Below are two methods that are declared in different style. Both are doing the same work. I am wondering, why
different syntax is required to check type of function (yellow
block)
different syntax is required to call function (green block)
declaration of both methods on scala repl gives different results
(red block)
Also, please suggest, which one is the preferred way to declare
methods or are there any special use cases for both the styles of
method declaration?
Edit 1:
Below are the commands from screenshot :-
Welcome to Scala version 2.11.7 (Java HotSpot(TM) 64-Bit Server VM, Java 1.7.0_79).
Type in expressions to have them evaluated.
Type :help for more information.
scala> def add(x:Int, y :Int): Int = {x+y}
add: (x: Int, y: Int)Int
scala> def sum = (x:Int, y:Int) => {x+y}
sum: (Int, Int) => Int
scala> :t add
<console>:12: error: missing arguments for method add;
follow this method with `_' if you want to treat it as a partially applied function
add
^
scala> :t add(_, _)
(Int, Int) => Int
scala> :t sum
(Int, Int) => Int
scala> add
<console>:12: error: missing arguments for method add;
follow this method with `_' if you want to treat it as a partially applied function
add
^
scala> add(_, _)
res1: (Int, Int) => Int = <function2>
scala> sum
res2: (Int, Int) => Int = <function2>
Edit 2:
#Shadowlands
i have read on dzone, that states "when a function is expected but a method is provided, it will be automatically converted into a function. This is called the ETA expansion.".
Now, if ETA takes care to convert your methods to function. Is it really required to use sum style, because it looks like an additional overhead of Function<> object.
Simply put, add is a function that takes two Int arguments and returns an Int result, while sum is a 0-argument function that returns a new function that, in turn, takes two Int arguments and returns an Int result. Indeed, sum could readily be defined as:
def sum = add _
As to which way to define your functions, this depends on how it is to be used. Generally, use the add style most of the time, as it is easier to understand and work with. If, however, you will be passing the function as an argument to another function, then the sum-style formulation can be more convenient and may convey more of your intent on how the function is expected to be used.
This is how REPL is implemented. The code that you type is wrapped in object. Thus your def add becomes method of this object, while def sum is a method that returns Function. When you are addressing add like this: add(_, _) you are addressing method, while when you are addressing sum as sum you are addressing result of sum's execution, which is function.
Differences between methods and functions are described here.
PS. Try to check the type of sum _
Related
I noticed that I get the same effect if I define this trivial function:
fun double ( i: Int ) = i*2
and if I define a variable and assign a lambda (with an identical body) to it:
var double = { i : Int -> i*2 }
I get the same result if I call double(a) with either declaration.
This leaves me confused. When is it needed, recommended, advantageous to define a variable as a lambda rather than define a function to it?
When is it needed, recommended, advantageous to define a variable as a lambda rather than define a function to it?
Whenever you have the choice of either, you should use a fun declaration. Even with a fun you can still get a first-class callable object from it by using a function reference.
On the JVM, a fun is significantly more lightweight, both in terms of RAM and invocation overhead. It compiles into a Java method, whereas a val compiles into an instance field + getter + a synthetic class that implements a functional interface + a singleton instance of that class that you must fetch, dereference, and invoke a method on it.
You should consider a function-typed val or var only when something is forcing you to do it. One example is that you can dynamically replace a var and effectively change the definition of the function. You may also receive function objects from the outside, or you may need to comply with an API that needs them.
In any case, if you ever use a function-typed property of a class, you'll know why you're doing it.
First, if I understand you right, your question is "Why are functions first-class citizens in Kotlin -- And when to use them as such?", right?
Kotlin functions are first-class, which means that they can be stored in variables and data structures, passed as arguments to and returned from other higher-order functions. You can operate with functions in any way that is possible for other non-function values. (see here)
As stated in the docs, one use case are higher-order functions. As a first step, I will leave the wikipedia link here: https://en.wikipedia.org/wiki/Higher-order_function
Basically, a higher-order function is a function that takes functions as parameters, or returns a function.
