Scala: understanding anonymous function syntax

Question

I am trying to make sense of a custom Iterator in Scala written by another programmer. I am having trouble understanding the function declarations. They look like anonymous functions to me, but I simply can't wrap my head around them fully.

I did some reading about Anonymous Functions in Scala , and I found this resource [http://www.scala-lang.org/old/node/133] helpful, but I still cannot read the above functions and make sense of them completely.

Here is the code:

class MyCustomIterator(somePath: Path, someInt: Int, aMaxNumber: Int) {
      def customFilter:(Path) => Boolean = (p) => true
       // Path is from java.nio.files.Path
      def doSomethingWithPath:(Path) => Path = (p) => p
}

I would like to understand these understand these functions. What is the return type really? What is the body of the function?

.

Justin Pihony · Accepted Answer · 2015-03-29 22:49:27Z

5

(For the first def) The parts after the colon and before the equals sign are the return type. So, the return type is:

Path => Boolean

Which denotes a function signature.

Now, breaking that down, the item on the left of the arrow is the parameters of a function. The right hand side is the return type of the function.

So, it is returning a function that accepts a Path and returns a Boolean. In this case, it is returning a function that will accept a Path and return true no matter what.

The second def returns a function that accepts a Path and returns another Path (the same Path in this case)

An example usage would be to use them as follows:

First method:

iter.customFilter(myPath) //returns true

or

val pathFunction = iter.customFilter;
pathFunction(myPath) //returns true

Second method:

iter.doSomethingWithPath(myPath) //returns myPath

or

val pathFunction = iter.doSomethingWithPath
pathFunction(myPath) //returns myPath

edited Mar 29, 2015 at 22:49

answered Mar 29, 2015 at 22:43

Justin Pihony

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4 Comments

som-snytt Over a year ago

Could add, more usual to see def customFilter: Path => Boolean = _ => true and def doSomethingWithPath: Path => Path = identity. Is that easier to read?

user3825558 Over a year ago

Justin, I like your explanation. However, let me get my understanding fully right: "The item on the left of the arrow is the parameters of a function. " - is it "Boolean = (p) => true" If that is so, how do we interpret this syntax.

user3825558 Over a year ago

@som-snytt, that is easier to read. Path is the return type, and this thing "Boolean = _ => true" is the return type? What is the underscore _ ? It is like a placeholder, I feel. And what did you mean by: Path = identity I can't get the Identity part.

Vladimir Matveev Over a year ago

@user3825558, I think something like this is clearer: def customFilter: (Path => Boolean) = (_ => true) or def customFilter: (Path => Boolean) = ((p) => true). These are exactly identical to the ones in the original code. Now the parentheses clearly delimit types and expressions, and you can see that customFilter is a method returning a function.

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