I am new to Scala and trying to understand a following method:
def method1 = {
val key = "k1"
val value = "v1"
basicSetup() { (a, b, c) =>
val json = s"""{"field1":"$value"}"""
someMethodTest.send(a, b, json, c)
} { (record, avroObject, schema) =>
if (avroObject.get("field1").toString != value) {
failure("failed")
} else {
success
}
}
}
So far I worked on simple methods and understand when is a simple call and return but here it looks like is bundled stuff in it.
Need a help to understand how to read it from basicSetup line (just a general flow, signature and return).
e.g. Why is here 2 blocks of code: basicSetup() { ... } { ...} (how is it being executed?)
private def basicSetup()
(run: (Producer, String, Schema) => Unit)
(verify: (ProducerRecord[String, Array[Byte]], GenericRecord, Schema) => Result) = {
...
...
}
Thanks
It would be helpful to look at the definition of basicSetup, but it looks like a method with three parameter groups, the last two of which are themselves functions (making basicSetup a higher-order function).
The first parameter group is empty ().
The second and third are two "closures" or blocks of code or anonymous functions.
You could rewrite this as
// give names to these blocks
def doSomethingWithABC(a:A, b:B, c:C) = ???
def doSomethingWithAvro(record: R, avro: O, schema: S) = ???
basicSetup()(doSomethingWithABC)(doSomethingWithAvro)
Why is here 2 blocks of code ?
This is syntactic sugar to make function calls (especially higher-order function calls) look more like "built-in" constructs. So you can roll your own control flow methods. Keyword here is DSL.
These two blocks are parameters to basicSetup. They can appear as just bare blocks (without any parameter parentheses) to make it more concise (and natural, once you get used to it).
Update (now that we have the signature):
private def basicSetup()
(run: (Producer, String, Schema) => Unit)
(verify: (ProducerRecord[String, Array[Byte]], GenericRecord, Schema) => Result) = {
Indeed. The function takes three parameter groups.
The first one is actually empty, so you just call it with (). But it could have some parameters, even optionals, maybe to add configuration.
The second one is your "callback" to "run" (after this basic setup has been completed). It itself is a function that will be called with three parameters, a Producer, a String and a Schema.
The third one is your code to "verify" the results of all that. It looks at three parameters and returns a Result (presumably indicating that all is good or what went wrong).
Related
What is still unclear for is what's the advantage by-name parameters over anonymous functions in terms of lazy evaluation and other benefits if any:
def func1(a: => Int)
def func2(a: () => Int)
When should I use the first and when the second one?
This is not the copy of What's the difference between => , ()=>, and Unit=>
Laziness is the same in the both cases, but there are slight differences. Consider:
def generateInt(): Int = { ... }
def byFunc(a: () => Int) { ... }
def byName(a: => Int) { ... }
// you can pass method without
// generating additional anonymous function
byFunc(generateInt)
// but both of the below are the same
// i.e. additional anonymous function is generated
byName(generateInt)
byName(() => generateInt())
Functions with call-by-name however is useful for making DSLs. For instance:
def measured(block: ⇒ Unit): Long = {
val startTime = System.currentTimeMillis()
block
System.currentTimeMillis() - startTime
}
Long timeTaken = measured {
// any code here you like to measure
// written just as there were no "measured" around
}
def func1(a: => Int) {
val b = a // b is of type Int, and it`s value is the result of evaluation of a
}
def func2(a: () => Int) {
val b = a // b is of type Function0 (is a reference to function a)
}
An example might give a pretty thorough tour of the differences.
Consider that you wanted to write your own version of the veritable while loop in Scala. I know, I know... using while in Scala? But this isn't about functional programming, this is an example that demonstrates the topic well. So hang with me. We'll call our own version whyle. Furthermore, we want to implement it without using Scala's builtin while. To pull that off we can make our whyle construct recursive. Also, we'll add the #tailrec annotation to make sure that our implementation can be used as a real-world substitute for the built-in while. Here's a first go at it:
#scala.annotation.tailrec
def whyle(predicate: () => Boolean)(block: () => Unit): Unit = {
if (predicate()) {
block()
whyle(predicate)(block)
}
}
Let's see how this works. We can pass in parameterized code blocks to whyle. The first is the predicate parameterized function. The second is the block parameterized function. How would we use this?