This means that a higher-order function has at least one parameter of a function type or returns a value of a function type.
Following a short example of a higher-order function that receives a parameter of function type (Int) -> Boolean:
fun foo(pred: (Int) -> Boolean) : String = if(pred(x)) "SUCCESS" else "FAIL"
This higher-order function can now be called with any (Int) -> Boolean function.
The docs also state ... [can be used] in any way that is possible for other non-function values.
This means that you can, for example, assign different functions to a variable, depending on your current context.
For example:
// This example is verbose on purpose ;)
var checker: (Int) -> Boolean
if (POSITIVE_CHECK) {
checker = { x -> x > 0 } // Either store this function ...
} else {
checker = { x -> x < 0 } // ... or this one ...
}
if (checker(someNumber)) { // ... and use whatever function is now stored in variable "checker" here
print("Check was fine")
}
(Code untested)
You can define variable and assign it lambda when you want change behaviour for some reason. For example, you have different formula for several cases.
val formula: (Int) -> Int = when(value) {
CONDITION1 -> { it*2 }
CONDITION2 -> { it*3 }
else -> { it }
}
val x: Int = TODO()
val result = formula(x)
If you simply need helper function, you should define it as fun.
If you pass a lambda as a parameter of a function it will be stored in a variable. The calling application might need to save that (e.g. event listener for later use). Therefore you need to be able to store it as a variable as well. As said in the answer however, you should do this only when needed!
For me, I would write the Lambda variable as followed:
var double: (Int) -> Int = { i -> //no need to specify parameter name in () but in {}
i*2
}
So that you can easily know that its type is (i: Int) -> Int, read as takes an integer and returns an integer.
Then you can pass it to somewhere say a function like:
fun doSomething(double: (Int) -> Int) {
double(i)
}
Problem:
Something about implicit class, confuses reduce().
When inside implicit class, compiler complains on reduce() second parameter.
but when same code is inside non-implicit method it compiles and works fine.
What am I missing about implicit classes?
Code:
object ImpliCurri {
implicit class MySeq[Int](val l: Seq[Int]) {
//not compiling
final def mapSum(f:Int=>Int):Int = {
l.map(x=>f(x)).reduce(_+_)
//compile error on reduce: Type mismatch. Expected String, fount Int
}
}
// works fine
def mySum(l:Seq[Int], f:Int=>Int):Int = {
l.map(x=>f(x)).reduce(_+_)
// compiles and works no issues
}
}
You need to get rid of the type parameter Int. Int in the case of the implicit class is not actually the type Int, but instead it's a free type parameter that's shadowing the name of Int.
The reason for the cryptic compiler error is that the compiler is inferring the type Any from the lambda _ + _ (since the type parameter could be anything), and assuming the + will come from a toString on type Any. If you replace Int with T in the class declaration, you'll see the error is the same.
This will work:
implicit class MySeq(val l: Seq[Int]) {
final def mapSum(f: Int => Int): Int = {
l.map(x => f(x)).reduce(_ + _)
}
}
It doesn't actually have anything to do with implicits. You get the same error if it's just a regular class.
The reason is that you have declared a generic type: MySeq[Int] that just happens to be called Int. So when you say f: Int => Int you think "Oh, that's an integer" and the compiler thinks, "Oh, that means you could fill in any type there!". (Replace all your Ints with A and it would work the same.)
Now the compiler is in a bind. What + can you apply to any pair of types? Well, you can convert anything to a String, and + is defined on a String. So you get a very misleading error message when the compiler realizes that this approach won't work.
Just drop the [Int] and all your Ints will actually mean what you think they mean, and the implicit class version will work fine.
Replace MySeq[Int](val l: Seq[Int]) with MySeq(val l: Seq[Int]).
