I was trying to implement the closure example in Scala, from Neal Ford's Functional Thinking presentation which is in Groovy. Refer slide # 43 & 44 https://sea.ucar.edu/sites/default/files/Functional_Thinking.pdf
def makeCounter : Unit = {
var localVar = 0
return { localVar += 1 }
}
This code returns an anonymous function. Now I want to increment the localVar by invoking this anonymous function.
I have two questions:
1. How do I invoke the anonymous function?
2. After invoking it how do I check if the value of localVar is incremented or not?
First I tried this -
val c1 = makeCounter(). It threw below error:
error: makeCounter of type Unit does not take parameters
Then I tried this.
val c1 = makeCounter
This didn't give any error. Only printed c1: Unit = ().
Then,
print(c1) printed (), whereas c1() gave the same error.
First of all. Don't use return, its semantics is completely different in Scala than in Java or Groovy.
The Unit type isn't a synonym for anonymous function. It's more like a indication of side-effects.
The type for an anonymous function is () => A. In your case you want a function that doesn't return any thing, but causes a side-effect. So its type should be () => Unit.
Let's see some code:
def makeCounter : () => Unit = {
var x = 0
{ () => x = x + 1 }
}
val counter = makeCounter
counter(); counter(); counter()
Great! We made makeCounter give us a fresh counter!
There is only one problem. x is a local variable in the method makeCounter and since it's never returned we can't see its value! Ever! We could, for example, remove x from the method, making it public in the outer scope. But it's not very functional. Instead let's make the function return it:
def makeCounter : () => Int = { // Notice now, instead of Unit we use Int
var x = 0
{ () => x = x + 1; x }
}
val counter = makeCounter
println(counter(), counter(), counter())
val counter2 = makeCounter
println(counter2(), counter2(), counter2())
You will see "1,2,3" twice. Once for each counter.
I didn't look at the presentation, so I don't know if this is functional thinking or just one of the slides on the road to functional thinking, but:
scala> def f: () => Int = {
| var v = 0
| () => v += 1 ; v }
f: () => Int
scala> val g = f
g: () => Int = <function0>
scala> g()
res0: Int = 1
scala> g()
res1: Int = 2
scala> g()
res2: Int = 3
The function literal returned by f is, of course, everything after the parens.
Related
I'm trying to understand the differences between these 2 syntaxes in scala and why they have not the same result.
testVal
and
testVal2
are not defined in the same way
object FunctionVsVal2 extends App {
val testVal: () => Int = {
val r = util.Random.nextInt
() => r
}
val testVal2: () => Int = () => {
val r = util.Random.nextInt
r
}
println(testVal())
println(testVal())
println(testVal2())
println(testVal2())
}
Thanks
testVal2 is a lambda expression. Each time you call testVal2(), it evaluates
{
val r = util.Random.nextInt
r
}
and so returns a new value.
testVal is a block expression. It calculates r first and then returns () => r (the last expression in the block), so each time you call testVal(), the same r is used.
I wrote this simple program in my attempt to learn how Cats Writer works
import cats.data.Writer
import cats.syntax.applicative._
import cats.syntax.writer._
import cats.instances.vector._
object WriterTest extends App {
type Logged2[A] = Writer[Vector[String], A]
Vector("started the program").tell
val output1 = calculate1(10)
val foo = new Foo()
val output2 = foo.calculate2(20)
val (log, sum) = (output1 + output2).pure[Logged2].run
println(log)
println(sum)
def calculate1(x : Int) : Int = {
Vector("came inside calculate1").tell
val output = 10 + x
Vector(s"Calculated value ${output}").tell
output
}
}
class Foo {
def calculate2(x: Int) : Int = {
Vector("came inside calculate 2").tell
val output = 10 + x
Vector(s"calculated ${output}").tell
output
}
}
The program works and the output is
> run-main WriterTest
[info] Compiling 1 Scala source to /Users/Cats/target/scala-2.11/classes...
[info] Running WriterTest
Vector()
50
[success] Total time: 1 s, completed Jan 21, 2017 8:14:19 AM
But why is the vector empty? Shouldn't it contain all the strings on which I used the "tell" method?
When you call tell on your Vectors, each time you create a Writer[Vector[String], Unit]. However, you never actually do anything with your Writers, you just discard them. Further, you call pure to create your final Writer, which simply creates a Writer with an empty Vector. You have to combine the writers together in a chain that carries your value and message around.
type Logged[A] = Writer[Vector[String], A]
val (log, sum) = (for {
_ <- Vector("started the program").tell
output1 <- calculate1(10)
foo = new Foo()
output2 <- foo.calculate2(20)
} yield output1 + output2).run
def calculate1(x: Int): Logged[Int] = for {
_ <- Vector("came inside calculate1").tell
output = 10 + x
_ <- Vector(s"Calculated value ${output}").tell
} yield output
class Foo {
def calculate2(x: Int): Logged[Int] = for {
_ <- Vector("came inside calculate2").tell
output = 10 + x
_ <- Vector(s"calculated ${output}").tell
} yield output
}
Note the use of for notation. The definition of calculate1 is really
def calculate1(x: Int): Logged[Int] = Vector("came inside calculate1").tell.flatMap { _ =>
val output = 10 + x
Vector(s"calculated ${output}").tell.map { _ => output }
}
flatMap is the monadic bind operation, which means it understands how to take two monadic values (in this case Writer) and join them together to get a new one. In this case, it makes a Writer containing the concatenation of the logs and the value of the one on the right.
