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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 myScala Syntax PrimerI 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 asituationin 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 aboutone sixthof 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 theapplymethod, 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 theapplymethod 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 theapplymethod 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 filetest.scalawith 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.classandtest$$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 tof1. If you use more than one function expression in yourtestclass, there will be more than one anonymous class file, even if you write the same function expression over again.

If you runjavapon thetestclass, 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; } Runningjavapon the function classtest$$anonfun$1yields this: Compiled from "test.scala" public final class test$$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 theFunction1interface we know it is a function of one argument. You can see that it contains a handful of methods, including theapplymethod.

You can also define a function in terms of an existing method by referencing that method name followed by a space and an underscore. Modifytest.scalato add another line that does this: class test { def m1(x:Int) = x+3 val f1 = (x:Int) => x+3 val f2 = m1 _ } Them1 _syntax tells Scala to treatm1as a function rather than taking the value generated by a call to that method. Alternatively, you can explicitly declare the type off2, 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 compiletest.scalathere will be two anonymous class files, one for thef1class and one for thef2class. You can use either definition forf2, they generate identical class files.

If you use the-coption tojavapto get the code of the second anonymous class, you can see the call to them1method of thetestclass in theapplymethod: 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 inboldand 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 betweenm1andf1: the signature(Int)Intis for amethodthat takes oneIntargument and returns anIntvalue, whereas the signature(Int) => Intis for afunctionthat takes oneIntargument and returns anIntvalue.

At this point we seem to have a methodm1and two functionsf1andf2that all do the same thing. Butf1andf2are actually variables that contain an instance of a generated class that implements theFunction1interface, and that object instance has methods thatm1does not have. scala> f1.toString res3: java.lang.String = <function> scala> (f1 andThen f2)(2) res4: Int = 8 Becausem1is itself a method, unlikef1you 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 thoughf2andf3both refer tom1, 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 expressionm1 _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 abugin previous versions that causes this syntax to give atype mismatcherror).

There are some other differences between methods and functions. A methodcan betype-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.