Explanation of the compiler message:
The MySeq[Int] part defines an abstract type parameter for class MySeq named Int, which is (automatically) a subclass of Any and shadows the actual scala.Int. Then the compiler tries to call + method of an instance of Any. It sees a declaration of an implicit class scala.Predef.any2stringadd, which has a method with signature def +(other: String): String, so the compiler thinks the second parameter to + should be a String.
I'm trying to understand every piece of the implementation of class OWritesOps explained here: http://kailuowang.blogspot.mx/2013/11/addremove-fields-to-plays-default-case.html
The way scala / play doc is structured doesn't really help (in this area Java(doc) beats Scala(doc) hands down).
Let's start with: addField:
def addField[T: Writes](fieldName: String, field: A => T): OWrites[A] =
(writes ~ (__ \ fieldName).write[T])((a: A) => (a, field(a)))
I could only guess that the ~ is an equivalent of "and" (I gather it from here: http://mandubian.com/2012/10/01/unveiling-play-2-dot-1-json-api-part2-writes-format-combinators/ )
Alright, so let's just take it this way: (writes ~ (__ \ fieldName).write[T]) create a new instance of Writes (combo).
What about (__ \ fieldName) ? After a lot of digging Play Doc, I came to a (shaky) conclusion that it produces an instance of JsPath? How did I arrive to that conclusion? It's for that .write[T] that follows; JsPath has that method.
Beside, I read somewhere that __ is a shortcut for JsPath (I forgot where, I guess it's in Play Doc as well, but using Scala doc it's almost impossible to find my way back to that page).
Ok..., so (writes ~ (__ \ fieldName).write[T]) produces an instance of OWrites. Next: what's this?: ((a: A) => (a, field(a)))
Oh..., it's for the apply the parameter to the "apply" method of OWrites; it takes an lambda function with the following signature ((A) => JsObject)
Ok...., so.... (a, field(a)) is supposed to create an instance of JsObject. So, I opened the play doc, JsObject. Hmm..., I guess it's this constructor (?): JsObject(fields: Seq[(String, JsValue)]) .... But,... I really doubt it.... I'm lost here.
Anyway, let's see an example how it is used:
val customWrites: Writes[Person] = Json.Writes[Person]. addField("isAdult", _.isAdult)
So..., _.isAdult is supposed to map to a block with the following signature: A => T ... But..., if that's really the case, I was expecting {x => x.isAdult} .... so I guess _.isAdult is a syntactic-sugar provided by Scala (?).
So... back to the definition of addField: field(a) executes the block "_.isAdult". I can infer here that a is an instance of Person then.
Can you help me decipher all this?
Scala is cool, but it's doc system (still) sucks :(
Can you help me decipher all this?
I could only guess that the ~ is an equivalent of "and"
~ is the name of a method invoked on the instance of OWrites. writes ~ (__ \ fieldName).write[T]) is the same as writes.~((__ \ fieldName).write[T])). Note the brackets around the argument of ~.
There are two things relevant to this equivalent form:
In Scala, any method with one argument can be written using infix notation. For example, if you have an instance list: java.util.ArrayList[Int], you could write list add 5.
While a method name need not be alphanumeric (as in Java), not all operators have the same precedence. For example, 1 + 5 * 2 translates to 1.+(5.*(2)) and not to 1.+(5).*(2). Here's a summary of operator precedence in Scala.
However, OWrites has no method called ~ in its interface. If you import play.api.libs.functional.syntax._, there's an implicit conversion
implicit def toFunctionalBuilderOps[M[_], A](a: M[A])(implicit fcb: FunctionalCanBuild[M]) = new FunctionalBuilderOps[M, A](a)(fcb)
and FunctionalBuilderOps defines the method
def ~[B](mb: M[B]): FunctionalBuilder[M]#CanBuild2[A, B] = {
val b = new FunctionalBuilder(fcb)
new b.CanBuild2[A, B](ma, mb)
}
and an alias for it: def and[B](mb: M[B]): FunctionalBuilder[M]#CanBuild2[A, B] = this.~(mb)
What about (__ \ fieldName) ?