Note how there are no side effects. There is no global state by which Writer can remember all your calls to tell. You instead make many Writers and join them together with flatMap to get one big one at the end.
The problem with your example code is that you're not using the result of the tell method.
If you take a look at its signature, you'll see this:
final class WriterIdSyntax[A](val a: A) extends AnyVal {
def tell: Writer[A, Unit] = Writer(a, ())
}
it is clear that tell returns a Writer[A, Unit] result which is immediately discarded because you didn't assign it to a value.
The proper way to use a Writer (and any monad in Scala) is through its flatMap method. It would look similar to this:
println(
Vector("started the program").tell.flatMap { _ =>
15.pure[Logged2].flatMap { i =>
Writer(Vector("ended program"), i)
}
}
)
The code above, when executed will give you this:
WriterT((Vector(started the program, ended program),15))
As you can see, both messages and the int are stored in the result.
Now this is a bit ugly, and Scala actually provides a better way to do this: for-comprehensions. For-comprehension are a bit of syntactic sugar that allows us to write the same code in this way:
println(
for {
_ <- Vector("started the program").tell
i <- 15.pure[Logged2]
_ <- Vector("ended program").tell
} yield i
)
Now going back to your example, what I would recommend is for you to change the return type of compute1 and compute2 to be Writer[Vector[String], Int] and then try to make your application compile using what I wrote above.
I'm confused about anonymous function definitions as following:
var plusOne = (x:Int)=>x+1
// or val plusOne=(x:Int)=>x+1
println(plusOne(2))
Or
def plusOne = (x:Int)=>x+1
println(plusOne(2))
What's the difference please in var/val and def for a function name.
val declares an "immutable variable or rather symbol" that doesn't allow reassignment, right hand side of the assignment is evaluated immediately
var declares a "mutable variable" that allows reassignments later to the symbol, right hand side of the assignment is evaluated immediately just like val
def declares an "immutable symbol" that doesn't allow reassignment, right hand side is evaluated lazily, i.e. whenever that symbol is referenced later in the code
Example -
var plusOneVar = (x:Int)=>x+1
val plusOneVal = (x:Int)=>x+1
def plusOneDef = (x:Int)=>x+1
plusOneVar = (x:Int)=>x+2 // Reassignment to var is valid
plusOneVal = (x:Int)=>x+2 // Compile time error, reassignment to val
plusOneDef = (x:Int)=>x+2 // Compile time error, reassignment to val
Because you are looking at an example with functions, it is hard to understand. Let's try to understand it with simple variables.
var symbolVar = 100 // line 1
val symbolVal = symbolVar // line 2
def symbolDef = symbolVar // line 3
println(symbolVar) // prints 100
println(symbolVal) // prints 100
println(symbolDef) // prints 100 - no surprise yet
symbolVar = symbolVar + 1
println(symbolVal) // still prints 100 which was evaluated and assigned on line 2
println(symbolDef) // prints 101 as symbolDef is a def and it depends on symbolVar, line 3 is evaluated again
var plusOne can be reassigned. val plusOne cannot be reassigned. Both are evaluated once. def plusOne is evaluated each time it is called
Note also with val (or var) one instance of the function is created and is used for any number of invocations to that function, whereas with def a new instance of the function is created for each invocation. Yet, for
def f (i:Int) = i+1
f: (i: Int)Int
note
val g = f _
g: Int => Int = <function1>
It might be clear from the Class File Disassembler results using javap. Save the following code as Test.scala
class Test {
val fVal: Int => Int = x => x + 1
var fVar: Int => Int = x => x + 1
def fDef(x: Int): Int = { x + 1 }
}
and do scalac Test.scala; javap Test will show
Compiled from "Test.scala"
public class Test {
public scala.Function1<java.lang.Object, java.lang.Object> fVal();
public scala.Function1<java.lang.Object, java.lang.Object> fVar();
public void fVar_$eq(scala.Function1<java.lang.Object, java.lang.Object>);
public int fDef(int);
public Test();
}
As is shown in the results above, val and fVar are represented as methods that return a Function1 object. The difference is fVar has an additional "setter". While fDef is like normal Java method.
