Understanding higher-kinded types - implementing trait functionality

Question

Trying to follow this example (taken from scala in action) for creating generic sum function for all collections

trait Summable[A] { def plus(a1: A, a2: A): A def init: A } object IntSummable extends Summable[Int] { def plus(a1: Int, a2: Int): Int = a1 + a2 def init: Int = 0 } trait Foldable[F[_]] { def foldLeft[A](xs: F[A], m: Summable[A]) : A }

implementing foldLeft like this works fine :

 object ListFoldLeft extends Foldable[List] { def foldLeft[A](xs:List[A],m:Summable[A]) = xs.foldLeft(m.init)(m.plus) }

however, however, although the foldLeft function accepts two parameters (xs:List[A],m:Summable[A]), we send them seperatly xs.foldLeft(m.init)(m.plus) and it works fine

but if I try to send them like this :

 object ListFoldLeft extends Foldable[List] { def foldLeft[A](xs:List[A],m:Summable[A]) = xs.foldLeft(m.init,m.plus) }

I get

<console>:18: error: too many arguments for method foldLeft: (z: B)(op: (B, A) => B)B xs.foldLeft(m.init,m.plus)

why ?

barczajozsef · Accepted Answer · 2014-02-06 15:21:51Z

2

Maybe I'm wrong, but it looks like you try to call List . foldLeft[B](z: B)(f: (B, A) ⇒ B): B curried function with only one parameter list, and with invalid number of parameters in that defined parameter list.

List.foldLeft is a curried function

more about curried functions here:

http://www.codecommit.com/blog/scala/function-currying-in-scala

edited Feb 6, 2014 at 15:21

answered Feb 6, 2014 at 14:35

barczajozsef

4835 silver badges12 bronze badges

Sign up to request clarification or add additional context in comments.

6 Comments

igx Over a year ago

I think you are missing my point : why do I get too many arguments when trying to implement it like this :object ListFoldLeft extends Foldable[List] { def foldLeft[A](xs:List[A],m:Summable[A]) = xs.foldLeft(m.init,m.plus) }

barczajozsef Over a year ago

xs is a List and you call foldLeft with one parameter list and in that parameterlist with invalid number of parameters.

Daenyth Over a year ago

@igx I believe you are invoking List.foldLeft instead of ListFoldLeft.foldLeft on xs

barczajozsef Over a year ago

@Daenyth is right, you invoke foldLeft on a different type than you would like to

barczajozsef Over a year ago

because foldLeft is a curried function, foldLeft has two parameter lists with specified number of parameters and you try to use the first parameter list with invalid numebr of parameters. please check my original comment about curried functions

|

Collectives™ on Stack Overflow

Understanding higher-kinded types - implementing trait functionality

1 Answer 1

6 Comments

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

6 Comments

Related