The package play.api.libs.json defines a value val __ = JsPath, so the double underscores are just an alias for the singleton object JsPath. The singleton object JsPath is not to be confused with the equally-named class. This construct is called a companion object in Scala. While Java puts static members next to the regular members in a class, Scala puts them inside the companion object.
Apparently, the object JsPath is at the same time the companion object of the case class JsPath and an instance of JsPath(List.empty).
fieldName is of type String and JsPath defines a method
def \(child: String) = JsPath(path :+ KeyPathNode(child))
which is equivalent to
def \(child: String) = JsPath.apply(path :+ KeyPathNode(child))
The default apply method of a case class' companion object creates a new instance, so this is equivalent to
def \(child: String) = new JsPath(path :+ KeyPathNode(child))
As our singleton object __ is an instance of JsPath with the empty list for path, this returns us a new JsPath(KeyPathNode(child)), where child == fieldName
what's this?: ((a: A) => (a, field(a)))
It's a function taking an instance of A. The function maps the argument a to the tuple (a, field(a), where field is a function provided as argument to your method addField. It has the signature field: A => T, so it is a function that takes an instance of A and returns an instance of T.
This may be a bit confusing, so let me give you an example function that does not operate on generics. Say you want to define a function f that squares an integer number. For example, f(1) should be 1, f(3) should be 9. As it takes an integer and returns an integer, its signature is f: Int => Int. It's implementation is f = (x: Int) => x * x.
Oh..., it's for the apply the parameter to the "apply" method of OWrites; it takes an lambda function with the following signature ((A) => JsObject)
Actually, this is not correct - the signatures do not match. The given lambda function is of type A => (A, T). Also, as pointed out in the first part of the answer, the method ~ returns an instance of FunctionalBuilder#CanBuild2. Unfortunatly, I am lost on what the called apply-method does, because it is highly generic and uses implicit arguments of a generic type.
so I guess _.isAdult is a syntactic-sugar provided by Scala (?).
Yes, the underscore in lambda functions is syntactic sugar for capturing the arguments. For example, _ + _ is syntactic sugar for (x, y) => x + y, meaning the following two values are equivalent:
val f: (Int, Int) => Int = (x, y) => x + y
val g: (Int, Int) => Int = _ + _
So _.isAdult really just translates to {x => x.isAdult} and in the example, x: Person.
Update
The idea behind this write builder seems to be to provide means of defining a mapping from one of your Scala classes to JSON. Let's take a look again at the case class Person(name: String, age: Int). Now, let's assume we have an instance of Person:
val odersky = Person("Martin Odersky", 56)
and want to serialize the instance to the following JSON:
{
"name": "Martin Odersky",
"age": 56
}
The key is always a string, but the value can be of a different type. So, basically, the serialization is a function taking a Person and returning a tuple of key-value pairs where the value is always a string, i.e., Person => ((String, String), (String, Int)). Or, in the general case where we do not know what we are going to serialize, simply that it has two fields, it is A => ((String, B), (String, C)) for arbitrary types A, B, and C.
This is what CanBuild2 emulates - the 2 standing for the number of members to serialize. There's also a CanBuild3, CanBuild4 and so on.
We can instantiate a CanBuild2 that is appropriate to our example, by calling
val serializer = (__ \ "name").write[String] ~ (__ \ "age").write[Int]
Here, the type of serializer is FunctionalBuilder[OWrites]#CanBuild2[String, Int]
Now, the apply method of CanBuild2 can be used to generate an instance of the first generic argument - OWrites. The argument to the apply method is a function that maps an instance of our type to serialize (here Person) to a tuple of the desired value types, here String and Int (representing name and age).
val personWriter: OWrites[Person] = serializer((p: Person) => (p.name, p.age))
This writer can now be used to generate a JSON string from any instance of Person. So we can pass our person and get the desired output:
println(toJson(odersky)(personWriter))
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Possible Duplicate:
Difference between method and function in Scala
Two ways of defining functions in Scala. What is the difference?
