on the function class test$$anonfun$1 yields this:

Compiled from "test.scala"
public final class test$$anonfun$1 extends java.lang.Object implements scala.Function1,scala.ScalaObject{
    public test$$anonfun$1(test);
    public final java.lang.Object apply(java.lang.Object);
    public final int apply(int);
    public int $tag()       throws java.rmi.RemoteException;
    public scala.Function1 andThen(scala.Function1);
    public scala.Function1 compose(scala.Function1);
    public java.lang.String toString();
}

Because this class implements the Function1 interface we know it is a function of one argument. You can see that it contains a handful of methods, including the apply method.

You can also define a function in terms of an existing method by referencing that method name followed by a space and an underscore. Modify test.scala to add another line that does this:

class test {
    def m1(x:Int) = x+3
    val f1 = (x:Int) => x+3
    val f2 = m1 _
}

The m1 _ syntax tells Scala to treat m1 as a function rather than taking the value generated by a call to that method. Alternatively, you can explicitly declare the type of f2, in which case you don't need to include the trailing underscore:

    val f2 : (Int) => Int = m1

In general, if Scala expects a function type, you can pass it a method name and have it automatically converted to a function. For example, if you are calling a method that accepts a function as one of its parameters, you can supply as that argument a method of the appropriate signature without having to include the trailing underscore.

Back to our test file, now when you compile test.scala there will be two anonymous class files, one for the f1 class and one for the f2 class. You can use either definition for f2, they generate identical class files.

If you use the -c option to javap to get the code of the second anonymous class, you can see the call to the m1 method of the test class in the apply method:

public final int apply(int);
  Code:
   0:   aload_0
   1:   getfield        #17; //Field $outer:Ltest;
   4:   astore_2
   5:   aload_0
   6:   getfield        #17; //Field $outer:Ltest;
   9:   iload_1
   10:  invokevirtual   #51; //Method test.m1:(I)I
   13:  ireturn

Let's fire up the scala interpreter and see how this works. In the following examples, input text is shown in bold and output text in regular weight.

scala> def m1(x:Int) = x+3
m1: (Int)Int

scala> val f1 = (x:Int) => x+3
f1: (Int) => Int = <function>

scala> val f2 = m1 _
f2: (Int) => Int = <function>

scala> m1(2)
res0: Int = 5

scala> f1(2)
res1: Int = 5

scala> f2(2)
res2: Int = 5

Note the difference in the signatures between m1 and f1: the signature (Int)Int is for a method that takes one Int argument and returns an Int value, whereas the signature (Int) => Int is for a function that takes one Int argument and returns an Int value.

At this point we seem to have a method m1 and two functions f1 and f2 that all do the same thing. But f1 and f2 are actually variables that contain an instance of a generated class that implements the Function1 interface, and that object instance has methods that m1 does not have.

scala> f1.toString
res3: java.lang.String = <function>

scala> (f1 andThen f2)(2)
res4: Int = 8

Because m1 is itself a method, unlike f1 you can't call methods on it:

scala> m1.toString
<console>:6: error: missing arguments for method m1 in object $iw;
follow this method with `_' if you want to treat it as a partially applied function
       m1.toString
       ^

Note that each time you separately reference a method as a function Scala will create a separate object.

scala> val f3 = m1 _
f3: (Int) => Int = <function>

scala> f2 == f3
res6: Boolean = false

Even though f2 and f3 both refer to m1, and both do the same thing, they are not considered equal by Scala because function objects inherit the default equality method that compares identity, and these are two different objects. If you want two function values to be equal, you must ensure that they refer to the same instance of function object:

scala> val f4 = f2
f4: (Int) => Int = <function>

scala> f2 == f4
res7: Boolean = true

Here are a few other examples showing that the expression m1 _ is in fact a function object:

scala> m1 _
res8: (Int) => Int = <function>

scala> (m1 _).toString
res9: java.lang.String = <function>

scala> (m1 _).apply(3)
res10: Int = 6

As of Scala version 2.8.0, the expression (m1 _)(3) will also return the same value (there is a bug in previous versions that causes this syntax to give a type mismatch error).

There are some other differences between methods and functions. A method can be type-parameterized, but an anonymous function can not:

scala> def m2[T](x:T) = x.toString.substring(0,4)
m2: [T](T)java.lang.String

scala> m2("abcdefg")
res11: java.lang.String = abcd

scala> m2(1234567)
res12: java.lang.String = 1234

However, if you are willing to define an explicit class for your function, then you can type-parameterize it similarly:

scala> class myfunc[T] extends Function1[T,String] {
     |     def apply(x:T) = x.toString.substring(0,4)
     | }
defined class myfunc

scala> val f5 = new myfunc[String]
f5: myfunc[String] = <function>

scala> f5("abcdefg")
res13: java.lang.String = abcd

scala> val f6 = new myfunc[Int]
f6: myfunc[Int] = <function>

scala> f6(1234567)
res14: java.lang.String = 1234

So go ahead and keep converting pounds to kilograms by dividing by 2.2 (unless you are an astronaut), but when you start mixing functions and methods in Scala, keep in mind that they are not quite the same thing.

Updated 2010-12-08: clarify memory usage in response to comment by jqb.