Can someone explain what is going on here?

Question

Scala beginner, here. I'm pretty confused by this entire expression, to be honest. I vaguely understand what this is supposed to do, but what are the ~s doing?

val simple = {
   get[String]("user.email") ~
   get[String]("user.name") ~
   get[String]("user.password") map {
     case email~name~password => User(email, name, password)
   }
}

Here's the github source if you're curious or need more info.

Thanks in advance!

Answer 1

Yes the method RowParser.~ combines two separate RowParsers for separate columns, giving result RowParser[anorm.~[Col1Type, Col2Type]]. Here's the processing details:

1. Singleton object anorm.SqlParser has all members imported.

2. It has a method get[T]:

   def get[T](columnName: String)(implicit extractor: Column[T]): RowParser[T] 
   

   • when called, generic param T is instantiated as String
   • input param columnName is populated as "user.email", etc
   • input param extractor is implicit. It's not provided in calls. So, the compiler picks up an implicit value using the type signature matching within the Column singleton object. It finds method rowToString: Column[String].
   • output params RowParser[T] is instantiated RowParser[String]

3. anorm.RowParser[String] has method "~" called. It's signature is:

   def ~[B](p: RowParser[B]): RowParser[~[String, B]]  
   

4. But what is the input (parameter p) for "~"? It's on the next line: get[String]("user.name") ~.

5. Because of this "chaining" of method calls, the method call order is actually bottom-up:

   • first

     get[String]("user.password") map {
       case email~name~password => User(email, name, password)
     

     which returns type RowParser[String]

   • which is passed into

     get[String]("user.name") ~
     

     which returns type RowParser[~[String, String]]. Here, this uses a different "~" - the type anorm.~[A,B] which is defined as a case class:

     case class ~[+A, +B](_1: A, _2: B) extends Product with Serializable 
     

     representing the result of conjoining two parsed columns in the row.

   • This RowParser is passed into:

     get[String]("user.email") ~
     

     which returns type RowParser[~[String, RowParser[~[String, String]]]]

Answer 2

Figured it out. The '~' is a parser combinator that operates on a row. It validates the existence of user.emal, user.name, user.password, the result of which (if successful) is a ~[A, B, C] which can be mapped. In this case, if all 3 exist, a new User object is created and returned.