There are 2 ways to define a function.
scala> def a : (Int, Int) => Int = {(s:Int, t:Int) => s + t}
a: (Int, Int) => Int
scala> a
res15: (Int, Int) => Int = <function2>
scala> def b(s:Int, t:Int) : Int = s+t
b: (s: Int, t: Int)Int
scala> b
<console>:9: error: missing arguments for method b in object $iw;
follow this method with `_' if you want to treat it as a partially applied function
b
^
scala> b(3,4)
res17: Int = 7
scala> a(3,4)
res18: Int = 7
Is there any difference in how I define functions a and b? Why do I have missing arguments error with b?
b is not a function, but a method. You can however turn b into a function by adding the _ as the compiler mentions. val f = b _. But you should only do this, if you want to pass a method of an object to a method/function that takes a function as parameter. If you just want to define a function go the normal way.
But to answer your question, there is another way:
val f = new Function2[Int,Int,Int] {
def apply(x: Int, y: Int) = x + y
}
Object Oriented languages (e.g. java) typically have classes and classes have methods, while functions are not "first-class citizens", meaning you can't assign a function directly to a variable or put a few functions in a list or send them as arguments to other functions.
If you want to send a function around in java, you have to create a class with a method and send the class around.
For instance, if you want to have a function that calculates the double of its input, you have to put it in a class, like this:
class Doubler
{
public int apply(int a)
{
return a * 2;
}
}
In functional programming languages, like Haskell, functions are "first-class", and you can store them in variables and send them around.
doubleIt :: Integer -> Integer
doubleIt x = 2 * x
As a beautiful combination of functional and object oriented, scala has both methods and functions.
// a function like in haskell
val doubleIt = (x:Int) => x * 2
// a method in an object like in java
object Doubler {
def apply(x:Int) = x * 2
}
In scala def always defines a method. Note that the REPL wraps everything you write in an object with a main (in order to be able to compile and execute it on the fly), but writing a def something not wrapped in a class/object/trait in a program would not compile.
Scala also has a few more perks to offer, to decrease the disconnect between object and first-class functions.
For one thing, the "apply" method over there in the Doubler object definition in scala is sort of magic.
Given the definition above, you can write Doubler(2) and scala's compiler will transform this into Doubler.apply(2) and happily return 4.
So you can sort of use objects as functions.
But also, you can (by putting a _ sign after the call to a method) transform the method into a real function and (say) assign it to a val.
scala> val d = Doubler.apply _
d: Int => Int = <function1>
scala> d(2)
res1: Int = 4
On the other hand, the way scala makes a function like val doubleIt = (x:Int) => x * 2 into a first class "thing" is by turning it into something like this behind your back.
object Doubler extends Function[Int, Int] {
def apply(x: Int) = x*2
}
val doubleIt = Doubler
So, yeah... a function is actually still a class with a method, more or less like in java. Except that scala does it for you and gives you a lot of syntactic sugar to use that generated class as you would use an actual function.
To make things even more interesting, since functions are first-class, one of the things you can do with them in scala is use them as return values from other functions or methods.
So when you wrote def a : (Int, Int) => Int = {(s:Int, t:Int) => s + t} you actually defined a method, called a that returns a function. It may be confusing at first (and at second, and at third....) but around the fourth time around it will probably start looking beautiful. At least that's what it did for me.
I read Scala Functions (part of Another tour of Scala). In that post he stated:
Methods and functions are not the same thing
But he didn't explain anything about it. What was he trying to say?
Jim has got this pretty much covered in his blog post, but I'm posting a briefing here for reference.
First, let's see what the Scala Specification tell us. Chapter 3 (types) tell us about Function Types (3.2.9) and Method Types (3.3.1). Chapter 4 (basic declarations) speaks of Value Declaration and Definitions (4.1), Variable Declaration and Definitions (4.2) and Functions Declarations and Definitions (4.6). Chapter 6 (expressions) speaks of Anonymous Functions (6.23) and Method Values (6.7). Curiously, function values is spoken of one time on 3.2.9, and no where else.
