How to get value out of anonymous function in scala? - scala

Question

I am trying to learn scala and understand difference between functions and methods.
Here is very simple code I wrote -
scala> class C ( acc:Int) {
| val minc = ( acc + 1 )
| val func = { () => acc += 3 }
| }
scala> val c1 = new C(3)
c1: C = C#55e610e3
scala> c1.minc
res2: Int = 4
scala> c1.func
res3: () => Int = <function0>
I understand that result of calling function func on instantiated object c1 is stored as another expression res3.
However I want to get value out of of anonymous function () = acc +3 that is inside class C.
If I try to pass argument to res3 expression scala throws an error
scala> res3(4)
<console>:11: error: too many arguments for method apply: ()Int in trait Function0
res3(4)
^
How to get value out of it ?
PS - I have just started with scala and don't know if this is at all possible or not ?

Answer 1

This is your definition of func:
val func = { () => acc += 3 }
Let's take a look in the REPL at what is the type of func.
scala> val c1 = new C(3)
val c1 = new C(3)
c1: C = C#58db88e9
scala> c1.func
c1.func
res29: () => Unit = <function0>
In plain English, this means "func refers to a function that accepts no arguments and doesn't return a value." Unit means the method doesn't return anything. If you're coming from Java, then it's analogous to void as the return type. function0 means it accepts 0 arguments.
Next, let's take a look at the failing call in your example.
scala> c1.func(4)
c1.func(4)
<console>:10: error: too many arguments for method apply: ()Unit in trait Function0
c1.func(4)
^
Now that we know the method signature of func, this error message should make more sense. We know that func refers to a function that accepts no arguments, yet in this call, you have attempted to call it with a single integer argument. Since the method call has too many arguments, Scala correctly reports this as an error.
I am not entirely sure what you were trying to do by passing 4 as an argument. My best guess is that you are trying to apply the function to calculate its result by adding 3 to acc and then returning it to the caller. If I'm right, then we can redefine C as this:
class C(var acc:Int) {
val minc = ( acc + 1 )
val func = () => {
acc += 3
acc
}
scala> val c1 = new C(3)
val c1 = new C(3)
c1: C = C#58db88e9
scala> c1.func()
c1.func()
res44: Int = 6
When we call c1.func(), no arguments are passed, so it correctly matches the defined method signature.
Another possibility is that you were trying to parameterize the increment amount and pass 4 to it in the call. If so, then you can do this:
class C(var acc:Int) {
val minc = ( acc + 1 )
val func = (delta: Int) => {
acc += delta
acc
}
}
scala> c1.func(4)
c1.func(4)
res45: Int = 7
In this case, we have declared that the anonymous function accepts 1 argument of type Int, so when we pass 4 in the method call, it correctly matches the method signature.

Answer 2

You're not declaring an anonymous function in the way you think. This line:
val func = { () => acc += 3 }
That actually declares 2 anonymous functions. The brackets are the first anonymous function (that takes no arguments), and the () => acc += 3 is the second function. Scala allows you to declare a 0-arity anonymous function by just using brackets. If you drop those, you'll have what you want:
val func = () => acc += 3
As a bit of a side note, you can see this in the type signature. Your current signature for func is:
() => Int = <function0>
That's a function that takes 0 arguments and returns an integer. That's not what you want. When you change it to what I gave you, you'll get this:
Int => Int = <function1>
Now func takes a single argument, and Integer, and returns another Integer, which is what you wanted.