What we want is for our end user to use the whyle just like you would the while control structure:
// Using the vanilla 'while'
var i = 0
while(i < args.length) {
println(args(i))
i += 1
}
But since our code blocks are parameterized, the end-user of our whyle loop must add some ugly syntactic sugar to get it to work:
// Ouch, our whyle is hideous
var i = 0
whyle( () => i < args.length) { () =>
println(args(i))
i += 1
}
So. It appears that if we want the end-user to be able to call our whyle loop in a more familiar, native looking style, we'll need to use parameterless functions. But then we have a really big problem. As soon as you use parameterless functions, you can no longer have your cake and eat it too. You can only eat your cake. Behold:
#scala.annotation.tailrec
def whyle(predicate: => Boolean)(block: => Unit): Unit = {
if (predicate) {
block
whyle(predicate)(block) // !!! THIS DOESN'T WORK LIKE YOU THINK !!!
}
}
Wow. Now the user can call our whyle loop the way they expect... but our implementation doesn't make any sense. You have no way of both calling a parameterless function and passing the function itself around as a value. You can only call it. That's what I mean by only eating your cake. You can't have it, too. And therefore our recursive implementation now goes out the window. It only works with the parameterized functions which is unfortunately pretty ugly.
We might be tempted at this point to cheat. We could rewrite our whyle loop to use Scala's built-in while:
def whyle(pred: => Boolean)(block: => Unit): Unit = while(pred)(block)
Now we can use our whyle exactly like while, because we only needed to be able to eat our cake... we didn't need to have it, too.
var i = 0
whyle(i < args.length) {
println(args(i))
i += 1
}
But we cheated! Actually, here's a way to have our very own tail-optimized version of the while loop:
def whyle(predicate: => Boolean)(block: => Unit): Unit = {
#tailrec
def whyle_internal(predicate2: () => Boolean)(block2: () => Unit): Unit = {
if (predicate2()) {
block2()
whyle_internal(predicate2)(block2)
}
}
whyle_internal(predicate _)(block _)
}
Can you figure out what we just did?? We have our original (but ugly) parameterized functions in the inner function here. We have it wrapped with a function that takes as arguments parameterless functions. It then calls the inner function and converts the parameterless functions into parameterized functions (by turning them into partially applied functions).
Let's try it out and see if it works:
var i = 0
whyle(i < args.length) {
println(args(i))
i += 1
}
And it does!
Thankfully, since in Scala we have closures we can clean this up big time:
def whyle(predicate: => Boolean)(block: => Unit): Unit = {
#tailrec
def whyle_internal: Unit = {
if (predicate) {
block
whyle_internal
}
}
whyle_internal
}
Cool. Anyways, those are some really big differences between parameterless and parameterized functions. I hope this gives you some ideas!
The two formats are used interchangeably, but there are some cases where we can use only one of theme.
let's explain by example, suppose that we need to define a case class with two parameters :
{
.
.
.
type Action = () => Unit;
case class WorkItem(time : Int, action : Action);
.
.
.
}
as we can see, the second parametre of the WorkItem class has a type Action.
if we try to replace this parameter with the other format =>,
case class WorkItem1(time : Int, s : => Unit) the compiler will show a message error :
Multiple markers at this line:
`val' parameters may not be call-by-name Call-by-name parameter
creation: () ⇒
so as we have see the format ()=> is more generic and we can use it to define Type, as class field or method parameter, in the other side => format can used as method parameter but not as class field.
A by-name type, in which the empty parameter list, (), is left out, is only
allowed for parameters. There is no such thing as a by-name variable or a
by-name field.
You should use the first function definition if you want to pass as the argument an Int by name.
Use the second definition if you want the argument to be a parameterless function returning an Int.
I am developing Play application and I've just started with Scala. I see that there is this word Action after the equals sign in the function below and before curly brace.
def index = Action {
Ok(views.html.index("Hi there"))
}
What does this code do? I've seen it used with def index = { but not with the word before the curly brace.
I would assume that the name of the function is index. But I do not know what the word Action does in this situation.
This word is a part of Play Framework, and it's an object, which has method apply(block: ⇒ Result), so your code is actually:
def index: Action[AnyContent] = Action.apply({
Ok.apply(views.html.index("Hi there"))
})
Your index method returns an instance of the class Action[AnyContent].