A Function Type is (roughly) a type of the form (T1, ..., Tn) => U, which is a shorthand for the trait FunctionN in the standard library. Anonymous Functions and Method Values have function types, and function types can be used as part of value, variable and function declarations and definitions. In fact, it can be part of a method type.
A Method Type is a non-value type. That means there is no value - no object, no instance - with a method type. As mentioned above, a Method Value actually has a Function Type. A method type is a def declaration - everything about a def except its body.
Value Declarations and Definitions and Variable Declarations and Definitions are val and var declarations, including both type and value - which can be, respectively, Function Type and Anonymous Functions or Method Values. Note that, on the JVM, these (method values) are implemented with what Java calls "methods".
A Function Declaration is a def declaration, including type and body. The type part is the Method Type, and the body is an expression or a block. This is also implemented on the JVM with what Java calls "methods".
Finally, an Anonymous Function is an instance of a Function Type (ie, an instance of the trait FunctionN), and a Method Value is the same thing! The distinction is that a Method Value is created from methods, either by postfixing an underscore (m _ is a method value corresponding to the "function declaration" (def) m), or by a process called eta-expansion, which is like an automatic cast from method to function.
That is what the specs say, so let me put this up-front: we do not use that terminology! It leads to too much confusion between so-called "function declaration", which is a part of the program (chapter 4 -- basic declarations) and "anonymous function", which is an expression, and "function type", which is, well a type -- a trait.
The terminology below, and used by experienced Scala programmers, makes one change from the terminology of the specification: instead of saying function declaration, we say method. Or even method declaration. Furthermore, we note that value declarations and variable declarations are also methods for practical purposes.
So, given the above change in terminology, here's a practical explanation of the distinction.
A function is an object that includes one of the FunctionX traits, such as Function0, Function1, Function2, etc. It might be including PartialFunction as well, which actually extends Function1.
Let's see the type signature for one of these traits:
trait Function2[-T1, -T2, +R] extends AnyRef
This trait has one abstract method (it has a few concrete methods as well):
def apply(v1: T1, v2: T2): R
And that tell us all that there is to know about it. A function has an apply method which receives N parameters of types T1, T2, ..., TN, and returns something of type R. It is contra-variant on the parameters it receives, and co-variant on the result.
That variance means that a Function1[Seq[T], String] is a subtype of Function1[List[T], AnyRef]. Being a subtype means it can be used in place of it. One can easily see that if I'm going to call f(List(1, 2, 3)) and expect an AnyRef back, either of the two types above would work.
Now, what is the similarity of a method and a function? Well, if f is a function and m is a method local to the scope, then both can be called like this:
val o1 = f(List(1, 2, 3))
val o2 = m(List(1, 2, 3))
These calls are actually different, because the first one is just a syntactic sugar. Scala expands it to:
val o1 = f.apply(List(1, 2, 3))
Which, of course, is a method call on object f. Functions also have other syntactic sugars to its advantage: function literals (two of them, actually) and (T1, T2) => R type signatures. For example:
val f = (l: List[Int]) => l mkString ""
val g: (AnyVal) => String = {
case i: Int => "Int"
case d: Double => "Double"
case o => "Other"
}
Another similarity between a method and a function is that the former can be easily converted into the latter:
val f = m _
Scala will expand that, assuming m type is (List[Int])AnyRef into (Scala 2.7):
val f = new AnyRef with Function1[List[Int], AnyRef] {
def apply(x$1: List[Int]) = this.m(x$1)
}
On Scala 2.8, it actually uses an AbstractFunction1 class to reduce class sizes.
Notice that one can't convert the other way around -- from a function to a method.
