def vs val

[Luigi Plinge]

Does it matter whether I use def or val in these functions? Both seem to do the same thing.

def greater(n:Int, lst:List[Int]):List[Int] = {
def fun(x:Int) = if(x > n) true else false
lst.filter(fun)
}

def greater2(n:Int, lst:List[Int]):List[Int] = {
val fun = (x:Int) => if(x > n) true else false
lst.filter(fun)
}

[Samuel Cox]

I don't think it matters. My guess is one is syntactic sugar for the other. I'll further guess def is the sugar...

I think most people use the def keyword to define functions unless they are anonymous.

Here are the anonymous function versions:

def greater(n: Int, lst: List[Int]): List[Int] = lst.filter( (x:Int) => x > n)

// and the fill-in-the-blank thingy:

def greater(n: Int, lst: List[Int]) = lst.filter (_ > n)

// As an aside, you almost always want to replace
if (boolean_expression) true else false

// with

boolean_expression

[Luigi Plinge]

Thanks, very helpful.

I note we can also use both of the following:

def greater(n: Int, lst: List[Int]) = lst.filter (_ > n)

val greater2 = (n: Int, lst: List[Int]) => lst.filter (_ > n)

which both seem to do the same thing, although the REPL gives slightly diffrent messages after you type each in.

I don't think it's just syntactic sugar because if you just type in the name of the function in each case, it comes back with:

scala> greater
<console>:8: error: missing arguments for method greater in object $iw;
follow this method with `_' if you want to treat it as a partially applied function
greater
^

scala> greater2
res31: (Int, List[Int]) => List[Int] = <function2>

So is one being treated as a method and the other a function?

[Mohamed Sanaulla]

In the above example- greater is a function which has to be invoked with the parameters, where as greater2 is a function value- A closure?

Am just not sure about this though.

[Samuel Cox]

I'm glad that I specified I was guessing:)

I think both constructs can classify as closures, since both are essentially creating functions that capture their surrounding scope.

def foo = {
  val i = 1;
  def a() = i + 1; // closure
  a()
}

def bar = {
  val i = 1;
  val a = () => i + 1; // closure
  a()
}

http://jim-mcbeath.blogspot.com/2009/05/scala-functions-vs-methods.html has some info on it, but I don't think it is a 100% match to your question. You might try the mailing list if you can't get a satisfactory answer (but please come back;).

[Jesper de Jong]

def and val are two different things.

With def, you declare a method. The expression won't be evaluated until you call the method.

With val, you create a value (a constant). The expression is evaluated at the point where you declare it.

But you've written some code that makes it a bit confusing.

Note that the val fun in your method greater2 is a function: it has the type Int => Boolean. The syntax:

val fun = (x: Int) => if(x > n) true else false

is syntactic sugar for this:

val fun: Function1[Int, Boolean] = new Function1[Int, Boolean] {
  def apply(x: Int): Boolean = if(x > n) true else false
}

So, in that case, fun is really a Function1 object with an apply method that gets called when you make a "call" on fun.

Note by the way that an expression like if(x > n) true else false is overly verbose. You can just write x > n, which is by itself an expression that evaluates to a Boolean:

val fun = (x: Int) => x > n

[Mohamed Sanaulla]

I was waiting for Jesper to answer this Thanks a lot for the explanation.

[Samuel Cox]

Ok, I stand corrected.

Furthermore, I just had a colleague tell me that my def b() = i + 1 was not technically a closure; whereas, val b = () => i + 1 is.

Clearly, I have a lot to learn!

The good news is you can do a lot of good stuff with Scala without understanding these finer points. At least, that's my opinion.

[Marimuthu Madasamy]

To add, With 'def' you are creating a method which means you cannot pass it around as first class value whereas 'val's containing functions are first class values so you can pass it around.

Thats why you are getting the following error since the method is not passed enough arguments hence it is not a method call and also it cannot be assigned to the REPL generated variable since it is not a first class value:

scala> greater 
<console>:8: error: missing arguments for method greater in object $iw; 
follow this method with `_' if you want to treat it as a partially applied function 
greater 
^ 

If we follow this method with '_', a function is created which can be passed around/assigned to a variable.

[Luigi Plinge]

Thanks for the explanations.

The link Samuel Cox gives above is also quite helpful.