This is inside the chisel library
object Module {
// returns a new Module of type T, initialized with a Parameters instance if _p !=None.
def apply[T<:Module](c: =>T)(implicit _p: Option[Parameters] = None):T
}
I don't understand the =sign in the parameters. What does it represents?
The = in (implicit _p: Option[Parameters] = None) is assigning a default value of None to the parameter _p. That means that unless the otherwise specified there is no Parameter instance assigned to _p.
Just in case you are asking about the => in (c: =>T), the => is means that the first parameter c is a reference to a function that returns an instance of T, where T is a subclass of Module.
There's a bunch of idiomatic features of Scala being employed here: Function Currying, implicit parameters, Functions as first class citizens of the language. It's worth a taking a bit of time to learn the syntax of these things. Check out Chisel's generator-bootcamp tutorial particularly section 3.2 and 3.3 for some of the ways Chisel takes advantage of Scala's syntax
This example has two = signs. The first corresponds to By-name parameters: https://docs.scala-lang.org/tour/by-name-parameters.html.
The former is important because Modules in Chisel must wrapped in Module(...) when they are constructed. We generally accomplish using call by-name:
class MyModule extends Module {
...
}
// This works!
def func(mod: => MyModule) = {
val instance = Module(mod) // The module is constructed inside Module(...)
}
func(new MyModule)
// This doesn't work!
def func(mod: MyModule) = {
val instance = Module(mod)
}
func(new MyModule) // The module is constructed too early, here!
The second is a Default parameter: https://docs.scala-lang.org/tour/default-parameter-values.html. It's mainly a convenience thing:
def func(x: Int = 3) = { println(x) }
func(5) // prints 5
func() // prints 3
Given an overloaded function, with one taking a function as parameter. This parameter-function takes no arguments:
def func(param: () => Unit): Unit = {
param()
}
def func(param: Int): Unit = {
println(param)
}
While calling func with an anonymous function works perfect:
func(() => println("it works"))
Using a plain function fails:
def functionAsParam(): Unit = {
println("it works")
}
func(functionAsParam)
Obviously, Scala evaluates functionAsParam and don't pass the function itself to func. Question: How can I (as a user of a library which provides funcs) pass in a non-anonymous function?
There are several ways to do this. Either you explicitly pass the function as parameter:
scala> func(() => functionAsParam)
it works
scala> func (functionAsParam _ )
it works
(These two cases are slightly different though, in the sense that in the first example, you construct new anonymous function with your other function, and in the second example, you indicate that this function should not yet be evaluated, by adding _)
Or you create a variable which is a function and pass it along:
val fval = () => println("It works")
scala> func(fval)
It works
The error comes from the fact that you defined a method, but your func expects a function. Yeah, there is a problem with overloading in scala (in other areas as well). To fix it you need manually convert your method into a function (it's called eta-expantion):
func(functionAsParam _)
If you turn on -Xprint:typer you'll see that scalac expands you method into a function:
val res4: Unit = func({
(() => functionAsParam())
});
I have some recursion code I want to refactor to use tail recursive from Enumerator, I can simplify that recursion to look like this, please ignore what functionality this functionality wants to achieve.
#tailrec
def doStuff: List[Int] => Int = {
case Nil => 0
case x :: xs => doStuff(xs)
}
If I get rid of tailrec annotation, it is working fine, the structure looks like this doStuff(doStuff(doStuff(..))). It will have stackoverflow exception.
So how can I make it tail recursive if it is a function
Anonymous functions cannot be made tail-recursive. Let us first do a very simple rewrite of your code to introduce a val to hold the resulting function.
#tailrec
def doStuff: List[Int] => Int = {
val result: List[Int] => Int = {
case Nil => 0
case x :: xs => doStuff(xs)
}
result
}
It should be clear, from there, that doStuff(xs) is not calling the anonymous function itself. It is calling the method doStuff, which returns the function you want to call. Worse, since it is a def, it actually returns a different function on every call. Hence the anonymous function is definitely not calling itself.
The problem generalizes to this simple fact: anonymous functions are, well, anonymous. So there's no way they can call themselves directly: they're always calling some other def or val that might return themselves, but the compiler doesn't know that.
For this reason, only proper defs like the one proposed by #dhg will truly be tail-recursive.
Now, if you really want to return a function value, that happens to be implemented with a tail-recursive case, you can simply convert a method into a function using theMethod _. Hence, the solution to your initial problem would be the following:
val doStuff = {
#tailrec
def rec(list: List[Int]): Int = list match {
case Nil => 0
case x :: xs => rec(xs)
}
rec _
}
Note that we declare a proper tail-recursive method (rec), that we then transform into a function value with rec _.
Probably you mean this?
#tailrec
def doStuff(list: List[Int]): Int = list match {
case Nil => 0
case x :: xs => doStuff(xs)
}
So how can I make it tail recursive if it is a function
You can't. I mean, of course you can write a tail-recursive function in Scala, but it won't help you because it won't get optimized.
Scala only optimizes direct tail-recursive method calls.
This question already has answers here:
Closed 10 years ago.
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.