Methods, however, have one big advantage (well, two -- they can be slightly faster): they can receive type parameters. For instance, while f above can necessarily specify the type of List it receives (List[Int] in the example), m can parameterize it:
def m[T](l: List[T]): String = l mkString ""
I think this pretty much covers everything, but I'll be happy to complement this with answers to any questions that may remain.
One big practical difference between a method and a function is what return means. return only ever returns from a method. For example:
scala> val f = () => { return "test" }
<console>:4: error: return outside method definition
val f = () => { return "test" }
^
Returning from a function defined in a method does a non-local return:
scala> def f: String = {
| val g = () => { return "test" }
| g()
| "not this"
| }
f: String
scala> f
res4: String = test
Whereas returning from a local method only returns from that method.
scala> def f2: String = {
| def g(): String = { return "test" }
| g()
| "is this"
| }
f2: String
scala> f2
res5: String = is this
function A function can be invoked with a list of arguments to produce a
result. A function has a parameter list, a body, and a result type.
Functions that are members of a class, trait, or singleton object are
called methods. Functions defined inside other functions are called
local functions. Functions with the result type of Unit are called procedures.
Anonymous functions in source code are called function literals.
At run time, function literals are instantiated into objects called
function values.
Programming in Scala Second Edition.
Martin Odersky - Lex Spoon - Bill Venners
Let Say you have a List
scala> val x =List.range(10,20)
x: List[Int] = List(10, 11, 12, 13, 14, 15, 16, 17, 18, 19)
Define a Method
scala> def m1(i:Int)=i+2
m1: (i: Int)Int
Define a Function
scala> (i:Int)=>i+2
res0: Int => Int = <function1>
scala> x.map((x)=>x+2)
res2: List[Int] = List(12, 13, 14, 15, 16, 17, 18, 19, 20, 21)
Method Accepting Argument
scala> m1(2)
res3: Int = 4
Defining Function with val
scala> val p =(i:Int)=>i+2
p: Int => Int = <function1>
Argument to function is Optional
scala> p(2)
res4: Int = 4
scala> p
res5: Int => Int = <function1>
Argument to Method is Mandatory
scala> m1
<console>:9: error: missing arguments for method m1;
follow this method with `_' if you want to treat it as a partially applied function
Check the following Tutorial that explains passing other differences with examples like other example of diff with Method Vs Function, Using function as Variables, creating function that returned function
Functions don't support parameter defaults. Methods do. Converting from a method to a function loses parameter defaults. (Scala 2.8.1)
There is a nice article here from which most of my descriptions are taken.
Just a short comparison of Functions and Methods regarding my understanding. Hope it helps:
Functions:
They are basically an object. More precisely, functions are objects with an apply method; Therefore, they are a little bit slower than methods because of their overhead. It is similar to static methods in the sense that they are independent of an object to be invoked.
A simple example of a function is just like bellow:
val f1 = (x: Int) => x + x
f1(2) // 4
The line above is nothing except assigning one object to another like object1 = object2. Actually the object2 in our example is an anonymous function and the left side gets the type of an object because of that. Therefore, now f1 is an object(Function). The anonymous function is actually an instance of Function1[Int, Int] that means a function with 1 parameter of type Int and return value of type Int.
Calling f1 without the arguments will give us the signature of the anonymous function (Int => Int = )
Methods:
They are not objects but assigned to an instance of a class,i.e., an object. Exactly the same as method in java or member functions in c++ (as Raffi Khatchadourian pointed out in a comment to this question) and etc.
A simple example of a method is just like bellow:
def m1(x: Int) = x + x
m1(2) // 4
The line above is not a simple value assignment but a definition of a method. When you invoke this method with the value 2 like the second line, the x is substituted with 2 and the result will be calculated and you get 4 as an output. Here you will get an error if just simply write m1 because it is method and need the input value. By using _ you can assign a method to a function like bellow:
val f2 = m1 _ // Int => Int = <function1>
Here is a great post by Rob Norris which explains the difference, here is a TL;DR
Methods in Scala are not values, but functions are. You can construct a function that delegates to a method via η-expansion (triggered by the trailing underscore thingy).
with the following definition:
a method is something defined with def and a value is something you can assign to a val
In a nutshell (extract from the blog):
When we define a method we see that we cannot assign it to a val.
scala> def add1(n: Int): Int = n + 1
add1: (n: Int)Int
scala> val f = add1
<console>:8: error: missing arguments for method add1;
follow this method with `_' if you want to treat it as a partially applied function
val f = add1
Note also the type of add1, which doesn’t look normal; you can’t declare a variable of type (n: Int)Int. Methods are not values.