By the way, you're passing a block of code {Ok(...)} to apply method, which (block of code) is actually acts as anonymous function here, because the required type for apply's input is not just Result but ⇒ Result, which means that it takes an anonymous function with no input parameters, which returns Result. So, your Ok-block will be executed when container, received your instance of class Action (from index method), decided to execute this block. Which simply means that you're just describing an action here - not executing - it will be actually executed when Play received your request - and find binding to your action inside routing file.
Also, you don't have to use def here as you always return same action - val or lazy val is usually enough. You will need a def only if you actually want to pass some parameter from routing table (for instance):
GET /clients/:id controllers.SomeController.index(id: Long)
def index(id: Long) = Action { ... } // new action generated for every new request here
Another possible approach is to choose Action, based on parameter:
def index(id: Long) = {
if (id == 0) Action {...} else Action{...}
}
But uasually you can use routing table itself for that, which is better for decoupling. This example just shows that Action is nothing more than return value.
Update for #Kazuya
val method1 = Action{...} //could be def too, no big difference here
// this (code inside Action) gonna be called separately after "index" (if method2 is requested of course)
// notice that it needs the whole request, so it (request) should be completely parsed at the time
val method2 = Action{ req => // you can extract additional params from request
val param1 = req.headers("header1")
...
}
//This is gonna be called first, notice that Play doesn't need the whole request body here, so it might not even be parsed on this stage
def index(methodName: String) = methodName match {
case "method1" => method1
case "method2" => method2
}
GWT/Scala.js use simillar approach for client-server interaction. This is just one possible solution to explain importance of the parameter "methodName" passed from routing table. So, action could be thought as a wrapper over function that in its turn represents a reference to OOP-method, which makes it useful for both REST and RPC purposes.
The other answers deal with your specific case. You asked about the general case, however, so I'll attempt to answer from that perspective.
First off, def is used to define a method, not a function (better to learn that difference now). But, you're right, index is the name of that method.
Now, unlike other languages you might be familiar with (e.g., C, Java), Scala lets you define methods with an expression (as suggested by the use of the assignment operator syntax, =). That is, everything after the = is an expression that will be evaluated to a value each time the method is invoked.
So, whereas in Java you have to say:
public int three() { return 3; }
In Scala, you can just say:
def three = 3
Of course, the expression is usually more complicated (as in your case). It could be a block of code, like you're more used to seeing, in which case the value is that of the last expression in the block:
def three = {
val a = 1
val b = 2
a + b
}
Or it might involve a method invocation on some other object:
def three = Numbers.add(1, 2)
The latter is, in fact, exactly what's going on in your specific example, although it requires a bit more explanation to understand why. There are two bits of magic involved:
If an object has an apply method, then you can treat the object as if it were a function. You can say, for example, Add(1, 2) when you really mean Add.apply(1,2) (assuming there's an Add object with an apply method, of course). And just to be clear, it doesn't have to be an object defined with the object keyword. Any object with a suitable apply method will do.
If a method has a single by-name parameter (e.g., def ifWaterBoiling(fn: => Tea)), then you can invoke the method like ifWaterBoiling { makeTea }. The code in that block is evaluated lazily (and may not be evaluated at all). This would be equivalent to writing ifWaterBoiling({ makeTea }). The { makeTea } part just defines an expression that gets passed in, unevaluated, for the fn parameter.
Its the Action being called on with an expression block as argument. (The apply method is used under the hood).
Action.apply({
Ok("Hello world")
})
A simple example (from here) is as follows (look at comments in code):
case class Logging[A](action: Action[A]) extends Action[A] {
def apply(request: Request[A]): Result = {// apply method which is called on expression
Logger.info("Calling action")
action(request) // action being called on further with the request provided to Logging Action
}
lazy val parser = action.parser
}
Now you can use it to wrap any other action value:
def index = Logging { // Expression argument starts
Action { // Action argument (goes under request)
Ok("Hello World")
}
}
Also, the case you mentioned for def index = { is actually returning Unit like: def index: Unit = {.
I am new to Scala, and am learning it by going over some Play code. I have had a good read of the major concepts of Scala and am comfortable with functional programming having done some Haskell and ML.
I am really struggling to read this code, at the level of the syntax and the programming paradigms alone. I understand what the code is supposed to do, but not how it does it because I can't figure out the syntax.
