What does `(...)(...)` mean in Scala code?-CodePudding

I got this piece of code from https://index.scala-lang.org/playframework/play-json that I don't fully understand:

implicit val locationReads: Reads[Location] = (
  (JsPath \ "lat").read[Double](min(-90.0) keepAnd max(90.0)) and
  (JsPath \ "long").read[Double](min(-180.0) keepAnd max(180.0))
)(Location.apply _)

What does (...)(...) mean? I took a look at the Scala syntax cheat sheet but still cannot figure what it is. Thank you in advance!

CodePudding user response：

This is called currying, and it means that the function returns another function that takes one argument.

CodePudding user response：

Without syntax sugar, what you have is:

(JsPath \ "lat").read[Double](min(-90.0) keepAnd max(90.0))
 .and( (JsPath \ "long").read[Double](min(-180.0) keepAnd max(180.0)) )
 .apply(Location.apply _)

The parentheses around the (... and ...) in your post are just like the parentheses in an expression like (1 2) / 3. They were used so the and method could be called like an operator and then call a method on the result of that and method. Nothing really special there.

The parentheses around Location.apply _ are because you are passing Location.apply _ as an argument to a function. You are technically calling the (... and ...) object's apply method, and omitting the word "apply" thanks to a Scala syntax sugar.

In scala, objects with an apply method can be treated as functions thanks to the syntax sugar, i.e. the following two statements are equivalent:

foo.apply(1)
foo(1) // equivalent

Note that this is not "currying" as the other answer states.

Currying is a way to represent a function with multiple arguments as a series of "functions which return functions", e.g.

def myFunc(a: Int, b: Int, c: Int) = a   b   c

val sum = myFunc(1, 2, 3)

def myCurriedFunc(a: Int)(b: Int)(c: Int) = a   b   c
val sum = myCurriedFunc(1)(2)(3)
val addOneAndTwo = myCurriedFunc(1)(2)
val sumAgain = addOneAndTwo(3)