CODEWITHSUNDEEP
scala
MojoJojo
MojoJojo

Reputation: 4242

Can someone explain the scala code below?

scala> def lift3[A,B,C,D] (
     | f: Function3[A,B,C,D]): Function3[Option[A], Option[B], Option[C], Option
[D]] = {
     | (oa: Option[A], ob:Option[B], oc: Option[C]) =>
     | for(a <- oa; b <- ob; c <- oc) yield f(a,b,c)
     | }
lift3: [A, B, C, D](f: (A, B, C) => D)(Option[A], Option[B], Option[C]) => Option[D]

In particular, the following line:

def lift3[A,B,C,D] (
          f: Function3[A,B,C,D]): Function3[Option[A], Option[B], Option[C], Option
    [D]]

This is taken from the book Scala In Depth by Joshua D Suereth (Listing 2.1, chapter 2). I'm not sure what purpose the additional Option[D] serves. In the body of the function, the code only maps to the first three parameters to the output type D. When then, D is declared in the input parameter list? Am I missing something?

With my limited understanding, I would read the function declaration as a function that takes a function as a parameter (which in turn takes 4 parameters) and returns a function that takes 4 parameters. Also, why is there no mention of the return type? Thanks in advance!

Upvotes: 2

Views: 105

Answers (2)

Luka Jacobowitz
Luka Jacobowitz

Reputation: 23532

A function3 takes 3 parameters. The D is the return type of the function. What the code does, is take a function with 3 arguments and return a function with 3 arguments, where each argument and its return type is "lifted" to an Option.

You can check out the API docs for Function3 here. And an explanation for the R generic type can be found in the docs for Function2

Upvotes: 2

Alexey Romanov
Alexey Romanov

Reputation: 170859

With my limited understanding, I would read the function declaration as a function that takes a function as a parameter (which in turn takes 4 parameters) and returns a function that takes 4 parameters. Also, why is there no mention of the return type?

Function3[A,B,C,D] is a function with 3 parameters (of types A, B and C) and D is the return type (it can also be written as (A, B, C) => D; this is exactly the same type). So in Function3[Option[A], Option[B], Option[C], Option[D]], Option[D] is the return type, not a parameter type.

Upvotes: 3

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