// -- Home page
def index(ref: Option[String]): Action[AnyContent] = Prismic.action(ref) { implicit request =>
for {
someDocuments <- ctx.api.forms("everything").ref(ctx.ref).submit()
} yield {
Ok(views.html.index(someDocuments))
}
}
(Prismic is an API separate to Play and is not really that relevant). How would I describe this function (or is it a method??) to another developer over the phone: in other words, using English. For example in this code:
def add(a: Int, b: Int): Int = a + b
I would say "add is a function which takes two integers, adds them together and returns the result as another integer".
In the Play code above I don't even know how to describe it after getting to "index is a function which takes an Option of a String and returns an Action of type AnyContent by ....."
The bit after the '=' and then the curly braces and the '=>' scare me! How do I read them? And is the functional or OO?
Thanks for your assistance
Let's reduce it to this:
def index(ref: Option[String]): Action[AnyContent] = Prismic.action(ref)(function)
That's better, isn't it? index is a function from Option of String to Action of AnyContent (one word), which calls the action method of the object Prismic passing two curried parameters: ref, the parameter that index received, and a function (to be described).
So let's break down the anonymous function:
{ implicit request =>
for {
someDocuments <- ctx.api.forms("everything").ref(ctx.ref).submit()
} yield {
Ok(views.html.index(someDocuments))
}
}
First, it uses {} instead of () because Scala allows one to drop () as parameter delimiter if it's a single parameter (there are two parameter lists, but each has a single parameter), and that parameter is enclosed in {}.
So, what about {}? Well, it's an expression that contains declarations and statements, with semi-colon inference on new lines, whose value is that of the last statement. That is, the value of these two expressions is the same, 3:
{ 1; 2; 3 }
{
1
2
3
}
It's a syntactic convention to use {} when passing a function that extends for more than one line, even if, as in this case, that function could have been passed with just parenthesis.
The next thing confusing is the implicit request =>, Let's pick something simpler:
x => x * 2
That's pretty easy, right? It takes one parameter, x, and returns x * 2. In our case, it is the same thing: the function takes one parameter, request, and returns this:
for (someDocuments <- somethingSomething())
yield Ok(views.html.index(someDocuments))
That is, it calls some methods, iterate over the result, and map those results into a new value. This is a close equivalent to Haskell's do notation. You can rewrite it like below (I'm breaking it down into multiple lines for readability):
ctx
.api
.forms("everything")
.ref(ctx.ref)
.submit()
.map(someDocuments => Ok(views.html.index(someDocuments)))
So, back to our method definition, we have this:
def index(ref: Option[String]): Action[AnyContent] = Prismic.action(ref)(
implicit request =>
ctx
.api
.forms("everything")
.ref(ctx.ref)
.submit()
.map(someDocuments => Ok(views.html.index(someDocuments)))
)
The only remaining question here is what that implicit is about. Basically, it makes that parameter implicitly available through the scope of the function. Presumably, at least one of these method calls require an implicit parameter which is properly fielded by request. I could drop the implicit there an pass request explicitly, if I knew which of these methods require it, but since I don't, I'm skipping that.
An alternate way of writing it would be:
def index(ref: Option[String]): Action[AnyContent] = Prismic.action(ref)({
request =>
implicit val req = request
ctx
.api
.forms("everything")
.ref(ctx.ref)
.submit()
.map(someDocuments => Ok(views.html.index(someDocuments)))
})
Here I added {} back because I added a declaration to the body of the function, though I decided not to drop the parenthesis, which I could have.
Something like this:
index is a function which takes an Option of a String and returns an Action of type AnyContent. It calls the method action that takes as a first argument an Option and as a second argument a method that assumes an implicit value request of type Request is in scope. This method uses a For-comprehension that calls the submit method which returns an Option or a Future and then in case its execution is successful, it yields the result Ok(...) that will be wrapped in the Action returned by the action method of Prismic.
Prismic.action is a method that takes 2 groups of arguments (a.k.a. currying).
The first is ref
The second is { implicit request => ...}, a function defined in a block of a code
more information on Action
I have a series of validation functions that returns an Option[Problem], if any, or None if no validation problems are found.
I would like to write a simple function that calls each validation function, stop and return the first not-None result.