However, by adding the η-expansion postfix operator (η is pronounced “eta”), we can turn the method into a function value. Note the type of f.
scala> val f = add1 _
f: Int => Int = <function1>
scala> f(3)
res0: Int = 4
The effect of _ is to perform the equivalent of the following: we construct a Function1 instance that delegates to our method.
scala> val g = new Function1[Int, Int] { def apply(n: Int): Int = add1(n) }
g: Int => Int = <function1>
scala> g(3)
res18: Int = 4
Practically, a Scala programmer only needs to know the following three rules to use functions and methods properly:
Methods defined by def and function literals defined by => are functions. It is defined in page 143, Chapter 8 in the book of Programming in Scala, 4th edition.
Function values are objects that can be passed around as any values. Function literals and partially applied functions are function values.
You can leave off the underscore of a partially applied function if a function value is required at a point in the code. For example: someNumber.foreach(println)
After four editions of Programming in Scala, it is still an issue for people to differentiate the two important concepts: function and function value because all editions don't give a clear explanation. The language specification is too complicated. I found the above rules are simple and accurate.
In Scala 2.13, unlike functions, methods can take/return
type parameters (polymorphic methods)
implicit parameters
dependent types
However, these restrictions are lifted in dotty (Scala 3) by Polymorphic function types #4672, for example, dotty version 0.23.0-RC1 enables the following syntax
Type parameters
def fmet[T](x: List[T]) = x.map(e => (e, e))
val ffun = [T] => (x: List[T]) => x.map(e => (e, e))
Implicit parameters (context parameters)
def gmet[T](implicit num: Numeric[T]): T = num.zero
val gfun: [T] => Numeric[T] ?=> T = [T] => (using num: Numeric[T]) => num.zero
Dependent types
class A { class B }
def hmet(a: A): a.B = new a.B
val hfun: (a: A) => a.B = hmet
For more examples, see tests/run/polymorphic-functions.scala
The difference is subtle but substantial and it is related to the type system in use (besides the nomenclature coming from Object Oriented or Functional paradigm).
When we talk about a function, we talk about the type Function: it being a type, an instance of it can be passed around as input or output to other functions (at least in the case of Scala).
When we talk about a method (of a class), we are actually talking about the type represented by the class it is part of: that is, the method is just a component of a larger type, and cannot be passed around by itself. It must be passed around with the instance of the type it is part of (i.e. the instance of the class).
A method belongs to an object (usually the class, trait or object in which you define it), whereas a function is by itself a value, and because in Scala every value is an object, therefore, a function is an object.
For example, given a method and a function below:
def timesTwoMethod(x :Int): Int = x * 2
def timesTwoFunction = (x: Int) => x * 2
The second def is an object of type Int => Int (the syntactic sugar for Function1[Int, Int]).
Scala made functions objects so they could be used as first-class entities. This way you can pass functions to other functions as arguments.
However, Scala can also treat methods as functions via a mechanism called Eta Expansion.
For example, the higher-order function map defined on List, receives another function f: A => B as its only parameter. The next two lines are equivalent:
List(1, 2, 3).map(timesTwoMethod)
List(1, 2, 3).map(timesTwoFunction)
When the compiler sees a def given in a place where a function is needed, it automatically converts the method into an equivalent function.
A method operates on an object but a function doesn't.
Scala and C++ has Fuction but in JAVA, you have to imitate them with static methods.