How do I interprete a sequence of => in Scala? - scala

Question

I'm currently starting with play framework, but my Scala knowledge is not sufficient.
As I know the => indicates that IsAuthenticated has some kind of functions as parameter.
I found out as well the f: => String... is a function with no input value. But how do I interprete the complete line with its 3 => ?
And further down, what exactly happens in the second line with => f(user)(request)? What is the target function for user and request object?
def IsAuthenticated(f: => String => Request[AnyContent] => Result) = Security.Authenticated(username, onUnauthorized) { user =>
Action(request => f(user)(request))
}

Answer 1

=> String => Request[AnyContent] => Result
is easier to read with parens added:
=> (String => (Request[AnyContent] => Result))
You can try this in the REPL. For example:
scala> def foo(f: => String => Int => Char): Char = f("abcdefgh")(4)
foo: (f: => String => (Int => Char))Char
In this example, f is a nullary function call-by-name parameter that returns a function (let's call that function g). g is a function that accepts a String parameter and returns yet another function (h). h is a function that accepts an Int parameter and returns a Char.
Example invocation:
scala> foo { s: String => { i: Int => s.charAt(i) } }
res0: Char = e
Let's walk through the type of each expression in the method body as it's evaluated:
f
Type: String => (Int => Char)
Value: { s: String => { i: Int => s.charAt(i) } }
f("abcdefgh")
Type: Int => Char
Value: { i: Int => "abcdefgh".charAt(i) }
f("abcdefgh")(4)
Type: Char
Value: 'e'

Answer 2

A couple of additions to the accepted answer
About Security.Authenticated
Security.Authenticated(username, onUnauthorized) { user =>
Action(request => f(user)(request))
}
Security.Authenticated - seems to be a function that takes two lists of parameters. The first list of parameters:
(username:String, onUnauthorized:Boolean)
and the second list is one argument:
(g:User => Action)
The complete signature can be something like:
def Authenticated(username:String, onUnauthorized:Boolean)(g:User => Action)
Action seems to be a class that takes a function in it's constructor:
class Action(h:Request => Result)
The argument to the constructor is a new function
def hActual(request:Request):Result = f(user)(request)
(the def should have user in the scope, because it is not given in arguments. For instance, hActual can be declared immediately before Action constructor call:
def IsAuthenticated(f: => String => Request[AnyContent] => Result) = Security.Authenticated(username, onUnauthorized) { user =>
def hActual(request:Request):Result = f(user)(request)
Action(hActual)
}
)
Also It seems so that the constructor can be called using curring:
def IsAuthenticated(f: => String => Request[AnyContent] => Result) = Security.Authenticated(username, onUnauthorized) { user =>
Action(f(user))
}