Naturally I can write this method in the "java-style", but I would like to know if a better approach exists.
EDIT
This was the original Java solution:
validate01(arg);
validate02(arg);
validate03(arg);
...
Each method throws an exception in case of problem. I would stay away from the exceptions while I'm writing Scala.
As an example, let's say we want to validate a String. Our validation function takes a String and a list of validators, which are functions from String to Option[Problem]. We can implement it in a functional manner like this:
def firstProblem(validators: List[String => Option[Problem]], s:String) =
validators.view.flatMap(_(s)).headOption
This creates a new list by applying each validation function to the string and keeping the result only if it is a Some. We then take the first element of this List. Because of the call to view, the list will be computed only as needed. So as soon as the first Problem is found, no further validators will be called.
If you have finite and known at compile time number of validations you may use .orElse on Options:
def foo(x: Int): Option[Problem] = ...
def bar(x: Int): Option[Problem] = ...
...
def baz(x: Int): Option[Problem] = ...
foo(1) orElse bar(2) orElse .. baz(n)
Maybe you want--assuming the validation functions take no arguments
def firstProblem(fs: (() => Option[Problem])*) = {
fs.iterator.map(f => f()).find(_.isDefined).flatten
}
You'll get an existing Option[Problem] if there are any, or None if they all succeed. If you need to pass arguments to the functions, then you need to explain what those arguments are. For example, you could
def firstProblem[A](a: A)(fs: (A => Option[Problem])*) = /* TODO */
if you can pass the same argument to all of them. You would use it like this:
firstProblem(myData)(
validatorA,
validatorB,
validatorC,
validatorD
)
I have a loan pattern that applies a function n times where 'i' is the incrementing variable. "Occasionally", I want the function passed in to have access to 'i'....but I don't want to require all functions passed in to require defining a param to accept 'i'. Example below...
def withLoaner = (n:Int) => (op:(Int) => String) => {
val result = for(i <- 1 to n) yield op(i)
result.mkString("\n")
}
def bob = (x:Int) => "bob" // don't need access to i. is there a way use () => "bob" instead?
def nums = (x:Int) => x.toString // needs access to i, define i as an input param
println(withLoaner(3)(bob))
println(withLoaner(3)(nums))
def withLoaner(n: Int) = new {
def apply(op: Int => String) : String = (1 to n).map(op).mkString("\n")
def apply(op: () => String) : String = apply{i: Int => op()}
}
(not sure how it is related to the loan pattern)
Edit Little explanation as requested in comment.
Not sure what you know and don't know of scala and what you don't undestand in that code. so sorry if what I just belabor the obvious.
First, a scala program consist of traits/classes (also singleton object) and methods. Everything that is done is done by methods (leaving constructor aside). Functions (as opposed to methods) are instances of (subtypes of) the various FunctionN traits (N the number of arguments). Each of them has as apply method that is the actual implemention.
If you write
val inc = {i: Int => i + 1}
it is desugared to
val inc = new Function1[Int, Int] {def apply(i: Int) = i + 1}
(defines an anonymous class extending Function1, with given apply method and creating an instance)
So writing a function has rather more weight than a simple method. Also you cannot have overloading (several methods with the same name, differing by the signature, just what I did above), nor use named arguments, or default value for arguments.
On the other hand, functions are first classes values (they can be passed as arguments, returned as result) while methods are not. They are automatically converted to functions when needed, however there may be some edges cases when doing that. If a method is intended solely to be used as a function value, rather than called as a method, it might be better to write a function.
A function f, with its apply method, is called with f(x) rather than f.apply(x) (which works too), because scala desugars function call notation on a value (value followed by parentheses and 0 or more args) to a call to method apply. f(x) is syntactic sugar for f.apply(x). This works whatever the type of f, it does not need to be one of the FunctionN.
What is done in withLoaner is returning an object (of an anonymous type, but one could have defined a class separately and returned an instance of it). The object has two apply methods, one accepting an Int => String, the other one an () => String. When you do withLoaner(n)(f) it means withLoaner(n).apply(f). The appropriate apply method is selected, if f has the proper type for one of them, otherwise, compile error.
Just in case you wonder withLoaner(n) does not mean withLoaner.apply(n) (or it would never stop, that could just as well mean withLoaner.apply.apply(n)), as withLoaner is a method, not a value.