I'm trying to write a class where when you call a function defined in the class, it will store it in an array of functions instead of executing it right away, then user calls exec() to execute it:
class TestA(val a: Int, newAction: Option[ArrayBuffer[(Int) => Int]]) {
val action: ArrayBuffer[(Int) => Int] = if (newAction.isEmpty) ArrayBuffer.empty[(Int) => Int] else newAction.get
def add(b: Int): TestA = {action += (a => a + b); new TestA(a, Some(action))}
def exec(): Int = {
var result = 0
action.foreach(r => result += r.apply(a))
result
}
def this(a:Int) = this(a, None)
}
Then this is my test code:
"delayed action" should "delay action till ready" in {
val test = new TestA(3)
val result = test.add(5).add(5)
println(result.exec())
}
This gives me a result of 16 because 3 was passed in twice and got added twice. I guess the easy way for me to solve this problem is to not pass in value for the second round, like change val a: Int to val a: Option[Int]. It helps but it doesn't solve my real problem: letting the second function know the result of the first execution.
Does anyone have a better solution to this?? Or if this is a pattern, can anyone share a tutorial of it?
Just save the result of the action in the 'result' variable (instatiate it with 'a') and use the previous result as input for the current iteration
def exec(): Int = {
var result = a
action.foreach(r => result = r.apply(result))
result
}
or use the more functional oriented solution that does the same
def exec(): Int = {
action.foldLeft(a)((r, f) => f.apply(r))
}
Does anyone know the type of => Unit in scala? I don't know the meaning of => Unit and how to use it. I defined a function like below:
def test(code: => Unit){
print("start ...")
code
print("end ....")
}
test(print(1))
Does it means a function with any arguments returning Unit?
Thanks
This kind of parameter are called by-name parameter
=> B represents a block a of code which return a B value, their purpose is that they are evaluated only when you call the parameter.
def foo(code: => Int) {
println("Not yet evaluated")
val result = code
println("parameter evaluated %s, is it an int ? %s " format (
result, result.isInstanceOf[Int]) )
}
And you can call foo in the following way :
foo(1)
or
val a = 3
val b = 5
foo {
val c = a * a
c * b
}
The other style of passing parameter is by-value : parameters are evaluated before they are sent to the method
def foo(code : Int) {
println("Parameter already evaluated")
val result = code
println("parameter evaluated : " + result)
}
References
Extract from the Book Functionnal Programming in Scala
More differences between by-name parameter and by-value parameter illustrated
In x: => Type the x is a call by name parameter. That's different than taking an argument which is a function taking no arguments: x: () => Type
This is called a by name parameter, as related to call-by-name parameter evaluation strategy. Please see the linked wikipedia article for similar, but not identical, ways of passing parameters.
To explain it better, let's consider first the two most common parameter evaluation strategies: call by value and call by reference.
Call by value is by far the most common evaluation strategy. It is the sole strategy in Java, for instance, and the default strategy in C. Consider, for instance, this simple Java program:
public class ByValue {
static public void inc(int y) {
y++;
}
static public void main(String... args) {
int x = 0;
inc(x);
System.out.println(x);
}
}
It will print 0, because x's value is copied to y, so that when y is incremented it doesn't change the original value in x. Contrast this with this C++ program with call-by-reference:
#include <stdio.h>
void inc(int &y) {
y++;
}
int main() {
int x = 0;
inc(x);
printf("%d\n", x);
}
This will print 1, because the reference to x is passed to inc, instead of x's value.
Note that Java passes objects references by value, which leads some to claim it does call by reference. This is not true, to the extent that if you were to assign a new object to a parameter of a function, it would not be reflected in the function's caller.
So, what does a call by name looks like? In a call by name, neither value nor reference is passed. Instead, the whole code is passed, and everywhere the parameter is used, the code is executed and its result used. For example:
object ByName {
def incIfZero(y: => Int): Int = if (y == 0) y + 1 else y
def main(args: Array[String]) {
var x = 0
x = incIfZero( { val tmp = x; x += 1; tmp } )
println(x)
}
}
This example prints 2 instead of 1, because the block of code passed as parameter is evaluted twice. When executing, it's as if the second line in the main was written like this:
x = if ({ val tmp = x; x += 1; tmp }) { val tmp = x; x += 1; tmp } + 1 else { val tmp = x; x += 1; tmp }
Now, by name parameters have at least three interesting uses:
It can be used to delay the execution of something until the proper time.
It can be used to avoid the execution in some situations.
It can be used to execute some block of code multiple times.
The first and last cases are pretty obvious, I think. Here's an example of the second case:
implicit def fromBoolean(b: Boolean) = new {
def and(that: => Boolean): Boolean = if (b) that else b }
val x = 0
(x > 0) and (10 / x > 0)
If that was not a by name parameter, there would be an exception thrown at the last line. As it is, it will just return false.
It means call-by-name, which basically means that the value is calculated when used in your function. As opposed to call-by-value which is default in Java (and Scala with regular type syntax), where the value is calculated before the method is called. A Unit type would not make much sense in call-by-value I guess..
This is typically used to create functions that behave as custom control structures, or things like timing or logging as in your example. Stuff you often use AOP to do in Java.